take more than one argument. Thus, a comma terminates the argument of
a unary operator, but merely separates the arguments of a list
operator. A unary operator generally provides a scalar context to its
-argument, while a list operator may provide either scalar and list
+argument, while a list operator may provide either scalar or list
contexts for its arguments. If it does both, the scalar arguments will
-be first, and the list argument will follow. (Note that there can only
-ever be one list argument.) For instance, splice() has three scalar
-arguments followed by a list.
+be first, and the list argument will follow. (Note that there can ever
+be only one such list argument.) For instance, splice() has three scalar
+arguments followed by a list, whereas gethostbyname() has four scalar
+arguments.
In the syntax descriptions that follow, list operators that expect a
list (and provide list context for the elements of the list) are shown
Any function in the list below may be used either with or without
parentheses around its arguments. (The syntax descriptions omit the
-parens.) If you use the parens, the simple (but occasionally
-surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
+parentheses.) If you use the parentheses, the simple (but occasionally
+surprising) rule is this: It I<looks> like a function, therefore it I<is> a
function, and precedence doesn't matter. Otherwise it's a list
operator or unary operator, and precedence does matter. And whitespace
between the function and left parenthesis doesn't count--so you need to
be careful sometimes:
- print 1+2+3; # Prints 6.
- print(1+2) + 3; # Prints 3.
- print (1+2)+3; # Also prints 3!
- print +(1+2)+3; # Prints 6.
- print ((1+2)+3); # Prints 6.
+ print 1+2+4; # Prints 7.
+ print(1+2) + 4; # Prints 3.
+ print (1+2)+4; # Also prints 3!
+ print +(1+2)+4; # Prints 7.
+ print ((1+2)+4); # Prints 7.
If you run Perl with the B<-w> switch it can warn you about this. For
example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
+A few functions take no arguments at all, and therefore work as neither
+unary nor list operators. These include such functions as C<time>
+and C<endpwent>. For example, C<time+86_400> always means
+C<time() + 86_400>.
+
For functions that can be used in either a scalar or list context,
-non-abortive failure is generally indicated in a scalar context by
+nonabortive failure is generally indicated in a scalar context by
returning the undefined value, and in a list context by returning the
null list.
-Remember the following rule:
+Remember the following important rule: There is B<no rule> that relates
+the behavior of an expression in list context to its behavior in scalar
+context, or vice versa. It might do two totally different things.
+Each operator and function decides which sort of value it would be most
+appropriate to return in scalar context. Some operators return the
+length of the list that would have been returned in list context. Some
+operators return the first value in the list. Some operators return the
+last value in the list. Some operators return a count of successful
+operations. In general, they do what you want, unless you want
+consistency.
+
+An named array in scalar context is quite different from what would at
+first glance appear to be a list in scalar context. You can't get a list
+like C<(1,2,3)> into being in scalar context, because the compiler knows
+the context at compile time. It would generate the scalar comma operator
+there, not the list construction version of the comma. That means it
+was never a list to start with.
-=over 5
+In general, functions in Perl that serve as wrappers for system calls
+of the same name (like chown(2), fork(2), closedir(2), etc.) all return
+true when they succeed and C<undef> otherwise, as is usually mentioned
+in the descriptions below. This is different from the C interfaces,
+which return C<-1> on failure. Exceptions to this rule are C<wait>,
+C<waitpid>, and C<syscall>. System calls also set the special C<$!>
+variable on failure. Other functions do not, except accidentally.
-=item *
+=head2 Perl Functions by Category
+
+Here are Perl's functions (including things that look like
+functions, like some keywords and named operators)
+arranged by category. Some functions appear in more
+than one place.
+
+=over
+
+=item Functions for SCALARs or strings
+
+C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
+C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
+C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
+
+=item Regular expressions and pattern matching
+
+C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
+
+=item Numeric functions
+
+C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
+C<sin>, C<sqrt>, C<srand>
+
+=item Functions for real @ARRAYs
+
+C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
+
+=item Functions for list data
+
+C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
+
+=item Functions for real %HASHes
+
+C<delete>, C<each>, C<exists>, C<keys>, C<values>
+
+=item Input and output functions
+
+C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
+C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
+C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
+C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
+C<warn>, C<write>
+
+=item Functions for fixed length data or records
+
+C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
+
+=item Functions for filehandles, files, or directories
+
+C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
+C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
+C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
+C<unlink>, C<utime>
+
+=item Keywords related to the control flow of your perl program
+
+C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
+C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
+
+=item Keywords related to scoping
+
+C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
-I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
+=item Miscellaneous functions
+
+C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
+C<scalar>, C<undef>, C<wantarray>
+
+=item Functions for processes and process groups
+
+C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
+C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
+C<times>, C<wait>, C<waitpid>
+
+=item Keywords related to perl modules
+
+C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
+
+=item Keywords related to classes and object-orientedness
+
+C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
+C<untie>, C<use>
+
+=item Low-level socket functions
+
+C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
+C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
+C<socket>, C<socketpair>
+
+=item System V interprocess communication functions
+
+C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
+C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
+
+=item Fetching user and group info
+
+C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
+C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
+C<getpwuid>, C<setgrent>, C<setpwent>
+
+=item Fetching network info
+
+C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
+C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
+C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
+C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
+C<setnetent>, C<setprotoent>, C<setservent>
+
+=item Time-related functions
+
+C<gmtime>, C<localtime>, C<time>, C<times>
+
+=item Functions new in perl5
+
+C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
+C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
+C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
+C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
+
+* - C<sub> was a keyword in perl4, but in perl5 it is an
+operator, which can be used in expressions.
+
+=item Functions obsoleted in perl5
+
+C<dbmclose>, C<dbmopen>
=back
-Each operator and function decides which sort of value it would be most
-appropriate to return in a scalar context. Some operators return the
-length of the list that would have been returned in a list context. Some
-operators return the first value in the list. Some operators return the
-last value in the list. Some operators return a count of successful
-operations. In general, they do what you want, unless you want
-consistency.
+=head2 Portability
+
+Perl was born in Unix and can therefore access all common Unix
+system calls. In non-Unix environments, the functionality of some
+Unix system calls may not be available, or details of the available
+functionality may differ slightly. The Perl functions affected
+by this are:
+
+C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
+C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
+C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
+C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
+C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
+C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
+C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
+C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
+C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
+C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
+C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
+C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
+C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
+C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
+C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
+C<utime>, C<wait>, C<waitpid>
+
+For more information about the portability of these functions, see
+L<perlport> and other available platform-specific documentation.
+
+=head2 Alphabetical Listing of Perl Functions
=over 8
-=item -X FILEHANDLE
+=item I<-X> FILEHANDLE
-=item -X EXPR
+=item I<-X> EXPR
-=item -X
+=item I<-X>
A file test, where X is one of the letters listed below. This unary
operator takes one argument, either a filename or a filehandle, and
tests the associated file to see if something is true about it. If the
-argument is omitted, tests $_, except for C<-t>, which tests STDIN.
-Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
+argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
+Unless otherwise documented, it returns C<1> for true and C<''> for false, or
the undefined value if the file doesn't exist. Despite the funny
names, precedence is the same as any other named unary operator, and
the argument may be parenthesized like any other unary operator. The
operator may be any of:
+X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
+X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-e File exists.
-z File has zero size.
- -s File has non-zero size (returns size).
+ -s File has nonzero size (returns size).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
- -p File is a named pipe (FIFO).
+ -p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-g File has setgid bit set.
-k File has sticky bit set.
- -T File is a text file.
- -B File is a binary file (opposite of -T).
+ -T File is an ASCII text file.
+ -B File is a "binary" file (opposite of -T).
-M Age of file in days when script started.
-A Same for access time.
-C Same for inode change time.
-The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
-C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
-uids and gids of the user. There may be other reasons you can't actually
-read, write or execute the file. Also note that, for the superuser,
-C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
-1 if any execute bit is set in the mode. Scripts run by the superuser may
-thus need to do a stat() in order to determine the actual mode of the
-file, or temporarily set the uid to something else.
-
Example:
while (<>) {
chop;
next unless -f $_; # ignore specials
- ...
+ #...
}
+The interpretation of the file permission operators C<-r>, C<-R>,
+C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
+of the file and the uids and gids of the user. There may be other
+reasons you can't actually read, write, or execute the file. Such
+reasons may be for example network filesystem access controls, ACLs
+(access control lists), read-only filesystems, and unrecognized
+executable formats.
+
+Also note that, for the superuser on the local filesystems, the C<-r>,
+C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
+if any execute bit is set in the mode. Scripts run by the superuser
+may thus need to do a stat() to determine the actual mode of the file,
+or temporarily set their effective uid to something else.
+
+If you are using ACLs, there is a pragma called C<filetest> that may
+produce more accurate results than the bare stat() mode bits.
+When under the C<use filetest 'access'> the above-mentioned filetests
+will test whether the permission can (not) be granted using the
+access() family of system calls. Also note that the C<-x> and C<-X> may
+under this pragma return true even if there are no execute permission
+bits set (nor any extra execute permission ACLs). This strangeness is
+due to the underlying system calls' definitions. Read the
+documentation for the C<filetest> pragma for more information.
+
Note that C<-s/a/b/> does not do a negated substitution. Saying
C<-exp($foo)> still works as expected, however--only single letters
following a minus are interpreted as file tests.
The C<-T> and C<-B> switches work as follows. The first block or so of the
file is examined for odd characters such as strange control codes or
-characters with the high bit set. If too many odd characters (>30%)
+characters with the high bit set. If too many strange characters (E<gt>30%)
are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
containing null in the first block is considered a binary file. If C<-T>
or C<-B> is used on a filehandle, the current stdio buffer is examined
-rather than the first block. Both C<-T> and C<-B> return TRUE on a null
-file, or a file at EOF when testing a filehandle.
+rather than the first block. Both C<-T> and C<-B> return true on a null
+file, or a file at EOF when testing a filehandle. Because you have to
+read a file to do the C<-T> test, on most occasions you want to use a C<-f>
+against the file first, as in C<next unless -f $file && -T $file>.
-If any of the file tests (or either the stat() or lstat() operators) are given the
-special filehandle consisting of a solitary underline, then the stat
+If any of the file tests (or either the C<stat> or C<lstat> operators) are given
+the special filehandle consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator) is used, saving
a system call. (This doesn't work with C<-t>, and you need to remember
that lstat() and C<-l> will leave values in the stat structure for the
=item abs VALUE
+=item abs
+
Returns the absolute value of its argument.
+If VALUE is omitted, uses C<$_>.
=item accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the accept(2) system call
-does. Returns the packed address if it succeeded, FALSE otherwise.
-See example in L<perlipc>.
+does. Returns the packed address if it succeeded, false otherwise.
+See the example in L<perlipc/"Sockets: Client/Server Communication">.
=item alarm SECONDS
+=item alarm
+
Arranges to have a SIGALRM delivered to this process after the
-specified number of seconds have elapsed. (On some machines,
+specified number of seconds have elapsed. If SECONDS is not specified,
+the value stored in C<$_> is used. (On some machines,
unfortunately, the elapsed time may be up to one second less than you
specified because of how seconds are counted.) Only one timer may be
counting at once. Each call disables the previous timer, and an
-argument of 0 may be supplied to cancel the previous timer without
+argument of C<0> may be supplied to cancel the previous timer without
starting a new one. The returned value is the amount of time remaining
on the previous timer.
-For sleeps of finer granularity than one second, you may use Perl's
-syscall() interface to access setitimer(2) if your system supports it,
-or else see L</select()> below.
+For delays of finer granularity than one second, you may use Perl's
+four-argument version of select() leaving the first three arguments
+undefined, or you might be able to use the C<syscall> interface to
+access setitimer(2) if your system supports it. The Time::HiRes module
+from CPAN may also prove useful.
+
+It is usually a mistake to intermix C<alarm>
+and C<sleep> calls.
+
+If you want to use C<alarm> to time out a system call you need to use an
+C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
+fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
+restart system calls on some systems. Using C<eval>/C<die> always works,
+modulo the caveats given in L<perlipc/"Signals">.
+
+ eval {
+ local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
+ alarm $timeout;
+ $nread = sysread SOCKET, $buffer, $size;
+ alarm 0;
+ };
+ if ($@) {
+ die unless $@ eq "alarm\n"; # propagate unexpected errors
+ # timed out
+ }
+ else {
+ # didn't
+ }
=item atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
+For the tangent operation, you may use the C<POSIX::tan()>
+function, or use the familiar relation:
+
+ sub tan { sin($_[0]) / cos($_[0]) }
+
=item bind SOCKET,NAME
Binds a network address to a socket, just as the bind system call
-does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
-packed address of the appropriate type for the socket. See example in
-L<perlipc>.
+does. Returns true if it succeeded, false otherwise. NAME should be a
+packed address of the appropriate type for the socket. See the examples in
+L<perlipc/"Sockets: Client/Server Communication">.
=item binmode FILEHANDLE
-Arranges for the file to be read or written in "binary" mode in
-operating systems that distinguish between binary and text files.
-Files that are not in binary mode have CR LF sequences translated to LF
-on input and LF translated to CR LF on output. Binmode has no effect
-under Unix; in DOS, it may be imperative. If FILEHANDLE is an expression,
-the value is taken as the name of the filehandle.
-
-=item bless REF,PACKAGE
+Arranges for FILEHANDLE to be read or written in "binary" mode on
+systems whose run-time libraries force the programmer to guess
+between binary and text files. If FILEHANDLE is an expression, the
+value is taken as the name of the filehandle. binmode() should be
+called after the C<open> but before any I/O is done on the filehandle.
+The only way to reset binary mode on a filehandle is to reopen the
+file.
+
+The operating system, device drivers, C libraries, and Perl run-time
+system all conspire to let the programmer conveniently treat a
+simple, one-byte C<\n> as the line terminator, irrespective of its
+external representation. On Unix and its brethren, the native file
+representation exactly matches the internal representation, making
+everyone's lives unbelievably simpler. Consequently, L<binmode>
+has no effect under Unix, Plan9, or Mac OS, all of which use C<\n>
+to end each line. (Unix and Plan9 think C<\n> means C<\cJ> and
+C<\r> means C<\cM>, whereas the Mac goes the other way--it uses
+C<\cM> for c<\n> and C<\cJ> to mean C<\r>. But that's ok, because
+it's only one byte, and the internal and external representations
+match.)
+
+In legacy systems like MS-DOS and its embellishments, your program
+sees a C<\n> as a simple C<\cJ> (just as in Unix), but oddly enough,
+that's not what's physically stored on disk. What's worse, these
+systems refuse to help you with this; it's up to you to remember
+what to do. And you mustn't go applying binmode() with wild abandon,
+either, because if your system does care about binmode(), then using
+it when you shouldn't is just as perilous as failing to use it when
+you should.
+
+That means that on any version of Microsoft WinXX that you might
+care to name (or not), binmode() causes C<\cM\cJ> sequences on disk
+to be converted to C<\n> when read into your program, and causes
+any C<\n> in your program to be converted back to C<\cM\cJ> on
+output to disk. This sad discrepancy leads to no end of
+problems in not just the readline operator, but also when using
+seek(), tell(), and read() calls. See L<perlport> for other painful
+details. See the C<$/> and C<$\> variables in L<perlvar> for how
+to manually set your input and output line-termination sequences.
+
+=item bless REF,CLASSNAME
=item bless REF
-This function tells the referenced object (passed as REF) that it is now
-an object in PACKAGE--or the current package if no PACKAGE is specified,
-which is the usual case. It returns the reference for convenience, since
-a bless() is often the last thing in a constructor. See L<perlobj> for
-more about the blessing (and blessings) of objects.
+This function tells the thingy referenced by REF that it is now an object
+in the CLASSNAME package. If CLASSNAME is omitted, the current package
+is used. Because a C<bless> is often the last thing in a constructor,
+it returns the reference for convenience. Always use the two-argument
+version if the function doing the blessing might be inherited by a
+derived class. See L<perltoot> and L<perlobj> for more about the blessing
+(and blessings) of objects.
+
+Consider always blessing objects in CLASSNAMEs that are mixed case.
+Namespaces with all lowercase names are considered reserved for
+Perl pragmata. Builtin types have all uppercase names, so to prevent
+confusion, you may wish to avoid such package names as well. Make sure
+that CLASSNAME is a true value.
+
+See L<perlmod/"Perl Modules">.
=item caller EXPR
=item caller
-Returns the context of the current subroutine call. In a scalar context,
-returns TRUE if there is a caller, that is, if we're in a subroutine or
-eval() or require(), and FALSE otherwise. In a list context, returns
+Returns the context of the current subroutine call. In scalar context,
+returns the caller's package name if there is a caller, that is, if
+we're in a subroutine or C<eval> or C<require>, and the undefined value
+otherwise. In list context, returns
- ($package,$filename,$line) = caller;
+ ($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger uses to
print a stack trace. The value of EXPR indicates how many call frames
to go back before the current one.
+ ($package, $filename, $line, $subroutine,
+ $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
+
+Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
+call, but an C<eval>. In such a case additional elements $evaltext and
+C<$is_require> are set: C<$is_require> is true if the frame is created by a
+C<require> or C<use> statement, $evaltext contains the text of the
+C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
+$filename is C<"(eval)">, but $evaltext is undefined. (Note also that
+each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
+frame.
+
+Furthermore, when called from within the DB package, caller returns more
+detailed information: it sets the list variable C<@DB::args> to be the
+arguments with which the subroutine was invoked.
+
+Be aware that the optimizer might have optimized call frames away before
+C<caller> had a chance to get the information. That means that C<caller(N)>
+might not return information about the call frame you expect it do, for
+C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
+previous time C<caller> was called.
+
=item chdir EXPR
-Changes the working directory to EXPR, if possible. If EXPR is
-omitted, changes to home directory. Returns TRUE upon success, FALSE
-otherwise. See example under die().
+Changes the working directory to EXPR, if possible. If EXPR is omitted,
+changes to the user's home directory. Returns true upon success,
+false otherwise. See the example under C<die>.
=item chmod LIST
Changes the permissions of a list of files. The first element of the
-list must be the numerical mode. Returns the number of files
-successfully changed.
+list must be the numerical mode, which should probably be an octal
+number, and which definitely should I<not> a string of octal digits:
+C<0644> is okay, C<'0644'> is not. Returns the number of files
+successfully changed. See also L</oct>, if all you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
+ $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
+ # --w----r-T
+ $mode = '0644'; chmod oct($mode), 'foo'; # this is better
+ $mode = 0644; chmod $mode, 'foo'; # this is best
=item chomp VARIABLE
=item chomp
-This is a slightly safer version of chop (see below). It removes any
-line ending that corresponds to the current value of C<$/> (also known as
-$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
-of characters removed. It's often used to remove the newline from the
-end of an input record when you're worried that the final record may be
-missing its newline. When in paragraph mode (C<$/ = "">), it removes all
-trailing newlines from the string. If VARIABLE is omitted, it chomps
-$_. Example:
+This safer version of L</chop> removes any trailing string
+that corresponds to the current value of C<$/> (also known as
+$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
+number of characters removed from all its arguments. It's often used to
+remove the newline from the end of an input record when you're worried
+that the final record may be missing its newline. When in paragraph
+mode (C<$/ = "">), it removes all trailing newlines from the string.
+When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
+a reference to an integer or the like, see L<perlvar>) chomp() won't
+remove anything.
+If VARIABLE is omitted, it chomps C<$_>. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
- ...
+ # ...
}
You can actually chomp anything that's an lvalue, including an assignment:
Chops off the last character of a string and returns the character
chopped. It's used primarily to remove the newline from the end of an
input record, but is much more efficient than C<s/\n//> because it neither
-scans nor copies the string. If VARIABLE is omitted, chops $_.
+scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
Example:
while (<>) {
chop; # avoid \n on last field
@array = split(/:/);
- ...
+ #...
}
You can actually chop anything that's an lvalue, including an assignment:
chop($answer = <STDIN>);
If you chop a list, each element is chopped. Only the value of the
-last chop is returned.
+last C<chop> is returned.
+
+Note that C<chop> returns the last character. To return all but the last
+character, use C<substr($string, 0, -1)>.
=item chown LIST
Changes the owner (and group) of a list of files. The first two
-elements of the list must be the I<NUMERICAL> uid and gid, in that order.
-Returns the number of files successfully changed.
+elements of the list must be the I<numeric> uid and gid, in that
+order. A value of -1 in either position is interpreted by most
+systems to leave that value unchanged. Returns the number of files
+successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
-Here's an example that looks up non-numeric uids in the passwd file:
+Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
- chop($user = <STDIN>);
- print "Files: "
- chop($pattern = <STDIN>);
+ chomp($user = <STDIN>);
+ print "Files: ";
+ chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
- @ary = <${pattern}>; # expand filenames
+ @ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
+On most systems, you are not allowed to change the ownership of the
+file unless you're the superuser, although you should be able to change
+the group to any of your secondary groups. On insecure systems, these
+restrictions may be relaxed, but this is not a portable assumption.
+On POSIX systems, you can detect this condition this way:
+
+ use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
+ $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
+
=item chr NUMBER
+=item chr
+
Returns the character represented by that NUMBER in the character set.
-For example, C<chr(65)> is "A" in ASCII.
+For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
+chr(0x263a) is a Unicode smiley face (but only within the scope of
+a C<use utf8>). For the reverse, use L</ord>.
+See L<utf8> for more about Unicode.
+
+If NUMBER is omitted, uses C<$_>.
=item chroot FILENAME
-Does the same as the system call of that name. If you don't know what
-it does, don't worry about it. If FILENAME is omitted, does chroot to
-$_.
+=item chroot
+
+This function works like the system call by the same name: it makes the
+named directory the new root directory for all further pathnames that
+begin with a C<"/"> by your process and all its children. (It doesn't
+change your current working directory, which is unaffected.) For security
+reasons, this call is restricted to the superuser. If FILENAME is
+omitted, does a C<chroot> to C<$_>.
=item close FILEHANDLE
-Closes the file or pipe associated with the file handle, returning TRUE
+=item close
+
+Closes the file or pipe associated with the file handle, returning true
only if stdio successfully flushes buffers and closes the system file
-descriptor. You don't have to close FILEHANDLE if you are immediately
-going to do another open on it, since open will close it for you. (See
-open().) However, an explicit close on an input file resets the line
-counter ($.), while the implicit close done by open() does not. Also,
-closing a pipe will wait for the process executing on the pipe to
-complete, in case you want to look at the output of the pipe
-afterwards. Closing a pipe explicitly also puts the status value of
-the command into C<$?>. Example:
-
- open(OUTPUT, '|sort >foo'); # pipe to sort
- ... # print stuff to output
- close OUTPUT; # wait for sort to finish
- open(INPUT, 'foo'); # get sort's results
-
-FILEHANDLE may be an expression whose value gives the real filehandle name.
+descriptor. Closes the currently selected filehandle if the argument
+is omitted.
+
+You don't have to close FILEHANDLE if you are immediately going to do
+another C<open> on it, because C<open> will close it for you. (See
+C<open>.) However, an explicit C<close> on an input file resets the line
+counter (C<$.>), while the implicit close done by C<open> does not.
+
+If the file handle came from a piped open C<close> will additionally
+return false if one of the other system calls involved fails or if the
+program exits with non-zero status. (If the only problem was that the
+program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
+also waits for the process executing on the pipe to complete, in case you
+want to look at the output of the pipe afterwards, and
+implicitly puts the exit status value of that command into C<$?>.
+
+Prematurely closing the read end of a pipe (i.e. before the process
+writing to it at the other end has closed it) will result in a
+SIGPIPE being delivered to the writer. If the other end can't
+handle that, be sure to read all the data before closing the pipe.
+
+Example:
+
+ open(OUTPUT, '|sort >foo') # pipe to sort
+ or die "Can't start sort: $!";
+ #... # print stuff to output
+ close OUTPUT # wait for sort to finish
+ or warn $! ? "Error closing sort pipe: $!"
+ : "Exit status $? from sort";
+ open(INPUT, 'foo') # get sort's results
+ or die "Can't open 'foo' for input: $!";
+
+FILEHANDLE may be an expression whose value can be used as an indirect
+filehandle, usually the real filehandle name.
=item closedir DIRHANDLE
-Closes a directory opened by opendir().
+Closes a directory opened by C<opendir> and returns the success of that
+system call.
+
+DIRHANDLE may be an expression whose value can be used as an indirect
+dirhandle, usually the real dirhandle name.
=item connect SOCKET,NAME
Attempts to connect to a remote socket, just as the connect system call
-does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
-package address of the appropriate type for the socket. See example in
-L<perlipc>.
+does. Returns true if it succeeded, false otherwise. NAME should be a
+packed address of the appropriate type for the socket. See the examples in
+L<perlipc/"Sockets: Client/Server Communication">.
+
+=item continue BLOCK
+
+Actually a flow control statement rather than a function. If there is a
+C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
+C<foreach>), it is always executed just before the conditional is about to
+be evaluated again, just like the third part of a C<for> loop in C. Thus
+it can be used to increment a loop variable, even when the loop has been
+continued via the C<next> statement (which is similar to the C C<continue>
+statement).
+
+C<last>, C<next>, or C<redo> may appear within a C<continue>
+block. C<last> and C<redo> will behave as if they had been executed within
+the main block. So will C<next>, but since it will execute a C<continue>
+block, it may be more entertaining.
+
+ while (EXPR) {
+ ### redo always comes here
+ do_something;
+ } continue {
+ ### next always comes here
+ do_something_else;
+ # then back the top to re-check EXPR
+ }
+ ### last always comes here
+
+Omitting the C<continue> section is semantically equivalent to using an
+empty one, logically enough. In that case, C<next> goes directly back
+to check the condition at the top of the loop.
=item cos EXPR
-Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
-takes cosine of $_.
+Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
+takes cosine of C<$_>.
+
+For the inverse cosine operation, you may use the C<POSIX::acos()>
+function, or use this relation:
+
+ sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
=item crypt PLAINTEXT,SALT
-Encrypts a string exactly like the crypt(3) function in the C library.
-Useful for checking the password file for lousy passwords, amongst
-other things. Only the guys wearing white hats should do this.
+Encrypts a string exactly like the crypt(3) function in the C library
+(assuming that you actually have a version there that has not been
+extirpated as a potential munition). This can prove useful for checking
+the password file for lousy passwords, amongst other things. Only the
+guys wearing white hats should do this.
+
+Note that C<crypt> is intended to be a one-way function, much like breaking
+eggs to make an omelette. There is no (known) corresponding decrypt
+function. As a result, this function isn't all that useful for
+cryptography. (For that, see your nearby CPAN mirror.)
+
+When verifying an existing encrypted string you should use the encrypted
+text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
+allows your code to work with the standard C<crypt> and with more
+exotic implementations. When choosing a new salt create a random two
+character string whose characters come from the set C<[./0-9A-Za-z]>
+(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
Here's an example that makes sure that whoever runs this program knows
their own password:
$pwd = (getpwuid($<))[1];
- $salt = substr($pwd, 0, 2);
system "stty -echo";
print "Password: ";
- chop($word = <STDIN>);
+ chomp($word = <STDIN>);
print "\n";
system "stty echo";
- if (crypt($word, $salt) ne $pwd) {
+ if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
- }
+ }
-Of course, typing in your own password to whoever asks you
-for it is unwise at best.
+Of course, typing in your own password to whoever asks you
+for it is unwise.
-=item dbmclose ASSOC_ARRAY
+The L<crypt> function is unsuitable for encrypting large quantities
+of data, not least of all because you can't get the information
+back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
+on your favorite CPAN mirror for a slew of potentially useful
+modules.
-[This function has been superseded by the untie() function.]
+=item dbmclose HASH
-Breaks the binding between a DBM file and an associative array.
+[This function has been largely superseded by the C<untie> function.]
-=item dbmopen ASSOC,DBNAME,MODE
+Breaks the binding between a DBM file and a hash.
-[This function has been superseded by the tie() function.]
+=item dbmopen HASH,DBNAME,MASK
-This binds a dbm(3) or ndbm(3) file to an associative array. ASSOC is the
-name of the associative array. (Unlike normal open, the first argument
-is I<NOT> a filehandle, even though it looks like one). DBNAME is the
-name of the database (without the F<.dir> or F<.pag> extension). If the
-database does not exist, it is created with protection specified by
-MODE (as modified by the umask()). If your system only supports the
-older DBM functions, you may perform only one dbmopen() in your program.
-If your system has neither DBM nor ndbm, calling dbmopen() produces a
-fatal error.
+[This function has been largely superseded by the C<tie> function.]
-If you don't have write access to the DBM file, you can only read
-associative array variables, not set them. If you want to test whether
-you can write, either use file tests or try setting a dummy array entry
-inside an eval(), which will trap the error.
+This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
+hash. HASH is the name of the hash. (Unlike normal C<open>, the first
+argument is I<not> a filehandle, even though it looks like one). DBNAME
+is the name of the database (without the F<.dir> or F<.pag> extension if
+any). If the database does not exist, it is created with protection
+specified by MASK (as modified by the C<umask>). If your system supports
+only the older DBM functions, you may perform only one C<dbmopen> in your
+program. In older versions of Perl, if your system had neither DBM nor
+ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
+sdbm(3).
-Note that functions such as keys() and values() may return huge array
-values when used on large DBM files. You may prefer to use the each()
+If you don't have write access to the DBM file, you can only read hash
+variables, not set them. If you want to test whether you can write,
+either use file tests or try setting a dummy hash entry inside an C<eval>,
+which will trap the error.
+
+Note that functions such as C<keys> and C<values> may return huge lists
+when used on large DBM files. You may prefer to use the C<each>
function to iterate over large DBM files. Example:
# print out history file offsets
}
dbmclose(%HIST);
+See also L<AnyDBM_File> for a more general description of the pros and
+cons of the various dbm approaches, as well as L<DB_File> for a particularly
+rich implementation.
+
+You can control which DBM library you use by loading that library
+before you call dbmopen():
+
+ use DB_File;
+ dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
+ or die "Can't open netscape history file: $!";
+
=item defined EXPR
-Returns a boolean value saying whether the lvalue EXPR has a real value
-or not. Many operations return the undefined value under exceptional
-conditions, such as end of file, uninitialized variable, system error
-and such. This function allows you to distinguish between an undefined
-null scalar and a defined null scalar with operations that might return
-a real null string, such as referencing elements of an array. You may
-also check to see if arrays or subroutines exist. Use of defined on
-predefined variables is not guaranteed to produce intuitive results.
+=item defined
-When used on a hash array element, it tells you whether the value
-is defined, not whether the key exists in the hash. Use exists() for that.
+Returns a Boolean value telling whether EXPR has a value other than
+the undefined value C<undef>. If EXPR is not present, C<$_> will be
+checked.
+
+Many operations return C<undef> to indicate failure, end of file,
+system error, uninitialized variable, and other exceptional
+conditions. This function allows you to distinguish C<undef> from
+other values. (A simple Boolean test will not distinguish among
+C<undef>, zero, the empty string, and C<"0">, which are all equally
+false.) Note that since C<undef> is a valid scalar, its presence
+doesn't I<necessarily> indicate an exceptional condition: C<pop>
+returns C<undef> when its argument is an empty array, I<or> when the
+element to return happens to be C<undef>.
+
+You may also use C<defined> to check whether a subroutine exists, by
+saying C<defined &func> without parentheses. On the other hand, use
+of C<defined> upon aggregates (hashes and arrays) is not guaranteed to
+produce intuitive results, and should probably be avoided.
+
+When used on a hash element, it tells you whether the value is defined,
+not whether the key exists in the hash. Use L</exists> for the latter
+purpose.
Examples:
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
- eval '@foo = ()' if defined(@foo);
- die "No XYZ package defined" unless defined %_XYZ;
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
+ $debugging = 0 unless defined $debugging;
+
+Note: Many folks tend to overuse C<defined>, and then are surprised to
+discover that the number C<0> and C<""> (the zero-length string) are, in fact,
+defined values. For example, if you say
-See also undef().
+ "ab" =~ /a(.*)b/;
+
+The pattern match succeeds, and C<$1> is defined, despite the fact that it
+matched "nothing". But it didn't really match nothing--rather, it
+matched something that happened to be zero characters long. This is all
+very above-board and honest. When a function returns an undefined value,
+it's an admission that it couldn't give you an honest answer. So you
+should use C<defined> only when you're questioning the integrity of what
+you're trying to do. At other times, a simple comparison to C<0> or C<""> is
+what you want.
+
+Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
+used to report whether memory for that aggregate has ever been
+allocated. This behavior may disappear in future versions of Perl.
+You should instead use a simple test for size:
+
+ if (@an_array) { print "has array elements\n" }
+ if (%a_hash) { print "has hash members\n" }
+
+See also L</undef>, L</exists>, L</ref>.
=item delete EXPR
-Deletes the specified value from its hash array. Returns the deleted
-value, or the undefined value if nothing was deleted. Deleting from
-C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM
-file deletes the entry from the DBM file. (But deleting from a tie()d
-hash doesn't necessarily return anything.)
+Deletes the specified key(s) and their associated values from a hash.
+For each key, returns the deleted value associated with that key, or
+the undefined value if there was no such key. Deleting from C<$ENV{}>
+modifies the environment. Deleting from a hash tied to a DBM file
+deletes the entry from the DBM file. (But deleting from a C<tie>d hash
+doesn't necessarily return anything.)
-The following deletes all the values of an associative array:
+The following deletes all the values of a hash:
- foreach $key (keys %ARRAY) {
- delete $ARRAY{$key};
+ foreach $key (keys %HASH) {
+ delete $HASH{$key};
}
-(But it would be faster to use the undef() command.) Note that the
-EXPR can be arbitrarily complicated as long as the final operation is
-a hash key lookup:
+And so does this:
+
+ delete @HASH{keys %HASH}
+
+But both of these are slower than just assigning the empty list
+or undefining it:
+
+ %hash = (); # completely empty %hash
+ undef %hash; # forget %hash every existed
+
+Note that the EXPR can be arbitrarily complicated as long as the final
+operation is a hash element lookup or hash slice:
delete $ref->[$x][$y]{$key};
+ delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
=item die LIST
-Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
-the current value of $! (errno). If $! is 0, exits with the value of
-C<($? E<gt>E<gt> 8)> (`command` status). If C<($? E<gt>E<gt> 8)> is 0,
-exits with 255. Inside an eval(), the error message is stuffed into C<$@>.
-and the eval() is terminated with the undefined value.
+Outside an C<eval>, prints the value of LIST to C<STDERR> and
+exits with the current value of C<$!> (errno). If C<$!> is C<0>,
+exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
+status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
+an C<eval(),> the error message is stuffed into C<$@> and the
+C<eval> is terminated with the undefined value. This makes
+C<die> the way to raise an exception.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
- chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
+ chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the value of EXPR does not end in a newline, the current script line
number and input line number (if any) are also printed, and a newline
-is supplied. Hint: sometimes appending ", stopped" to your message
-will cause it to make better sense when the string "at foo line 123" is
+is supplied. Note that the "input line number" (also known as "chunk")
+is subject to whatever notion of "line" happens to be currently in
+effect, and is also available as the special variable C<$.>.
+See L<perlvar/"$/"> and L<perlvar/"$.">.
+
+Hint: sometimes appending C<", stopped"> to your message
+will cause it to make better sense when the string C<"at foo line 123"> is
appended. Suppose you are running script "canasta".
die "/etc/games is no good";
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
-See also exit() and warn().
+See also exit(), warn(), and the Carp module.
+
+If LIST is empty and C<$@> already contains a value (typically from a
+previous eval) that value is reused after appending C<"\t...propagated">.
+This is useful for propagating exceptions:
+
+ eval { ... };
+ die unless $@ =~ /Expected exception/;
+
+If C<$@> is empty then the string C<"Died"> is used.
+
+die() can also be called with a reference argument. If this happens to be
+trapped within an eval(), $@ contains the reference. This behavior permits
+a more elaborate exception handling implementation using objects that
+maintain arbitary state about the nature of the exception. Such a scheme
+is sometimes preferable to matching particular string values of $@ using
+regular expressions. Here's an example:
+
+ eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
+ if ($@) {
+ if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
+ # handle Some::Module::Exception
+ }
+ else {
+ # handle all other possible exceptions
+ }
+ }
+
+Because perl will stringify uncaught exception messages before displaying
+them, you may want to overload stringification operations on such custom
+exception objects. See L<overload> for details about that.
+
+You can arrange for a callback to be run just before the C<die>
+does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
+handler will be called with the error text and can change the error
+message, if it sees fit, by calling C<die> again. See
+L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
+L<"eval BLOCK"> for some examples. Although this feature was meant
+to be run only right before your program was to exit, this is not
+currently the case--the C<$SIG{__DIE__}> hook is currently called
+even inside eval()ed blocks/strings! If one wants the hook to do
+nothing in such situations, put
+
+ die @_ if $^S;
+
+as the first line of the handler (see L<perlvar/$^S>). Because
+this promotes strange action at a distance, this counterintuitive
+behavior may be fixed in a future release.
=item do BLOCK
modifier, executes the BLOCK once before testing the loop condition.
(On other statements the loop modifiers test the conditional first.)
+C<do BLOCK> does I<not> count as a loop, so the loop control statements
+C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
+See L<perlsyn> for alternative strategies.
+
=item do SUBROUTINE(LIST)
A deprecated form of subroutine call. See L<perlsub>.
is just like
- eval `cat stat.pl`;
+ scalar eval `cat stat.pl`;
+
+except that it's more efficient and concise, keeps track of the current
+filename for error messages, searches the @INC libraries, and updates
+C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
+variables. It also differs in that code evaluated with C<do FILENAME>
+cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
+same, however, in that it does reparse the file every time you call it,
+so you probably don't want to do this inside a loop.
-except that it's more efficient, more concise, keeps track of the
-current filename for error messages, and searches all the B<-I>
-libraries if the file isn't in the current directory (see also the @INC
-array in L<perlvar/Predefined Names>). It's the same, however, in that it does
-reparse the file every time you call it, so you probably don't want to
-do this inside a loop.
+If C<do> cannot read the file, it returns undef and sets C<$!> to the
+error. If C<do> can read the file but cannot compile it, it
+returns undef and sets an error message in C<$@>. If the file is
+successfully compiled, C<do> returns the value of the last expression
+evaluated.
Note that inclusion of library modules is better done with the
-use() and require() operators.
+C<use> and C<require> operators, which also do automatic error checking
+and raise an exception if there's a problem.
+
+You might like to use C<do> to read in a program configuration
+file. Manual error checking can be done this way:
+
+ # read in config files: system first, then user
+ for $file ("/share/prog/defaults.rc",
+ "$ENV{HOME}/.someprogrc")
+ {
+ unless ($return = do $file) {
+ warn "couldn't parse $file: $@" if $@;
+ warn "couldn't do $file: $!" unless defined $return;
+ warn "couldn't run $file" unless $return;
+ }
+ }
=item dump LABEL
-This causes an immediate core dump. Primarily this is so that you can
-use the B<undump> program to turn your core dump into an executable binary
-after having initialized all your variables at the beginning of the
-program. When the new binary is executed it will begin by executing a
-C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
-it as a goto with an intervening core dump and reincarnation. If LABEL
-is omitted, restarts the program from the top. WARNING: any files
-opened at the time of the dump will NOT be open any more when the
-program is reincarnated, with possible resulting confusion on the part
-of Perl. See also B<-u> option in L<perlrun>.
-
-Example:
-
- #!/usr/bin/perl
- require 'getopt.pl';
- require 'stat.pl';
- %days = (
- 'Sun' => 1,
- 'Mon' => 2,
- 'Tue' => 3,
- 'Wed' => 4,
- 'Thu' => 5,
- 'Fri' => 6,
- 'Sat' => 7,
- );
-
- dump QUICKSTART if $ARGV[0] eq '-d';
-
- QUICKSTART:
- Getopt('f');
-
-=item each ASSOC_ARRAY
-
-Returns a 2 element array consisting of the key and value for the next
-value of an associative array, so that you can iterate over it.
-Entries are returned in an apparently random order. When the array is
-entirely read, a null array is returned (which when assigned produces a
-FALSE (0) value). The next call to each() after that will start
-iterating again. The iterator can be reset only by reading all the
-elements from the array. You should not add elements to an array while
-you're iterating over it. There is a single iterator for each
-associative array, shared by all each(), keys() and values() function
-calls in the program. The following prints out your environment like
-the printenv(1) program, only in a different order:
+=item dump
+
+This function causes an immediate core dump. See also the B<-u>
+command-line switch in L<perlrun>, which does the same thing.
+Primarily this is so that you can use the B<undump> program (not
+supplied) to turn your core dump into an executable binary after
+having initialized all your variables at the beginning of the
+program. When the new binary is executed it will begin by executing
+a C<goto LABEL> (with all the restrictions that C<goto> suffers).
+Think of it as a goto with an intervening core dump and reincarnation.
+If C<LABEL> is omitted, restarts the program from the top.
+
+B<WARNING>: Any files opened at the time of the dump will I<not>
+be open any more when the program is reincarnated, with possible
+resulting confusion on the part of Perl.
+
+This function is now largely obsolete, partly because it's very
+hard to convert a core file into an executable, and because the
+real compiler backends for generating portable bytecode and compilable
+C code have superseded it.
+
+If you're looking to use L<dump> to speed up your program, consider
+generating bytecode or native C code as described in L<perlcc>. If
+you're just trying to accelerate a CGI script, consider using the
+C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
+You might also consider autoloading or selfloading, which at least
+make your program I<appear> to run faster.
+
+=item each HASH
+
+When called in list context, returns a 2-element list consisting of the
+key and value for the next element of a hash, so that you can iterate over
+it. When called in scalar context, returns the key for only the "next"
+element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
+false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
+for this reason.)
+
+Entries are returned in an apparently random order. The actual random
+order is subject to change in future versions of perl, but it is guaranteed
+to be in the same order as either the C<keys> or C<values> function
+would produce on the same (unmodified) hash.
+
+When the hash is entirely read, a null array is returned in list context
+(which when assigned produces a false (C<0>) value), and C<undef> in
+scalar context. The next call to C<each> after that will start iterating
+again. There is a single iterator for each hash, shared by all C<each>,
+C<keys>, and C<values> function calls in the program; it can be reset by
+reading all the elements from the hash, or by evaluating C<keys HASH> or
+C<values HASH>. If you add or delete elements of a hash while you're
+iterating over it, you may get entries skipped or duplicated, so don't.
+
+The following prints out your environment like the printenv(1) program,
+only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
-See also keys() and values().
+See also C<keys>, C<values> and C<sort>.
=item eof FILEHANDLE
+=item eof ()
+
=item eof
Returns 1 if the next read on FILEHANDLE will return end of file, or if
FILEHANDLE is not open. FILEHANDLE may be an expression whose value
-gives the real filehandle name. (Note that this function actually
-reads a character and then ungetc()s it, so it is not very useful in an
-interactive context.) An C<eof> without an argument returns the eof status
-for the last file read. Empty parentheses () may be used to indicate
-the pseudo file formed of the files listed on the command line, i.e.
-C<eof()> is reasonable to use inside a while (<>) loop to detect the end
-of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
-test I<EACH> file in a while (<>) loop. Examples:
+gives the real filehandle. (Note that this function actually
+reads a character and then C<ungetc>s it, so isn't very useful in an
+interactive context.) Do not read from a terminal file (or call
+C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
+as terminals may lose the end-of-file condition if you do.
+
+An C<eof> without an argument uses the last file read as argument.
+Using C<eof()> with empty parentheses is very different. It indicates
+the pseudo file formed of the files listed on the command line,
+i.e., C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)>
+loop to detect the end of only the last file. Use C<eof(ARGV)> or
+C<eof> without the parentheses to test I<each> file in a while
+(E<lt>E<gt>) loop. Examples:
- # insert dashes just before last line of last file
+ # reset line numbering on each input file
while (<>) {
- if (eof()) {
- print "--------------\n";
- }
- print;
+ next if /^\s*#/; # skip comments
+ print "$.\t$_";
+ } continue {
+ close ARGV if eof; # Not eof()!
}
- # reset line numbering on each input file
+ # insert dashes just before last line of last file
while (<>) {
- print "$.\t$_";
- if (eof) { # Not eof().
- close(ARGV);
+ if (eof()) { # check for end of current file
+ print "--------------\n";
+ close(ARGV); # close or last; is needed if we
+ # are reading from the terminal
}
+ print;
}
Practical hint: you almost never need to use C<eof> in Perl, because the
-input operators return undef when they run out of data.
+input operators return false values when they run out of data, or if there
+was an error.
=item eval EXPR
=item eval BLOCK
-EXPR is parsed and executed as if it were a little Perl program. It
-is executed in the context of the current Perl program, so that any
-variable settings, subroutine or format definitions remain afterwards.
-The value returned is the value of the last expression evaluated, or a
-return statement may be used, just as with subroutines.
-
-If there is a syntax error or runtime error, or a die() statement is
-executed, an undefined value is returned by eval(), and C<$@> is set to the
+In the first form, the return value of EXPR is parsed and executed as if it
+were a little Perl program. The value of the expression (which is itself
+determined within scalar context) is first parsed, and if there weren't any
+errors, executed in the context of the current Perl program, so that any
+variable settings or subroutine and format definitions remain afterwards.
+Note that the value is parsed every time the eval executes. If EXPR is
+omitted, evaluates C<$_>. This form is typically used to delay parsing
+and subsequent execution of the text of EXPR until run time.
+
+In the second form, the code within the BLOCK is parsed only once--at the
+same time the code surrounding the eval itself was parsed--and executed
+within the context of the current Perl program. This form is typically
+used to trap exceptions more efficiently than the first (see below), while
+also providing the benefit of checking the code within BLOCK at compile
+time.
+
+The final semicolon, if any, may be omitted from the value of EXPR or within
+the BLOCK.
+
+In both forms, the value returned is the value of the last expression
+evaluated inside the mini-program; a return statement may be also used, just
+as with subroutines. The expression providing the return value is evaluated
+in void, scalar, or list context, depending on the context of the eval itself.
+See L</wantarray> for more on how the evaluation context can be determined.
+
+If there is a syntax error or runtime error, or a C<die> statement is
+executed, an undefined value is returned by C<eval>, and C<$@> is set to the
error message. If there was no error, C<$@> is guaranteed to be a null
-string. If EXPR is omitted, evaluates $_. The final semicolon, if
-any, may be omitted from the expression.
+string. Beware that using C<eval> neither silences perl from printing
+warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
+To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
+L</warn> and L<perlvar>.
-Note that, since eval() traps otherwise-fatal errors, it is useful for
-determining whether a particular feature (such as dbmopen() or symlink())
+Note that, because C<eval> traps otherwise-fatal errors, it is useful for
+determining whether a particular feature (such as C<socket> or C<symlink>)
is implemented. It is also Perl's exception trapping mechanism, where
the die operator is used to raise exceptions.
recompiling each time. The error, if any, is still returned in C<$@>.
Examples:
- # make divide-by-zero non-fatal
+ # make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
- eval { $answer = };
+ eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
-With an eval(), you should be especially careful to remember what's
+Due to the current arguably broken state of C<__DIE__> hooks, when using
+the C<eval{}> form as an exception trap in libraries, you may wish not
+to trigger any C<__DIE__> hooks that user code may have installed.
+You can use the C<local $SIG{__DIE__}> construct for this purpose,
+as shown in this example:
+
+ # a very private exception trap for divide-by-zero
+ eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
+ warn $@ if $@;
+
+This is especially significant, given that C<__DIE__> hooks can call
+C<die> again, which has the effect of changing their error messages:
+
+ # __DIE__ hooks may modify error messages
+ {
+ local $SIG{'__DIE__'} =
+ sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
+ eval { die "foo lives here" };
+ print $@ if $@; # prints "bar lives here"
+ }
+
+Because this promotes action at a distance, this counterintuitive behavior
+may be fixed in a future release.
+
+With an C<eval>, you should be especially careful to remember what's
being looked at when:
eval $x; # CASE 1
eval '$x'; # CASE 3
eval { $x }; # CASE 4
- eval "\$$x++" # CASE 5
+ eval "\$$x++"; # CASE 5
$$x++; # CASE 6
-Cases 1 and 2 above behave identically: they run the code contained in the
-variable $x. (Although case 2 has misleading double quotes making the
-reader wonder what else might be happening (nothing is).) Cases 3 and 4
-likewise behave in the same way: they run the code <$x>, which does
-nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
-is a place where normally you I<WOULD> like to use double quotes, except
-that in particular situation, you can just use symbolic references
-instead, as in case 6.
+Cases 1 and 2 above behave identically: they run the code contained in
+the variable $x. (Although case 2 has misleading double quotes making
+the reader wonder what else might be happening (nothing is).) Cases 3
+and 4 likewise behave in the same way: they run the code C<'$x'>, which
+does nothing but return the value of $x. (Case 4 is preferred for
+purely visual reasons, but it also has the advantage of compiling at
+compile-time instead of at run-time.) Case 5 is a place where
+normally you I<would> like to use double quotes, except that in this
+particular situation, you can just use symbolic references instead, as
+in case 6.
-=item exec LIST
+C<eval BLOCK> does I<not> count as a loop, so the loop control statements
+C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
-The exec() function executes a system command I<AND NEVER RETURNS>. Use
-the system() function if you want it to return.
+=item exec LIST
-If there is more than one argument in LIST, or if LIST is an array with
-more than one value, calls execvp(3) with the arguments in LIST. If
-there is only one scalar argument, the argument is checked for shell
-metacharacters. If there are any, the entire argument is passed to
-C</bin/sh -c> for parsing. If there are none, the argument is split
-into words and passed directly to execvp(), which is more efficient.
-Note: exec() (and system(0) do not flush your output buffer, so you may
-need to set C<$|> to avoid lost output. Examples:
+=item exec PROGRAM LIST
+
+The C<exec> function executes a system command I<and never returns>--
+use C<system> instead of C<exec> if you want it to return. It fails and
+returns false only if the command does not exist I<and> it is executed
+directly instead of via your system's command shell (see below).
+
+Since it's a common mistake to use C<exec> instead of C<system>, Perl
+warns you if there is a following statement which isn't C<die>, C<warn>,
+or C<exit> (if C<-w> is set - but you always do that). If you
+I<really> want to follow an C<exec> with some other statement, you
+can use one of these styles to avoid the warning:
+
+ exec ('foo') or print STDERR "couldn't exec foo: $!";
+ { exec ('foo') }; print STDERR "couldn't exec foo: $!";
+
+If there is more than one argument in LIST, or if LIST is an array
+with more than one value, calls execvp(3) with the arguments in LIST.
+If there is only one scalar argument or an array with one element in it,
+the argument is checked for shell metacharacters, and if there are any,
+the entire argument is passed to the system's command shell for parsing
+(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
+If there are no shell metacharacters in the argument, it is split into
+words and passed directly to C<execvp>, which is more efficient.
+Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
to the program you are executing about its own name, you can specify
the program you actually want to run as an "indirect object" (without a
comma) in front of the LIST. (This always forces interpretation of the
-LIST as a multi-valued list, even if there is only a single scalar in
+LIST as a multivalued list, even if there is only a single scalar in
the list.) Example:
$shell = '/bin/csh';
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
+When the arguments get executed via the system shell, results will
+be subject to its quirks and capabilities. See L<perlop/"`STRING`">
+for details.
+
+Using an indirect object with C<exec> or C<system> is also more
+secure. This usage (which also works fine with system()) forces
+interpretation of the arguments as a multivalued list, even if the
+list had just one argument. That way you're safe from the shell
+expanding wildcards or splitting up words with whitespace in them.
+
+ @args = ( "echo surprise" );
+
+ exec @args; # subject to shell escapes
+ # if @args == 1
+ exec { $args[0] } @args; # safe even with one-arg list
+
+The first version, the one without the indirect object, ran the I<echo>
+program, passing it C<"surprise"> an argument. The second version
+didn't--it tried to run a program literally called I<"echo surprise">,
+didn't find it, and set C<$?> to a non-zero value indicating failure.
+
+Note that C<exec> will not call your C<END> blocks, nor will it call
+any C<DESTROY> methods in your objects.
+
=item exists EXPR
-Returns TRUE if the specified hash key exists in its hash array, even
+Returns true if the specified hash key exists in its hash, even
if the corresponding value is undefined.
- print "Exists\n" if exists $array{$key};
- print "Defined\n" if defined $array{$key};
- print "True\n" if $array{$key};
+ print "Exists\n" if exists $array{$key};
+ print "Defined\n" if defined $array{$key};
+ print "True\n" if $array{$key};
-A hash element can only be TRUE if it's defined, and defined if
+A hash element can be true only if it's defined, and defined if
it exists, but the reverse doesn't necessarily hold true.
Note that the EXPR can be arbitrarily complicated as long as the final
operation is a hash key lookup:
- if (exists $ref->[$x][$y]{$key}) { ... }
+ if (exists $ref->{A}->{B}->{$key}) { }
+ if (exists $hash{A}{B}{$key}) { }
+
+Although the last element will not spring into existence just because
+its existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
+and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
+existence test for a $key element. This happens anywhere the arrow
+operator is used, including even
+
+ undef $ref;
+ if (exists $ref->{"Some key"}) { }
+ print $ref; # prints HASH(0x80d3d5c)
+
+This surprising autovivification in what does not at first--or even
+second--glance appear to be an lvalue context may be fixed in a future
+release.
=item exit EXPR
-Evaluates EXPR and exits immediately with that value. (Actually, it
-calls any defined C<END> routines first, but the C<END> routines may not
-abort the exit. Likewise any object destructors that need to be called
-are called before exit.) Example:
+Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
-See also die(). If EXPR is omitted, exits with 0 status.
+See also C<die>. If EXPR is omitted, exits with C<0> status. The only
+universally recognized values for EXPR are C<0> for success and C<1>
+for error; other values are subject to interpretation depending on the
+environment in which the Perl program is running. For example, exiting
+69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
+the mailer to return the item undelivered, but that's not true everywhere.
+
+Don't use C<exit> to abort a subroutine if there's any chance that
+someone might want to trap whatever error happened. Use C<die> instead,
+which can be trapped by an C<eval>.
+
+The exit() function does not always exit immediately. It calls any
+defined C<END> routines first, but these C<END> routines may not
+themselves abort the exit. Likewise any object destructors that need to
+be called are called before the real exit. If this is a problem, you
+can call C<POSIX:_exit($status)> to avoid END and destructor processing.
+See L<perlsub> for details.
=item exp EXPR
+=item exp
+
Returns I<e> (the natural logarithm base) to the power of EXPR.
If EXPR is omitted, gives C<exp($_)>.
use Fcntl;
-first to get the correct function definitions. Argument processing and
-value return works just like ioctl() below. Note that fcntl() will produce
-a fatal error if used on a machine that doesn't implement fcntl(2).
+first to get the correct constant definitions. Argument processing and
+value return works just like C<ioctl> below.
For example:
use Fcntl;
- fcntl($filehandle, F_GETLK, $packed_return_buffer);
+ fcntl($filehandle, F_GETFL, $packed_return_buffer)
+ or die "can't fcntl F_GETFL: $!";
+
+You don't have to check for C<defined> on the return from C<fnctl>.
+Like C<ioctl>, it maps a C<0> return from the system call into C<"0
+but true"> in Perl. This string is true in boolean context and C<0>
+in numeric context. It is also exempt from the normal B<-w> warnings
+on improper numeric conversions.
+
+Note that C<fcntl> will produce a fatal error if used on a machine that
+doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
+manpage to learn what functions are available on your system.
=item fileno FILEHANDLE
-Returns the file descriptor for a filehandle. This is useful for
-constructing bitmaps for select(). If FILEHANDLE is an expression, the
-value is taken as the name of the filehandle.
+Returns the file descriptor for a filehandle, or undefined if the
+filehandle is not open. This is mainly useful for constructing
+bitmaps for C<select> and low-level POSIX tty-handling operations.
+If FILEHANDLE is an expression, the value is taken as an indirect
+filehandle, generally its name.
-=item flock FILEHANDLE,OPERATION
+You can use this to find out whether two handles refer to the
+same underlying descriptor:
+
+ if (fileno(THIS) == fileno(THAT)) {
+ print "THIS and THAT are dups\n";
+ }
-Calls flock(2) on FILEHANDLE. See L<flock(2)> for
-definition of OPERATION. Returns TRUE for success, FALSE on failure.
-Will produce a fatal error if used on a machine that doesn't implement
-flock(2). Here's a mailbox appender for BSD systems.
+=item flock FILEHANDLE,OPERATION
- $LOCK_SH = 1;
- $LOCK_EX = 2;
- $LOCK_NB = 4;
- $LOCK_UN = 8;
+Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
+for success, false on failure. Produces a fatal error if used on a
+machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
+C<flock> is Perl's portable file locking interface, although it locks
+only entire files, not records.
+
+Two potentially non-obvious but traditional C<flock> semantics are
+that it waits indefinitely until the lock is granted, and that its locks
+B<merely advisory>. Such discretionary locks are more flexible, but offer
+fewer guarantees. This means that files locked with C<flock> may be
+modified by programs that do not also use C<flock>. See L<perlport>,
+your port's specific documentation, or your system-specific local manpages
+for details. It's best to assume traditional behavior if you're writing
+portable programs. (But if you're not, you should as always feel perfectly
+free to write for your own system's idiosyncrasies (sometimes called
+"features"). Slavish adherence to portability concerns shouldn't get
+in the way of your getting your job done.)
+
+OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
+LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
+you can use the symbolic names if import them from the Fcntl module,
+either individually, or as a group using the ':flock' tag. LOCK_SH
+requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
+releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
+LOCK_EX then C<flock> will return immediately rather than blocking
+waiting for the lock (check the return status to see if you got it).
+
+To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
+before locking or unlocking it.
+
+Note that the emulation built with lockf(3) doesn't provide shared
+locks, and it requires that FILEHANDLE be open with write intent. These
+are the semantics that lockf(3) implements. Most if not all systems
+implement lockf(3) in terms of fcntl(2) locking, though, so the
+differing semantics shouldn't bite too many people.
+
+Note also that some versions of C<flock> cannot lock things over the
+network; you would need to use the more system-specific C<fcntl> for
+that. If you like you can force Perl to ignore your system's flock(2)
+function, and so provide its own fcntl(2)-based emulation, by passing
+the switch C<-Ud_flock> to the F<Configure> program when you configure
+perl.
+
+Here's a mailbox appender for BSD systems.
+
+ use Fcntl ':flock'; # import LOCK_* constants
sub lock {
- flock(MBOX,$LOCK_EX);
+ flock(MBOX,LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
- flock(MBOX,$LOCK_UN);
+ flock(MBOX,LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
print MBOX $msg,"\n\n";
unlock();
-Note that flock() can't lock things over the network. You need to do
-locking with fcntl() for that.
+On systems that support a real flock(), locks are inherited across fork()
+calls, whereas those that must resort to the more capricious fcntl()
+function lose the locks, making it harder to write servers.
+
+See also L<DB_File> for other flock() examples.
=item fork
-Does a fork(2) system call. Returns the child pid to the parent process
-and 0 to the child process, or undef if the fork is unsuccessful.
-Note: unflushed buffers remain unflushed in both processes, which means
-you may need to set C<$|> ($AUTOFLUSH in English) or call the
-autoflush() FileHandle method to avoid duplicate output.
+Does a fork(2) system call to create a new process running the
+same program at the same point. It returns the child pid to the
+parent process, C<0> to the child process, or C<undef> if the fork is
+unsuccessful. File descriptors (and sometimes locks on those descriptors)
+are shared, while everything else is copied. On most systems supporting
+fork(), great care has gone into making it extremely efficient (for
+example, using copy-on-write technology on data pages), making it the
+dominant paradigm for multitasking over the last few decades.
-If you fork() without ever waiting on your children, you will accumulate
-zombies:
+All files opened for output are flushed before forking the child process.
- $SIG{'CHLD'} = sub { wait };
+If you C<fork> without ever waiting on your children, you will
+accumulate zombies. On some systems, you can avoid this by setting
+C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
+forking and reaping moribund children.
-There's also the double-fork trick (error checking on
-fork() returns omitted);
+Note that if your forked child inherits system file descriptors like
+STDIN and STDOUT that are actually connected by a pipe or socket, even
+if you exit, then the remote server (such as, say, a CGI script or a
+backgrounded job launched from a remote shell) won't think you're done.
+You should reopen those to F</dev/null> if it's any issue.
- unless ($pid = fork) {
- unless (fork) {
- exec "what you really wanna do";
- die "no exec";
- # ... or ...
- some_perl_code_here;
- exit 0;
- }
- exit 0;
- }
- waitpid($pid,0);
+=item format
+Declare a picture format for use by the C<write> function. For
+example:
-=item formline PICTURE, LIST
+ format Something =
+ Test: @<<<<<<<< @||||| @>>>>>
+ $str, $%, '$' . int($num)
+ .
-This is an internal function used by formats, though you may call it
+ $str = "widget";
+ $num = $cost/$quantity;
+ $~ = 'Something';
+ write;
+
+See L<perlform> for many details and examples.
+
+=item formline PICTURE,LIST
+
+This is an internal function used by C<format>s, though you may call it,
too. It formats (see L<perlform>) a list of values according to the
contents of PICTURE, placing the output into the format output
-accumulator, C<$^A>. Eventually, when a write() is done, the contents of
+accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
+Eventually, when a C<write> is done, the contents of
C<$^A> are written to some filehandle, but you could also read C<$^A>
-yourself and then set C<$^A> back to "". Note that a format typically
-does one formline() per line of form, but the formline() function itself
-doesn't care how many newlines are embedded in the PICTURE. Be careful
-if you put double quotes around the picture, since an "C<@>" character may
-be taken to mean the beginning of an array name. formline() always
-returns TRUE.
+yourself and then set C<$^A> back to C<"">. Note that a format typically
+does one C<formline> per line of form, but the C<formline> function itself
+doesn't care how many newlines are embedded in the PICTURE. This means
+that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
+You may therefore need to use multiple formlines to implement a single
+record format, just like the format compiler.
+
+Be careful if you put double quotes around the picture, because an C<@>
+character may be taken to mean the beginning of an array name.
+C<formline> always returns true. See L<perlform> for other examples.
=item getc FILEHANDLE
=item getc
Returns the next character from the input file attached to FILEHANDLE,
-or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
+or the undefined value at end of file, or if there was an error.
+If FILEHANDLE is omitted, reads from STDIN. This is not particularly
+efficient. However, it cannot be used by itself to fetch single
+characters without waiting for the user to hit enter. For that, try
+something more like:
+
+ if ($BSD_STYLE) {
+ system "stty cbreak </dev/tty >/dev/tty 2>&1";
+ }
+ else {
+ system "stty", '-icanon', 'eol', "\001";
+ }
+
+ $key = getc(STDIN);
+
+ if ($BSD_STYLE) {
+ system "stty -cbreak </dev/tty >/dev/tty 2>&1";
+ }
+ else {
+ system "stty", 'icanon', 'eol', '^@'; # ASCII null
+ }
+ print "\n";
+
+Determination of whether $BSD_STYLE should be set
+is left as an exercise to the reader.
+
+The C<POSIX::getattr> function can do this more portably on
+systems purporting POSIX compliance. See also the C<Term::ReadKey>
+module from your nearest CPAN site; details on CPAN can be found on
+L<perlmodlib/CPAN>.
=item getlogin
-Returns the current login from F</etc/utmp>, if any. If null, use
-getpwuid().
+Implements the C library function of the same name, which on most
+systems returns the current login from F</etc/utmp>, if any. If null,
+use C<getpwuid>.
+
+ $login = getlogin || getpwuid($<) || "Kilroy";
- $login = getlogin || (getpwuid($<))[0] || "Kilroy";
+Do not consider C<getlogin> for authentication: it is not as
+secure as C<getpwuid>.
=item getpeername SOCKET
Returns the packed sockaddr address of other end of the SOCKET connection.
- # An internet sockaddr
- $sockaddr = 'S n a4 x8';
- $hersockaddr = getpeername(S);
- ($family, $port, $heraddr) = unpack($sockaddr,$hersockaddr);
+ use Socket;
+ $hersockaddr = getpeername(SOCK);
+ ($port, $iaddr) = sockaddr_in($hersockaddr);
+ $herhostname = gethostbyaddr($iaddr, AF_INET);
+ $herstraddr = inet_ntoa($iaddr);
=item getpgrp PID
-Returns the current process group for the specified PID, 0 for the
-current process. Will produce a fatal error if used on a machine that
+Returns the current process group for the specified PID. Use
+a PID of C<0> to get the current process group for the
+current process. Will raise an exception if used on a machine that
doesn't implement getpgrp(2). If PID is omitted, returns process
-group of current process.
+group of current process. Note that the POSIX version of C<getpgrp>
+does not accept a PID argument, so only C<PID==0> is truly portable.
=item getppid
=item getpriority WHICH,WHO
-Returns the current priority for a process, a process group, or a
-user. (See L<getpriority(2)>.) Will produce a fatal error if used on a
+Returns the current priority for a process, a process group, or a user.
+(See L<getpriority(2)>.) Will raise a fatal exception if used on a
machine that doesn't implement getpriority(2).
=item getpwnam NAME
=item endservent
These routines perform the same functions as their counterparts in the
-system library. Within a list context, the return values from the
+system library. In list context, the return values from the
various get routines are as follows:
($name,$passwd,$uid,$gid,
- $quota,$comment,$gcos,$dir,$shell) = getpw*
+ $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
(If the entry doesn't exist you get a null list.)
-Within a scalar context, you get the name, unless the function was a
+In scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever it is.
(If the entry doesn't exist you get the undefined value.) For example:
- $uid = getpwnam
- $name = getpwuid
- $name = getpwent
- $gid = getgrnam
- $name = getgrgid
- $name = getgrent
- etc.
+ $uid = getpwnam($name);
+ $name = getpwuid($num);
+ $name = getpwent();
+ $gid = getgrnam($name);
+ $name = getgrgid($num;
+ $name = getgrent();
+ #etc.
+
+In I<getpw*()> the fields $quota, $comment, and $expire are
+special cases in the sense that in many systems they are unsupported.
+If the $quota is unsupported, it is an empty scalar. If it is
+supported, it usually encodes the disk quota. If the $comment
+field is unsupported, it is an empty scalar. If it is supported it
+usually encodes some administrative comment about the user. In some
+systems the $quota field may be $change or $age, fields that have
+to do with password aging. In some systems the $comment field may
+be $class. The $expire field, if present, encodes the expiration
+period of the account or the password. For the availability and the
+exact meaning of these fields in your system, please consult your
+getpwnam(3) documentation and your F<pwd.h> file. You can also find
+out from within Perl what your $quota and $comment fields mean
+and whether you have the $expire field by using the C<Config> module
+and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
+and C<d_pwexpire>. Shadow password files are only supported if your
+vendor has implemented them in the intuitive fashion that calling the
+regular C library routines gets the shadow versions if you're running
+under privilege. Those that incorrectly implement a separate library
+call are not supported.
The $members value returned by I<getgr*()> is a space separated list of
the login names of the members of the group.
For the I<gethost*()> functions, if the C<h_errno> variable is supported in
C, it will be returned to you via C<$?> if the function call fails. The
-@addrs value returned by a successful call is a list of the raw
+C<@addrs> value returned by a successful call is a list of the raw
addresses returned by the corresponding system library call. In the
Internet domain, each address is four bytes long and you can unpack it
by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
+The Socket library makes this slightly easier:
+
+ use Socket;
+ $iaddr = inet_aton("127.1"); # or whatever address
+ $name = gethostbyaddr($iaddr, AF_INET);
+
+ # or going the other way
+ $straddr = inet_ntoa($iaddr);
+
+If you get tired of remembering which element of the return list
+contains which return value, by-name interfaces are provided
+in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
+C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
+and C<User::grent>. These override the normal built-ins, supplying
+versions that return objects with the appropriate names
+for each field. For example:
+
+ use File::stat;
+ use User::pwent;
+ $is_his = (stat($filename)->uid == pwent($whoever)->uid);
+
+Even though it looks like they're the same method calls (uid),
+they aren't, because a C<File::stat> object is different from
+a C<User::pwent> object.
+
=item getsockname SOCKET
-Returns the packed sockaddr address of this end of the SOCKET connection.
+Returns the packed sockaddr address of this end of the SOCKET connection,
+in case you don't know the address because you have several different
+IPs that the connection might have come in on.
- # An internet sockaddr
- $sockaddr = 'S n a4 x8';
- $mysockaddr = getsockname(S);
- ($family, $port, $myaddr) =
- unpack($sockaddr,$mysockaddr);
+ use Socket;
+ $mysockaddr = getsockname(SOCK);
+ ($port, $myaddr) = sockaddr_in($mysockaddr);
+ printf "Connect to %s [%s]\n",
+ scalar gethostbyaddr($myaddr, AF_INET),
+ inet_ntoa($myaddr);
=item getsockopt SOCKET,LEVEL,OPTNAME
-Returns the socket option requested, or undefined if there is an error.
+Returns the socket option requested, or undef if there is an error.
=item glob EXPR
-Returns the value of EXPR with filename expansions such as a shell
-would do. This is the internal function implementing the <*.*>
-operator.
+=item glob
-=item gmtime EXPR
+Returns the value of EXPR with filename expansions such as the
+standard Unix shell F</bin/csh> would do. This is the internal function
+implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
+If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
+discussed in more detail in L<perlop/"I/O Operators">.
-Converts a time as returned by the time function to a 9-element array
-with the time analyzed for the Greenwich timezone. Typically used as
-follows:
+=item gmtime EXPR
+Converts a time as returned by the time function to a 9-element list
+with the time localized for the standard Greenwich time zone.
+Typically used as follows:
+ # 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(time);
-All array elements are numeric, and come straight out of a struct tm.
-In particular this means that $mon has the range 0..11 and $wday has
-the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
+All list elements are numeric, and come straight out of a struct tm.
+In particular this means that $mon has the range C<0..11> and $wday
+has the range C<0..6> with sunday as day C<0>. Also, $year is the
+number of years since 1900, that is, $year is C<123> in year 2023,
+I<not> simply the last two digits of the year. If you assume it is,
+then you create non-Y2K-compliant programs--and you wouldn't want to do
+that, would you?
+
+If EXPR is omitted, does C<gmtime(time())>.
+
+In scalar context, returns the ctime(3) value:
+
+ $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
+
+Also see the C<timegm> function provided by the C<Time::Local> module,
+and the strftime(3) function available via the POSIX module.
+
+This scalar value is B<not> locale dependent (see L<perllocale>), but
+is instead a Perl builtin. Also see the C<Time::Local> module, and the
+strftime(3) and mktime(3) functions available via the POSIX module. To
+get somewhat similar but locale dependent date strings, set up your
+locale environment variables appropriately (please see L<perllocale>)
+and try for example:
+
+ use POSIX qw(strftime);
+ $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
+
+Note that the C<%a> and C<%b> escapes, which represent the short forms
+of the day of the week and the month of the year, may not necessarily
+be three characters wide in all locales.
=item goto LABEL
+=item goto EXPR
+
=item goto &NAME
-The goto-LABEL form finds the statement labeled with LABEL and resumes
+The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
execution there. It may not be used to go into any construct that
-requires initialization, such as a subroutine or a foreach loop. It
-also can't be used to go into a construct that is optimized away. It
-can be used to go almost anywhere else within the dynamic scope,
+requires initialization, such as a subroutine or a C<foreach> loop. It
+also can't be used to go into a construct that is optimized away,
+or to get out of a block or subroutine given to C<sort>.
+It can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to use some other
-construct such as last or die. The author of Perl has never felt the
-need to use this form of goto (in Perl, that is--C is another matter).
+construct such as C<last> or C<die>. The author of Perl has never felt the
+need to use this form of C<goto> (in Perl, that is--C is another matter).
+
+The C<goto-EXPR> form expects a label name, whose scope will be resolved
+dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
+necessarily recommended if you're optimizing for maintainability:
-The goto-&NAME form is highly magical, and substitutes a call to the
+ goto ("FOO", "BAR", "GLARCH")[$i];
+
+The C<goto-&NAME> form is highly magical, and substitutes a call to the
named subroutine for the currently running subroutine. This is used by
-AUTOLOAD subroutines that wish to load another subroutine and then
+C<AUTOLOAD> subroutines that wish to load another subroutine and then
pretend that the other subroutine had been called in the first place
-(except that any modifications to @_ in the current subroutine are
-propagated to the other subroutine.) After the goto, not even caller()
+(except that any modifications to C<@_> in the current subroutine are
+propagated to the other subroutine.) After the C<goto>, not even C<caller>
will be able to tell that this routine was called first.
=item grep BLOCK LIST
=item grep EXPR,LIST
+This is similar in spirit to, but not the same as, grep(1) and its
+relatives. In particular, it is not limited to using regular expressions.
+
Evaluates the BLOCK or EXPR for each element of LIST (locally setting
-$_ to each element) and returns the list value consisting of those
-elements for which the expression evaluated to TRUE. In a scalar
-context, returns the number of times the expression was TRUE.
+C<$_> to each element) and returns the list value consisting of those
+elements for which the expression evaluated to true. In scalar
+context, returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
@foo = grep {!/^#/} @bar; # weed out comments
-Note that, since $_ is a reference into the list value, it can be used
-to modify the elements of the array. While this is useful and
-supported, it can cause bizarre results if the LIST is not a named
-array.
+Note that, because C<$_> is a reference into the list value, it can
+be used to modify the elements of the array. While this is useful and
+supported, it can cause bizarre results if the LIST is not a named array.
+Similarly, grep returns aliases into the original list, much as a for
+loop's index variable aliases the list elements. That is, modifying an
+element of a list returned by grep (for example, in a C<foreach>, C<map>
+or another C<grep>) actually modifies the element in the original list.
+This is usually something to be avoided when writing clear code.
+
+See also L</map> for a list composed of the results of the BLOCK or EXPR.
=item hex EXPR
-Returns the decimal value of EXPR interpreted as an hex string. (To
-interpret strings that might start with 0 or 0x see oct().) If EXPR is
-omitted, uses $_.
+=item hex
+
+Interprets EXPR as a hex string and returns the corresponding value.
+(To convert strings that might start with either 0, 0x, or 0b, see
+L</oct>.) If EXPR is omitted, uses C<$_>.
+
+ print hex '0xAf'; # prints '175'
+ print hex 'aF'; # same
+
+Hex strings may only represent integers. Strings that would cause
+integer overflow trigger a mandatory error message.
=item import
-There is no built-in import() function. It is merely an ordinary
-method subroutine defined (or inherited) by modules that wish to export
-names to another module. The use() function calls the import() method
-for the package used. See also L</use> below and L<perlmod>.
+There is no builtin C<import> function. It is just an ordinary
+method (subroutine) defined (or inherited) by modules that wish to export
+names to another module. The C<use> function calls the C<import> method
+for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
=item index STR,SUBSTR,POSITION
=item index STR,SUBSTR
-Returns the position of the first occurrence of SUBSTR in STR at or
-after POSITION. If POSITION is omitted, starts searching from the
-beginning of the string. The return value is based at 0, or whatever
-you've set the $[ variable to. If the substring is not found, returns
-one less than the base, ordinarily -1.
+The index function searches for one string within another, but without
+the wildcard-like behavior of a full regular-expression pattern match.
+It returns the position of the first occurrence of SUBSTR in STR at
+or after POSITION. If POSITION is omitted, starts searching from the
+beginning of the string. The return value is based at C<0> (or whatever
+you've set the C<$[> variable to--but don't do that). If the substring
+is not found, returns one less than the base, ordinarily C<-1>.
=item int EXPR
-Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
+=item int
+
+Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
+You should not use this function for rounding: one because it truncates
+towards C<0>, and two because machine representations of floating point
+numbers can sometimes produce counterintuitive results. For example,
+C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
+because it's really more like -268.99999999999994315658 instead. Usually,
+the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
+functions will serve you better than will int().
=item ioctl FILEHANDLE,FUNCTION,SCALAR
-Implements the ioctl(2) function. You'll probably have to say
+Implements the ioctl(2) function. You'll probably first have to say
- require "ioctl.ph"; # probably /usr/local/lib/perl/ioctl.ph
+ require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
-first to get the correct function definitions. If ioctl.ph doesn't
+to get the correct function definitions. If F<ioctl.ph> doesn't
exist or doesn't have the correct definitions you'll have to roll your
-own, based on your C header files such as <sys/ioctl.h>. (There is a
-Perl script called B<h2ph> that comes with the Perl kit which may help you
-in this.) SCALAR will be read and/or written depending on the
-FUNCTION--a pointer to the string value of SCALAR will be passed as the
-third argument of the actual ioctl call. (If SCALAR has no string
-value but does have a numeric value, that value will be passed rather
-than a pointer to the string value. To guarantee this to be TRUE, add
-a 0 to the scalar before using it.) The pack() and unpack() functions
-are useful for manipulating the values of structures used by ioctl().
-The following example sets the erase character to DEL.
-
- require 'ioctl.ph';
- $sgttyb_t = "ccccs"; # 4 chars and a short
- if (ioctl(STDIN,$TIOCGETP,$sgttyb)) {
- @ary = unpack($sgttyb_t,$sgttyb);
- $ary[2] = 127;
- $sgttyb = pack($sgttyb_t,@ary);
- ioctl(STDIN,$TIOCSETP,$sgttyb)
- || die "Can't ioctl: $!";
- }
-
-The return value of ioctl (and fcntl) is as follows:
+own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
+(There is a Perl script called B<h2ph> that comes with the Perl kit that
+may help you in this, but it's nontrivial.) SCALAR will be read and/or
+written depending on the FUNCTION--a pointer to the string value of SCALAR
+will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
+has no string value but does have a numeric value, that value will be
+passed rather than a pointer to the string value. To guarantee this to be
+true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
+functions may be needed to manipulate the values of structures used by
+C<ioctl>.
+
+The return value of C<ioctl> (and C<fcntl>) is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
-Thus Perl returns TRUE on success and FALSE on failure, yet you can
+Thus Perl returns true on success and false on failure, yet you can
still easily determine the actual value returned by the operating
system:
- ($retval = ioctl(...)) || ($retval = -1);
+ $retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
+The special string "C<0> but true" is exempt from B<-w> complaints
+about improper numeric conversions.
+
+Here's an example of setting a filehandle named C<REMOTE> to be
+non-blocking at the system level. You'll have to negotiate C<$|>
+on your own, though.
+
+ use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
+
+ $flags = fcntl(REMOTE, F_GETFL, 0)
+ or die "Can't get flags for the socket: $!\n";
+
+ $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
+ or die "Can't set flags for the socket: $!\n";
+
=item join EXPR,LIST
-Joins the separate strings of LIST or ARRAY into a single string with
-fields separated by the value of EXPR, and returns the string.
-Example:
+Joins the separate strings of LIST into a single string with fields
+separated by the value of EXPR, and returns that new string. Example:
+
+ $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
- $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
+See L</split>.
-See L<perlfunc/split>.
+=item keys HASH
-=item keys ASSOC_ARRAY
+Returns a list consisting of all the keys of the named hash. (In
+scalar context, returns the number of keys.) The keys are returned in
+an apparently random order. The actual random order is subject to
+change in future versions of perl, but it is guaranteed to be the same
+order as either the C<values> or C<each> function produces (given
+that the hash has not been modified). As a side effect, it resets
+HASH's iterator.
-Returns a normal array consisting of all the keys of the named
-associative array. (In a scalar context, returns the number of keys.)
-The keys are returned in an apparently random order, but it is the same
-order as either the values() or each() function produces (given that
-the associative array has not been modified). Here is yet another way
-to print your environment:
+Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
- while ($#keys >= 0) {
+ while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
print $key, '=', $ENV{$key}, "\n";
}
+To sort a hash by value, you'll need to use a C<sort> function.
+Here's a descending numeric sort of a hash by its values:
+
+ foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
+ printf "%4d %s\n", $hash{$key}, $key;
+ }
+
+As an lvalue C<keys> allows you to increase the number of hash buckets
+allocated for the given hash. This can gain you a measure of efficiency if
+you know the hash is going to get big. (This is similar to pre-extending
+an array by assigning a larger number to $#array.) If you say
+
+ keys %hash = 200;
+
+then C<%hash> will have at least 200 buckets allocated for it--256 of them,
+in fact, since it rounds up to the next power of two. These
+buckets will be retained even if you do C<%hash = ()>, use C<undef
+%hash> if you want to free the storage while C<%hash> is still in scope.
+You can't shrink the number of buckets allocated for the hash using
+C<keys> in this way (but you needn't worry about doing this by accident,
+as trying has no effect).
+
+See also C<each>, C<values> and C<sort>.
+
=item kill LIST
-Sends a signal to a list of processes. The first element of the list
-must be the signal to send. Returns the number of processes
-successfully signaled.
+Sends a signal to a list of processes. The first element of
+the list must be the signal to send. Returns the number of
+processes successfully signaled.
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
-Unlike in the shell, in Perl
-if the I<SIGNAL> is negative, it kills process groups instead of processes.
-(On System V, a negative I<PROCESS> number will also kill process
-groups, but that's not portable.) That means you usually want to use
-positive not negative signals. You may also use a signal name in quotes.
+Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
+process groups instead of processes. (On System V, a negative I<PROCESS>
+number will also kill process groups, but that's not portable.) That
+means you usually want to use positive not negative signals. You may also
+use a signal name in quotes. See L<perlipc/"Signals"> for details.
=item last LABEL
omitted, the command refers to the innermost enclosing loop. The
C<continue> block, if any, is not executed:
- line: while (<STDIN>) {
- last line if /^$/; # exit when done with header
- ...
+ LINE: while (<STDIN>) {
+ last LINE if /^$/; # exit when done with header
+ #...
}
+C<last> cannot be used to exit a block which returns a value such as
+C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
+a grep() or map() operation.
+
+See also L</continue> for an illustration of how C<last>, C<next>, and
+C<redo> work.
+
=item lc EXPR
+=item lc
+
Returns an lowercased version of EXPR. This is the internal function
-implementing the \L escape in double-quoted strings.
+implementing the C<\L> escape in double-quoted strings.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
+and L<utf8>.
+
+If EXPR is omitted, uses C<$_>.
=item lcfirst EXPR
+=item lcfirst
+
Returns the value of EXPR with the first character lowercased. This is
-the internal function implementing the \l escape in double-quoted strings.
+the internal function implementing the C<\l> escape in double-quoted strings.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
+
+If EXPR is omitted, uses C<$_>.
=item length EXPR
+=item length
+
Returns the length in characters of the value of EXPR. If EXPR is
-omitted, returns length of $_.
+omitted, returns length of C<$_>. Note that this cannot be used on
+an entire array or hash to find out how many elements these have.
+For that, use C<scalar @array> and C<scalar keys %hash> respectively.
=item link OLDFILE,NEWFILE
-Creates a new filename linked to the old filename. Returns 1 for
-success, 0 otherwise.
+Creates a new filename linked to the old filename. Returns true for
+success, false otherwise.
=item listen SOCKET,QUEUESIZE
-Does the same thing that the listen system call does. Returns TRUE if
-it succeeded, FALSE otherwise. See example in L<perlipc>.
+Does the same thing that the listen system call does. Returns true if
+it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
=item local EXPR
-In general, you should be using "my" instead of "local", because it's
-faster and safer. Format variables have to use "local" though, as
-do any other variables whose local value must be visible to called
-subroutines. This is known as dynamic scoping. Lexical scoping is
-done with "my", which works more like C's auto declarations.
-
-A local modifies the listed variables to be local to the enclosing block,
-subroutine, eval or "do". If more than one value is listed, the list
-must be placed in parens. All the listed elements must be legal
-lvalues. This operator works by saving the current values of those
-variables in LIST on a hidden stack and restoring them upon exiting the
-block, subroutine or eval. This means that called subroutines can also
-reference the local variable, but not the global one. The LIST may be
-assigned to if desired, which allows you to initialize your local
-variables. (If no initializer is given for a particular variable, it
-is created with an undefined value.) Commonly this is used to name the
-parameters to a subroutine. Examples:
-
- sub RANGEVAL {
- local($min, $max, $thunk) = @_;
- local $result = '';
- local $i;
-
- # Presumably $thunk makes reference to $i
-
- for ($i = $min; $i < $max; $i++) {
- $result .= eval $thunk;
- }
-
- $result;
- }
+You really probably want to be using C<my> instead, because C<local> isn't
+what most people think of as "local". See L<perlsub/"Private Variables
+via my()"> for details.
+A local modifies the listed variables to be local to the enclosing
+block, file, or eval. If more than one value is listed, the list must
+be placed in parentheses. See L<perlsub/"Temporary Values via local()">
+for details, including issues with tied arrays and hashes.
- if ($sw eq '-v') {
- # init local array with global array
- local @ARGV = @ARGV;
- unshift(@ARGV,'echo');
- system @ARGV;
- }
- # @ARGV restored
+=item localtime EXPR
+Converts a time as returned by the time function to a 9-element list
+with the time analyzed for the local time zone. Typically used as
+follows:
- # temporarily add to digits associative array
- if ($base12) {
- # (NOTE: not claiming this is efficient!)
- local(%digits) = (%digits,'t',10,'e',11);
- parse_num();
- }
+ # 0 1 2 3 4 5 6 7 8
+ ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
+ localtime(time);
-Note that local() is a run-time command, and so gets executed every
-time through a loop. In Perl 4 it used up more stack storage each
-time until the loop was exited. Perl 5 reclaims the space each time
-through, but it's still more efficient to declare your variables
-outside the loop.
+All list elements are numeric, and come straight out of a struct tm.
+In particular this means that $mon has the range C<0..11> and $wday
+has the range C<0..6> with sunday as day C<0>. Also, $year is the
+number of years since 1900, that is, $year is C<123> in year 2023,
+and I<not> simply the last two digits of the year. If you assume it is,
+then you create non-Y2K-compliant programs--and you wouldn't want to do
+that, would you?
-When you assign to a localized EXPR, the local doesn't change whether
-EXPR is viewed as a scalar or an array. So
+If EXPR is omitted, uses the current time (C<localtime(time)>).
- local($foo) = <STDIN>;
- local @FOO = <STDIN>;
+In scalar context, returns the ctime(3) value:
-both supply a list context to the righthand side, while
+ $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
- local $foo = <STDIN>;
+This scalar value is B<not> locale dependent, see L<perllocale>, but
+instead a Perl builtin. Also see the C<Time::Local> module, and the
+strftime(3) and mktime(3) function available via the POSIX module. To
+get somewhat similar but locale dependent date strings, set up your
+locale environment variables appropriately (please see L<perllocale>)
+and try for example:
-supplies a scalar context.
+ use POSIX qw(strftime);
+ $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
-=item localtime EXPR
+Note that the C<%a> and C<%b>, the short forms of the day of the week
+and the month of the year, may not necessarily be three characters wide.
-Converts a time as returned by the time function to a 9-element array
-with the time analyzed for the local timezone. Typically used as
-follows:
+=item lock
- ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
- localtime(time);
+ lock I<THING>
-All array elements are numeric, and come straight out of a struct tm.
-In particular this means that $mon has the range 0..11 and $wday has
-the range 0..6. If EXPR is omitted, does localtime(time).
+This function places an advisory lock on a variable, subroutine,
+or referenced object contained in I<THING> until the lock goes out
+of scope. This is a built-in function only if your version of Perl
+was built with threading enabled, and if you've said C<use Threads>.
+Otherwise a user-defined function by this name will be called. See
+L<Thread>.
-In a scalar context, prints out the ctime(3) value:
+=item log EXPR
- $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
+=item log
-See also L<perlmod/timelocal> and the strftime(3) function available
-via the POSIX modulie.
+Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
+returns log of C<$_>. To get the log of another base, use basic algebra:
+The base-N log of a number is equal to the natural log of that number
+divided by the natural log of N. For example:
-=item log EXPR
+ sub log10 {
+ my $n = shift;
+ return log($n)/log(10);
+ }
-Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
-of $_.
+See also L</exp> for the inverse operation.
=item lstat FILEHANDLE
=item lstat EXPR
-Does the same thing as the stat() function, but stats a symbolic link
-instead of the file the symbolic link points to. If symbolic links are
-unimplemented on your system, a normal stat() is done.
+=item lstat
+
+Does the same thing as the C<stat> function (including setting the
+special C<_> filehandle) but stats a symbolic link instead of the file
+the symbolic link points to. If symbolic links are unimplemented on
+your system, a normal C<stat> is done.
+
+If EXPR is omitted, stats C<$_>.
=item m//
=item map EXPR,LIST
-Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
-element) and returns the list value composed of the results of each such
-evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
-may produce zero, one, or more elements in the returned value.
+Evaluates the BLOCK or EXPR for each element of LIST (locally setting
+C<$_> to each element) and returns the list value composed of the
+results of each such evaluation. In scalar context, returns the
+total number of elements so generated. Evaluates BLOCK or EXPR in
+list context, so each element of LIST may produce zero, one, or
+more elements in the returned value.
@chars = map(chr, @nums);
translates a list of numbers to the corresponding characters. And
- %hash = map {&key($_), $_} @array;
+ %hash = map { getkey($_) => $_ } @array;
is just a funny way to write
%hash = ();
foreach $_ (@array) {
- $hash{&key($_)} = $_;
+ $hash{getkey($_)} = $_;
}
-=item mkdir FILENAME,MODE
+Note that, because C<$_> is a reference into the list value, it can
+be used to modify the elements of the array. While this is useful and
+supported, it can cause bizarre results if the LIST is not a named array.
+Using a regular C<foreach> loop for this purpose would be clearer in
+most cases. See also L</grep> for an array composed of those items of
+the original list for which the BLOCK or EXPR evaluates to true.
+
+=item mkdir FILENAME,MASK
-Creates the directory specified by FILENAME, with permissions specified
-by MODE (as modified by umask). If it succeeds it returns 1, otherwise
-it returns 0 and sets $! (errno).
+Creates the directory specified by FILENAME, with permissions
+specified by MASK (as modified by C<umask>). If it succeeds it
+returns true, otherwise it returns false and sets C<$!> (errno).
+
+In general, it is better to create directories with permissive MASK,
+and let the user modify that with their C<umask>, than it is to supply
+a restrictive MASK and give the user no way to be more permissive.
+The exceptions to this rule are when the file or directory should be
+kept private (mail files, for instance). The perlfunc(1) entry on
+C<umask> discusses the choice of MASK in more detail.
=item msgctl ID,CMD,ARG
-Calls the System V IPC function msgctl. If CMD is &IPC_STAT, then ARG
-must be a variable which will hold the returned msqid_ds structure.
-Returns like ioctl: the undefined value for error, "0 but true" for
-zero, or the actual return value otherwise.
+Calls the System V IPC function msgctl(2). You'll probably have to say
+
+ use IPC::SysV;
+
+first to get the correct constant definitions. If CMD is C<IPC_STAT>,
+then ARG must be a variable which will hold the returned C<msqid_ds>
+structure. Returns like C<ioctl>: the undefined value for error, C<"0 but
+true"> for zero, or the actual return value otherwise. See also
+C<IPC::SysV> and C<IPC::Semaphore> documentation.
=item msgget KEY,FLAGS
-Calls the System V IPC function msgget. Returns the message queue id,
-or the undefined value if there is an error.
+Calls the System V IPC function msgget(2). Returns the message queue
+id, or the undefined value if there is an error. See also C<IPC::SysV>
+and C<IPC::Msg> documentation.
=item msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the message MSG to the
message queue ID. MSG must begin with the long integer message type,
-which may be created with C<pack("L", $type)>. Returns TRUE if
-successful, or FALSE if there is an error.
+which may be created with C<pack("l", $type)>. Returns true if
+successful, or false if there is an error. See also C<IPC::SysV>
+and C<IPC::SysV::Msg> documentation.
=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a message from
message queue ID into variable VAR with a maximum message size of
-SIZE. Note that if a message is received, the message type will be the
-first thing in VAR, and the maximum length of VAR is SIZE plus the size
-of the message type. Returns TRUE if successful, or FALSE if there is
-an error.
+SIZE. Note that if a message is received, the message type will be
+the first thing in VAR, and the maximum length of VAR is SIZE plus the
+size of the message type. Returns true if successful, or false if
+there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
=item my EXPR
-A "my" declares the listed variables to be local (lexically) to the
-enclosing block, subroutine, eval or "do". If more than one value is
-listed, the list must be placed in parens. All the listed elements
-must be legal lvalues. Only alphanumeric identifiers may be lexically
-scoped--magical builtins like $/ must be localized with "local"
-instead. In particular, you're not allowed to say
-
- my $_; # Illegal.
-
-Unlike the "local" declaration, variables declared with "my"
-are totally hidden from the outside world, including any called
-subroutines (even if it's the same subroutine--every call gets its own
-copy).
-
-(An eval(), however, can see the lexical variables of the scope it is
-being evaluated in so long as the names aren't hidden by declarations within
-the eval() itself. See L<perlref>.)
-
-The EXPR may be assigned to if desired, which allows you to initialize
-your variables. (If no initializer is given for a particular
-variable, it is created with an undefined value.) Commonly this is
-used to name the parameters to a subroutine. Examples:
-
- sub RANGEVAL {
- my($min, $max, $thunk) = @_;
- my $result = '';
- my $i;
-
- # Presumably $thunk makes reference to $i
-
- for ($i = $min; $i < $max; $i++) {
- $result .= eval $thunk;
- }
-
- $result;
- }
-
-
- if ($sw eq '-v') {
- # init my array with global array
- my @ARGV = @ARGV;
- unshift(@ARGV,'echo');
- system @ARGV;
- }
- # Outer @ARGV again visible
-
-When you assign to the EXPR, the "my" doesn't change whether
-EXPR is viewed as a scalar or an array. So
-
- my($foo) = <STDIN>;
- my @FOO = <STDIN>;
-
-both supply a list context to the righthand side, while
-
- my $foo = <STDIN>;
-
-supplies a scalar context.
-
-Some users may wish to encourage the use of lexically scoped variables.
-As an aid to catching implicit references to package variables,
-if you say
-
- use strict 'vars';
-
-then any variable reference from there to the end of the enclosing
-block must either refer to a lexical variable, or must be fully
-qualified with the package name. A compilation error results
-otherwise. An inner block may countermand this with S<"no strict 'vars'">.
+A C<my> declares the listed variables to be local (lexically) to the
+enclosing block, file, or C<eval>. If
+more than one value is listed, the list must be placed in parentheses. See
+L<perlsub/"Private Variables via my()"> for details.
=item next LABEL
The C<next> command is like the C<continue> statement in C; it starts
the next iteration of the loop:
- line: while (<STDIN>) {
- next line if /^#/; # discard comments
- ...
+ LINE: while (<STDIN>) {
+ next LINE if /^#/; # discard comments
+ #...
}
Note that if there were a C<continue> block on the above, it would get
executed even on discarded lines. If the LABEL is omitted, the command
refers to the innermost enclosing loop.
+C<next> cannot be used to exit a block which returns a value such as
+C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
+a grep() or map() operation.
+
+See also L</continue> for an illustration of how C<last>, C<next>, and
+C<redo> work.
+
=item no Module LIST
-See the "use" function, which "no" is the opposite of.
+See the L</use> function, which C<no> is the opposite of.
=item oct EXPR
-Returns the decimal value of EXPR interpreted as an octal string. (If
-EXPR happens to start off with 0x, interprets it as a hex string
-instead.) The following will handle decimal, octal, and hex in the
-standard Perl or C notation:
+=item oct
+
+Interprets EXPR as an octal string and returns the corresponding
+value. (If EXPR happens to start off with C<0x>, interprets it as a
+hex string. If EXPR starts off with C<0b>, it is interpreted as a
+binary string.) The following will handle decimal, binary, octal, and
+hex in the standard Perl or C notation:
$val = oct($val) if $val =~ /^0/;
-If EXPR is omitted, uses $_.
+If EXPR is omitted, uses C<$_>. To go the other way (produce a number
+in octal), use sprintf() or printf():
+
+ $perms = (stat("filename"))[2] & 07777;
+ $oct_perms = sprintf "%lo", $perms;
+
+The oct() function is commonly used when a string such as C<644> needs
+to be converted into a file mode, for example. (Although perl will
+automatically convert strings into numbers as needed, this automatic
+conversion assumes base 10.)
=item open FILEHANDLE,EXPR
Opens the file whose filename is given by EXPR, and associates it with
FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
name of the real filehandle wanted. If EXPR is omitted, the scalar
-variable of the same name as the FILEHANDLE contains the filename. If
-the filename begins with "<" or nothing, the file is opened for input.
-If the filename begins with ">", the file is opened for output. If the
-filename begins with ">>", the file is opened for appending. (You can
-put a '+' in front of the '>' or '<' to indicate that you want both
-read and write access to the file.) If the filename begins with "|",
-the filename is interpreted as a command to which output is to be
-piped, and if the filename ends with a "|", the filename is interpreted
-as command which pipes input to us. (You may not have a command that
-pipes both in and out.) Opening '-' opens STDIN and opening '>-'
-opens STDOUT. Open returns non-zero upon success, the undefined
-value otherwise. If the open involved a pipe, the return value happens
-to be the pid of the subprocess. Examples:
+variable of the same name as the FILEHANDLE contains the filename.
+(Note that lexical variables--those declared with C<my>--will not work
+for this purpose; so if you're using C<my>, specify EXPR in your call
+to open.) See L<perlopentut> for a kinder, gentler explanation of opening
+files.
+
+If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
+If the filename begins with C<'E<gt>'>, the file is truncated and opened for
+output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
+the file is opened for appending, again being created if necessary.
+You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
+you want both read and write access to the file; thus C<'+E<lt>'> is almost
+always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
+file first. You can't usually use either read-write mode for updating
+textfiles, since they have variable length records. See the B<-i>
+switch in L<perlrun> for a better approach. The file is created with
+permissions of C<0666> modified by the process' C<umask> value.
+
+The prefix and the filename may be separated with spaces.
+These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
+C<'w+'>, C<'a'>, and C<'a+'>.
+
+If the filename begins with C<'|'>, the filename is interpreted as a
+command to which output is to be piped, and if the filename ends with a
+C<'|'>, the filename is interpreted as a command which pipes output to
+us. See L<perlipc/"Using open() for IPC">
+for more examples of this. (You are not allowed to C<open> to a command
+that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
+and L<perlipc/"Bidirectional Communication"> for alternatives.)
+
+Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
+nonzero upon success, the undefined value otherwise. If the C<open>
+involved a pipe, the return value happens to be the pid of the
+subprocess.
+
+If you're unfortunate enough to be running Perl on a system that
+distinguishes between text files and binary files (modern operating
+systems don't care), then you should check out L</binmode> for tips for
+dealing with this. The key distinction between systems that need C<binmode>
+and those that don't is their text file formats. Systems like Unix, MacOS, and
+Plan9, which delimit lines with a single character, and which encode that
+character in C as C<"\n">, do not need C<binmode>. The rest need it.
+
+When opening a file, it's usually a bad idea to continue normal execution
+if the request failed, so C<open> is frequently used in connection with
+C<die>. Even if C<die> won't do what you want (say, in a CGI script,
+where you want to make a nicely formatted error message (but there are
+modules that can help with that problem)) you should always check
+the return value from opening a file. The infrequent exception is when
+working with an unopened filehandle is actually what you want to do.
+
+Examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
+ # if the open fails, output is discarded
+
+ open(DBASE, '+<dbase.mine') # open for update
+ or die "Can't open 'dbase.mine' for update: $!";
- open(article, "caesar <$article |"); # decrypt article
+ open(ARTICLE, "caesar <$article |") # decrypt article
+ or die "Can't start caesar: $!";
- open(extract, "|sort >/tmp/Tmp$$"); # $$ is our process id
+ open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
+ or die "Can't start sort: $!";
# process argument list of files along with any includes
}
sub process {
- local($filename, $input) = @_;
+ my($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
+ local $_;
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
- ... # whatever
+ #... # whatever
}
}
You may also, in the Bourne shell tradition, specify an EXPR beginning
-with ">&", in which case the rest of the string is interpreted as the
-name of a filehandle (or file descriptor, if numeric) which is to be
-duped and opened. You may use & after >, >>, <, +>, +>> and +<. The
+with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
+name of a filehandle (or file descriptor, if numeric) to be
+duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
+C<+E<gt>E<gt>>, and C<+E<lt>>. The
mode you specify should match the mode of the original filehandle.
+(Duping a filehandle does not take into account any existing contents of
+stdio buffers.)
Here is a script that saves, redirects, and restores STDOUT and
STDERR:
#!/usr/bin/perl
- open(SAVEOUT, ">&STDOUT");
- open(SAVEERR, ">&STDERR");
+ open(OLDOUT, ">&STDOUT");
+ open(OLDERR, ">&STDERR");
open(STDOUT, ">foo.out") || die "Can't redirect stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";
close(STDOUT);
close(STDERR);
- open(STDOUT, ">&SAVEOUT");
- open(STDERR, ">&SAVEERR");
+ open(STDOUT, ">&OLDOUT");
+ open(STDERR, ">&OLDERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
-
-If you specify "<&=N", where N is a number, then Perl will do an
-equivalent of C's fdopen() of that file descriptor. For example:
+If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
+equivalent of C's C<fdopen> of that file descriptor; this is more
+parsimonious of file descriptors. For example:
open(FILEHANDLE, "<&=$fd")
-If you open a pipe on the command "-", i.e. either "|-" or "-|", then
+If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
there is an implicit fork done, and the return value of open is the pid
-of the child within the parent process, and 0 within the child
-process. (Use defined($pid) to determine whether the open was successful.)
+of the child within the parent process, and C<0> within the child
+process. (Use C<defined($pid)> to determine whether the open was successful.)
The filehandle behaves normally for the parent, but i/o to that
filehandle is piped from/to the STDOUT/STDIN of the child process.
In the child process the filehandle isn't opened--i/o happens from/to
the new STDOUT or STDIN. Typically this is used like the normal
piped open when you want to exercise more control over just how the
pipe command gets executed, such as when you are running setuid, and
-don't want to have to scan shell commands for metacharacters. The
-following pairs are more or less equivalent:
+don't want to have to scan shell commands for metacharacters.
+The following pairs are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, "cat -n '$file'|");
open(FOO, "-|") || exec 'cat', '-n', $file;
-Explicitly closing any piped filehandle causes the parent process to
-wait for the child to finish, and returns the status value in $?.
-Note: on any operation which may do a fork, unflushed buffers remain
-unflushed in both processes, which means you may need to set $| to
-avoid duplicate output.
-
-The filename that is passed to open will have leading and trailing
-whitespace deleted. In order to open a file with arbitrary weird
-characters in it, it's necessary to protect any leading and trailing
-whitespace thusly:
+See L<perlipc/"Safe Pipe Opens"> for more examples of this.
+
+NOTE: On any operation that may do a fork, all files opened for output
+are flushed before the fork is attempted. On systems that support a
+close-on-exec flag on files, the flag will be set for the newly opened
+file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
+
+Closing any piped filehandle causes the parent process to wait for the
+child to finish, and returns the status value in C<$?>.
+
+The filename passed to open will have leading and trailing
+whitespace deleted, and the normal redirection characters
+honored. This property, known as "magic open",
+can often be used to good effect. A user could specify a filename of
+F<"rsh cat file |">, or you could change certain filenames as needed:
+
+ $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
+ open(FH, $filename) or die "Can't open $filename: $!";
+
+However, to open a file with arbitrary weird characters in it, it's
+necessary to protect any leading and trailing whitespace:
+
+ $file =~ s#^(\s)#./$1#;
+ open(FOO, "< $file\0");
+
+If you want a "real" C C<open> (see L<open(2)> on your system), then you
+should use the C<sysopen> function, which involves no such magic. This is
+another way to protect your filenames from interpretation. For example:
+
+ use IO::Handle;
+ sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
+ or die "sysopen $path: $!";
+ $oldfh = select(HANDLE); $| = 1; select($oldfh);
+ print HANDLE "stuff $$\n");
+ seek(HANDLE, 0, 0);
+ print "File contains: ", <HANDLE>;
+
+Using the constructor from the C<IO::Handle> package (or one of its
+subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
+filehandles that have the scope of whatever variables hold references to
+them, and automatically close whenever and however you leave that scope:
+
+ use IO::File;
+ #...
+ sub read_myfile_munged {
+ my $ALL = shift;
+ my $handle = new IO::File;
+ open($handle, "myfile") or die "myfile: $!";
+ $first = <$handle>
+ or return (); # Automatically closed here.
+ mung $first or die "mung failed"; # Or here.
+ return $first, <$handle> if $ALL; # Or here.
+ $first; # Or here.
+ }
- $file =~ s#^(\s)#./$1#;
- open(FOO, "< $file\0");
+See L</seek> for some details about mixing reading and writing.
=item opendir DIRHANDLE,EXPR
-Opens a directory named EXPR for processing by readdir(), telldir(),
-seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
+Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
+C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.
=item ord EXPR
-Returns the numeric ascii value of the first character of EXPR. If
-EXPR is omitted, uses $_.
+=item ord
+
+Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
+EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
+See L<utf8> for more about Unicode.
=item pack TEMPLATE,LIST
-Takes an array or list of values and packs it into a binary structure,
+Takes a list of values and packs it into a binary structure,
returning the string containing the structure. The TEMPLATE is a
sequence of characters that give the order and type of values, as
follows:
+ a A string with arbitrary binary data, will be null padded.
A An ascii string, will be space padded.
- a An ascii string, will be null padded.
+ Z A null terminated (asciz) string, will be null padded.
+
b A bit string (ascending bit order, like vec()).
B A bit string (descending bit order).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char value.
- C An unsigned char value.
+ C An unsigned char value. Only does bytes. See U for Unicode.
+
s A signed short value.
S An unsigned short value.
+ (This 'short' is _exactly_ 16 bits, which may differ from
+ what a local C compiler calls 'short'.)
+
i A signed integer value.
I An unsigned integer value.
+ (This 'integer' is _at_least_ 32 bits wide. Its exact
+ size depends on what a local C compiler calls 'int',
+ and may even be larger than the 'long' described in
+ the next item.)
+
l A signed long value.
L An unsigned long value.
+ (This 'long' is _exactly_ 32 bits, which may differ from
+ what a local C compiler calls 'long'.)
- n A short in "network" order.
- N A long in "network" order.
+ n A short in "network" (big-endian) order.
+ N A long in "network" (big-endian) order.
v A short in "VAX" (little-endian) order.
V A long in "VAX" (little-endian) order.
+ (These 'shorts' and 'longs' are _exactly_ 16 bits and
+ _exactly_ 32 bits, respectively.)
+
+ q A signed quad (64-bit) value.
+ Q An unsigned quad value.
+ (Available only if your system supports 64-bit integer values
+ _and_ if Perl has been compiled to support those.
+ Causes a fatal error otherwise.)
f A single-precision float in the native format.
d A double-precision float in the native format.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
+ U A Unicode character number. Encodes to UTF-8 internally.
+ Works even if C<use utf8> is not in effect.
+
+ w A BER compressed integer. Its bytes represent an unsigned
+ integer in base 128, most significant digit first, with as
+ few digits as possible. Bit eight (the high bit) is set
+ on each byte except the last.
x A null byte.
X Back up a byte.
@ Null fill to absolute position.
-Each letter may optionally be followed by a number which gives a repeat
-count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
-pack function will gobble up that many values from the LIST. A * for the
-repeat count means to use however many items are left. The "a" and "A"
-types gobble just one value, but pack it as a string of length count,
-padding with nulls or spaces as necessary. (When unpacking, "A" strips
-trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
-fields pack a string that many bits long. The "h" and "H" fields pack a
-string that many nybbles long. The "P" packs a pointer to a structure of
-the size indicated by the length. Real numbers (floats and doubles) are
-in the native machine format only; due to the multiplicity of floating
-formats around, and the lack of a standard "network" representation, no
-facility for interchange has been made. This means that packed floating
-point data written on one machine may not be readable on another - even if
-both use IEEE floating point arithmetic (as the endian-ness of the memory
-representation is not part of the IEEE spec). Note that Perl uses doubles
-internally for all numeric calculation, and converting from double into
-float and thence back to double again will lose precision (i.e.
-C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
+The following rules apply:
+
+=over 8
+
+=item *
+
+Each letter may optionally be followed by a number giving a repeat
+count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
+C<"H">, and C<"P"> the pack function will gobble up that many values from
+the LIST. A C<*> for the repeat count means to use however many items are
+left.
+
+=item *
+
+The C<"a">, C<"A">, and C<"Z"> types gobble just one value, but pack it as a
+string of length count, padding with nulls or spaces as necessary. When
+unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
+after the first null, and C<"a"> returns data verbatim.
+
+=item *
+
+Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
+
+=item *
+
+The C<"h"> and C<"H"> fields pack a string that many nybbles long.
+
+=item *
+
+The C<"p"> type packs a pointer to a null-terminated string. You are
+responsible for ensuring the string is not a temporary value (which can
+potentially get deallocated before you get around to using the packed result).
+The C<"P"> type packs a pointer to a structure of the size indicated by the
+length. A NULL pointer is created if the corresponding value for C<"p"> or
+C<"P"> is C<undef>.
+
+=item *
+
+The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
+immediately followed by a C<"!"> to signify native shorts or longs--as
+you can see from above for example a bare C<"l"> does mean exactly 32
+bits, the native C<long> (as seen by the local C compiler) may be
+larger. This is an issue mainly in 64-bit platforms. You can see
+whether using C<"!"> makes any difference by
+
+ print length(pack("s")), " ", length(pack("s!")), "\n";
+ print length(pack("l")), " ", length(pack("l!")), "\n";
+
+C<"i!"> and C<"I!"> also work but only because of completeness;
+they are identical to C<"i"> and C<"I">.
+
+The actual sizes (in bytes) of native shorts, ints, and longs on
+the platform where Perl was built are also available via L<Config>:
+
+The actual sizes (in bytes) of native shorts, ints, longs, and long
+longs on the platform where Perl was built are also available via
+L<Config>:
+
+ use Config;
+ print $Config{shortsize}, "\n";
+ print $Config{intsize}, "\n";
+ print $Config{longsize}, "\n";
+ print $Config{longlongsize}, "\n";
+
+=item *
+
+The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
+are inherently non-portable between processors and operating systems
+because they obey the native byteorder and endianness. For example a
+4-byte integer 0x87654321 (2271560481 decimal) be ordered natively
+(arranged in and handled by the CPU registers) into bytes as
+
+ 0x12 0x34 0x56 0x78 # little-endian
+ 0x78 0x56 0x34 0x12 # big-endian
+
+Basically, the Intel, Alpha, and VAX CPUs and little-endian, while
+everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
+Power, and Cray are big-endian. MIPS can be either: Digital used it
+in little-endian mode; SGI uses it in big-endian mode.
+
+The names `big-endian' and `little-endian' are comic references to
+the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
+Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
+the egg-eating habits of the Lilliputians.
+
+Some systems may even have weird byte orders such as
+
+ 0x56 0x78 0x12 0x34
+ 0x34 0x12 0x78 0x56
+
+You can see your system's preference with
+
+ print join(" ", map { sprintf "%#02x", $_ }
+ unpack("C*",pack("L",0x12345678))), "\n";
+
+The byteorder on the platform where Perl was built is also available
+via L<Config>:
+
+ use Config;
+ print $Config{byteorder}, "\n";
+
+Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
+and C<'87654321'> are big-endian.
+
+If you want portable packed integers use the formats C<"n">, C<"N">,
+C<"v">, and C<"V">, their byte endianness and size is known.
+
+=item *
+
+Real numbers (floats and doubles) are in the native machine format only;
+due to the multiplicity of floating formats around, and the lack of a
+standard "network" representation, no facility for interchange has been
+made. This means that packed floating point data written on one machine
+may not be readable on another - even if both use IEEE floating point
+arithmetic (as the endian-ness of the memory representation is not part
+of the IEEE spec).
+
+Note that Perl uses doubles internally for all numeric calculation, and
+converting from double into float and thence back to double again will
+lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
+equal $foo).
+
+=back
Examples:
- $foo = pack("cccc",65,66,67,68);
+ $foo = pack("CCCC",65,66,67,68);
# foo eq "ABCD"
- $foo = pack("c4",65,66,67,68);
+ $foo = pack("C4",65,66,67,68);
# same thing
+ $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
+ # same thing with Unicode circled letters
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
+ $utmp_template = "Z8 Z8 Z16 L";
+ $utmp = pack($utmp_template, @utmp1);
+ # a struct utmp (BSDish)
+
+ @utmp2 = unpack($utmp_template, $utmp);
+ # "@utmp1" eq "@utmp2"
+
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
-The same template may generally also be used in the unpack function.
+The same template may generally also be used in unpack().
+
+=item package
+
+=item package NAMESPACE
+
+Declares the compilation unit as being in the given namespace. The scope
+of the package declaration is from the declaration itself through the end
+of the enclosing block, file, or eval (the same as the C<my> operator).
+All further unqualified dynamic identifiers will be in this namespace.
+A package statement affects only dynamic variables--including those
+you've used C<local> on--but I<not> lexical variables, which are created
+with C<my>. Typically it would be the first declaration in a file to
+be included by the C<require> or C<use> operator. You can switch into a
+package in more than one place; it merely influences which symbol table
+is used by the compiler for the rest of that block. You can refer to
+variables and filehandles in other packages by prefixing the identifier
+with the package name and a double colon: C<$Package::Variable>.
+If the package name is null, the C<main> package as assumed. That is,
+C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
+still seen in older code).
+
+If NAMESPACE is omitted, then there is no current package, and all
+identifiers must be fully qualified or lexicals. This is stricter
+than C<use strict>, since it also extends to function names.
+
+See L<perlmod/"Packages"> for more information about packages, modules,
+and classes. See L<perlsub> for other scoping issues.
=item pipe READHANDLE,WRITEHANDLE
Opens a pair of connected pipes like the corresponding system call.
Note that if you set up a loop of piped processes, deadlock can occur
unless you are very careful. In addition, note that Perl's pipes use
-stdio buffering, so you may need to set $| to flush your WRITEHANDLE
+stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
after each command, depending on the application.
+See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
+for examples of such things.
+
+On systems that support a close-on-exec flag on files, the flag will be set
+for the newly opened file descriptors as determined by the value of $^F.
+See L<perlvar/$^F>.
+
=item pop ARRAY
+=item pop
+
Pops and returns the last value of the array, shortening the array by
-1. Has a similar effect to
+one element. Has an effect similar to
- $tmp = $ARRAY[$#ARRAY--];
+ $ARRAY[$#ARRAY--]
-If there are no elements in the array, returns the undefined value.
+If there are no elements in the array, returns the undefined value
+(although this may happen at other times as well). If ARRAY is
+omitted, pops the C<@ARGV> array in the main program, and the C<@_>
+array in subroutines, just like C<shift>.
=item pos SCALAR
-Returns the offset of where the last m//g search left off for the variable
-in question. May be modified to change that offset.
+=item pos
+
+Returns the offset of where the last C<m//g> search left off for the variable
+is in question (C<$_> is used when the variable is not specified). May be
+modified to change that offset. Such modification will also influence
+the C<\G> zero-width assertion in regular expressions. See L<perlre> and
+L<perlop>.
=item print FILEHANDLE LIST
=item print
-Prints a string or a comma-separated list of strings. Returns non-zero
-if successful. FILEHANDLE may be a scalar variable name, in which case
-the variable contains the name of the filehandle, thus introducing one
-level of indirection. (NOTE: If FILEHANDLE is a variable and the next
-token is a term, it may be misinterpreted as an operator unless you
-interpose a + or put parens around the arguments.) If FILEHANDLE is
-omitted, prints by default to standard output (or to the last selected
-output channel--see select()). If LIST is also omitted, prints $_ to
-STDOUT. To set the default output channel to something other than
-STDOUT use the select operation. Note that, because print takes a
-LIST, anything in the LIST is evaluated in a list context, and any
-subroutine that you call will have one or more of its expressions
-evaluated in a list context. Also be careful not to follow the print
-keyword with a left parenthesis unless you want the corresponding right
-parenthesis to terminate the arguments to the print--interpose a + or
-put parens around all the arguments.
-
-=item printf FILEHANDLE LIST
-
-=item printf LIST
-
-Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
-of the list will be interpreted as the printf format.
+Prints a string or a list of strings. Returns true if successful.
+FILEHANDLE may be a scalar variable name, in which case the variable
+contains the name of or a reference to the filehandle, thus introducing
+one level of indirection. (NOTE: If FILEHANDLE is a variable and
+the next token is a term, it may be misinterpreted as an operator
+unless you interpose a C<+> or put parentheses around the arguments.)
+If FILEHANDLE is omitted, prints by default to standard output (or
+to the last selected output channel--see L</select>). If LIST is
+also omitted, prints C<$_> to the currently selected output channel.
+To set the default output channel to something other than STDOUT
+use the select operation. The current value of C<$,> (if any) is
+printed between each LIST item. The current value of C<$\> (if
+any) is printed after the entire LIST has been printed. Because
+print takes a LIST, anything in the LIST is evaluated in list
+context, and any subroutine that you call will have one or more of
+its expressions evaluated in list context. Also be careful not to
+follow the print keyword with a left parenthesis unless you want
+the corresponding right parenthesis to terminate the arguments to
+the print--interpose a C<+> or put parentheses around all the
+arguments.
+
+Note that if you're storing FILEHANDLES in an array or other expression,
+you will have to use a block returning its value instead:
+
+ print { $files[$i] } "stuff\n";
+ print { $OK ? STDOUT : STDERR } "stuff\n";
+
+=item printf FILEHANDLE FORMAT, LIST
+
+=item printf FORMAT, LIST
+
+Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
+(the output record separator) is not appended. The first argument
+of the list will be interpreted as the C<printf> format. If C<use locale> is
+in effect, the character used for the decimal point in formatted real numbers
+is affected by the LC_NUMERIC locale. See L<perllocale>.
+
+Don't fall into the trap of using a C<printf> when a simple
+C<print> would do. The C<print> is more efficient and less
+error prone.
+
+=item prototype FUNCTION
+
+Returns the prototype of a function as a string (or C<undef> if the
+function has no prototype). FUNCTION is a reference to, or the name of,
+the function whose prototype you want to retrieve.
+
+If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
+name for Perl builtin. If the builtin is not I<overridable> (such as
+C<qw//>) or its arguments cannot be expressed by a prototype (such as
+C<system>) returns C<undef> because the builtin does not really behave
+like a Perl function. Otherwise, the string describing the equivalent
+prototype is returned.
=item push ARRAY,LIST
=item qq/STRING/
+=item qr/STRING/
+
=item qx/STRING/
=item qw/STRING/
-Generalized quotes. See L<perlop>.
+Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
=item quotemeta EXPR
-Returns the value of EXPR with with all regular expression
-metacharacters backslashed. This is the internal function implementing
-the \Q escape in double-quoted strings.
+=item quotemeta
+
+Returns the value of EXPR with all non-alphanumeric
+characters backslashed. (That is, all characters not matching
+C</[A-Za-z_0-9]/> will be preceded by a backslash in the
+returned string, regardless of any locale settings.)
+This is the internal function implementing
+the C<\Q> escape in double-quoted strings.
+
+If EXPR is omitted, uses C<$_>.
=item rand EXPR
=item rand
-Returns a random fractional number between 0 and the value of EXPR.
-(EXPR should be positive.) If EXPR is omitted, returns a value between
-0 and 1. This function produces repeatable sequences unless srand()
-is invoked. See also srand().
+Returns a random fractional number greater than or equal to C<0> and less
+than the value of EXPR. (EXPR should be positive.) If EXPR is
+omitted, the value C<1> is used. Automatically calls C<srand> unless
+C<srand> has already been called. See also C<srand>.
-(Note: if your rand function consistently returns numbers that are too
+(Note: If your rand function consistently returns numbers that are too
large or too small, then your version of Perl was probably compiled
-with the wrong number of RANDBITS. As a workaround, you can usually
-multiply EXPR by the correct power of 2 to get the range you want.
-This will make your script unportable, however. It's better to recompile
-if you can.)
+with the wrong number of RANDBITS.)
=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
=item read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable SCALAR from the
-specified FILEHANDLE. Returns the number of bytes actually read, or
-undef if there was an error. SCALAR will be grown or shrunk to the
-length actually read. An OFFSET may be specified to place the read
-data at some other place than the beginning of the string. This call
-is actually implemented in terms of stdio's fread call. To get a true
-read system call, see sysread().
+specified FILEHANDLE. Returns the number of bytes actually read,
+C<0> at end of file, or undef if there was an error. SCALAR will be grown
+or shrunk to the length actually read. An OFFSET may be specified to
+place the read data at some other place than the beginning of the
+string. This call is actually implemented in terms of stdio's fread(3)
+call. To get a true read(2) system call, see C<sysread>.
=item readdir DIRHANDLE
-Returns the next directory entry for a directory opened by opendir().
-If used in a list context, returns all the rest of the entries in the
+Returns the next directory entry for a directory opened by C<opendir>.
+If used in list context, returns all the rest of the entries in the
directory. If there are no more entries, returns an undefined value in
-a scalar context or a null list in a list context.
+scalar context or a null list in list context.
+
+If you're planning to filetest the return values out of a C<readdir>, you'd
+better prepend the directory in question. Otherwise, because we didn't
+C<chdir> there, it would have been testing the wrong file.
+
+ opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
+ @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
+ closedir DIR;
+
+=item readline EXPR
+
+Reads from the filehandle whose typeglob is contained in EXPR. In scalar
+context, each call reads and returns the next line, until end-of-file is
+reached, whereupon the subsequent call returns undef. In list context,
+reads until end-of-file is reached and returns a list of lines. Note that
+the notion of "line" used here is however you may have defined it
+with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
+
+When C<$/> is set to C<undef>, when readline() is in scalar
+context (i.e. file slurp mode), and when an empty file is read, it
+returns C<''> the first time, followed by C<undef> subsequently.
+
+This is the internal function implementing the C<E<lt>EXPRE<gt>>
+operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
+operator is discussed in more detail in L<perlop/"I/O Operators">.
+
+ $line = <STDIN>;
+ $line = readline(*STDIN); # same thing
=item readlink EXPR
+=item readlink
+
Returns the value of a symbolic link, if symbolic links are
implemented. If not, gives a fatal error. If there is some system
-error, returns the undefined value and sets $! (errno). If EXPR is
-omitted, uses $_.
+error, returns the undefined value and sets C<$!> (errno). If EXPR is
+omitted, uses C<$_>.
-=item recv SOCKET,SCALAR,LEN,FLAGS
+=item readpipe EXPR
+
+EXPR is executed as a system command.
+The collected standard output of the command is returned.
+In scalar context, it comes back as a single (potentially
+multi-line) string. In list context, returns a list of lines
+(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
+This is the internal function implementing the C<qx/EXPR/>
+operator, but you can use it directly. The C<qx/EXPR/>
+operator is discussed in more detail in L<perlop/"I/O Operators">.
+
+=item recv SOCKET,SCALAR,LENGTH,FLAGS
Receives a message on a socket. Attempts to receive LENGTH bytes of
data into variable SCALAR from the specified SOCKET filehandle.
-Actually does a C recvfrom(), so that it can returns the address of the
+Actually does a C C<recvfrom>, so that it can return the address of the
sender. Returns the undefined value if there's an error. SCALAR will
be grown or shrunk to the length actually read. Takes the same flags
as the system call of the same name.
+See L<perlipc/"UDP: Message Passing"> for examples.
=item redo LABEL
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
- line: while (<STDIN>) {
+ LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
- s|^|$front{|;
- redo line;
+ s|^|$front\{|;
+ redo LINE;
}
}
}
print;
}
+C<redo> cannot be used to retry a block which returns a value such as
+C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
+a grep() or map() operation.
+
+See also L</continue> for an illustration of how C<last>, C<next>, and
+C<redo> work.
+
=item ref EXPR
-Returns a TRUE value if EXPR is a reference, FALSE otherwise. The value
-returned depends on the type of thing the reference is a reference to.
+=item ref
+
+Returns a true value if EXPR is a reference, false otherwise. If EXPR
+is not specified, C<$_> will be used. The value returned depends on the
+type of thing the reference is a reference to.
Builtin types include:
- REF
SCALAR
ARRAY
HASH
CODE
+ REF
GLOB
+ LVALUE
-If the referenced object has been blessed into a package, then that package
-name is returned instead. You can think of ref() as a typeof() operator.
+If the referenced object has been blessed into a package, then that package
+name is returned instead. You can think of C<ref> as a C<typeof> operator.
if (ref($r) eq "HASH") {
- print "r is a reference to an associative array.\n";
- }
- if (!ref ($r) {
+ print "r is a reference to a hash.\n";
+ }
+ unless (ref($r)) {
print "r is not a reference at all.\n";
+ }
+ if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
+ print "r is a reference to something that isa hash.\n";
}
See also L<perlref>.
=item rename OLDNAME,NEWNAME
-Changes the name of a file. Returns 1 for success, 0 otherwise. Will
-not work across filesystem boundaries.
+Changes the name of a file; an existing file NEWNAME will be
+clobbered. Returns true for success, false otherwise.
+
+Behavior of this function varies wildly depending on your system
+implementation. For example, it will usually not work across file system
+boundaries, even though the system I<mv> command sometimes compensates
+for this. Other restrictions include whether it works on directories,
+open files, or pre-existing files. Check L<perlport> and either the
+rename(2) manpage or equivalent system documentation for details.
=item require EXPR
=item require
-Demands some semantics specified by EXPR, or by $_ if EXPR is not
+Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
supplied. If EXPR is numeric, demands that the current version of Perl
-($] or $PERL_VERSION) be equal or greater than EXPR.
+(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
Otherwise, demands that a library file be included if it hasn't already
been included. The file is included via the do-FILE mechanism, which is
-essentially just a variety of eval(). Has semantics similar to the following
+essentially just a variety of C<eval>. Has semantics similar to the following
subroutine:
sub require {
- local($filename) = @_;
+ my($filename) = @_;
return 1 if $INC{$filename};
- local($realfilename,$result);
+ my($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
die $@ if $@;
die "$filename did not return true value" unless $result;
$INC{$filename} = $realfilename;
- $result;
+ return $result;
}
Note that the file will not be included twice under the same specified
-name. The file must return TRUE as the last statement to indicate
+name. The file must return true as the last statement to indicate
successful execution of any initialization code, so it's customary to
-end such a file with "1;" unless you're sure it'll return TRUE
-otherwise. But it's better just to put the "C<1;>", in case you add more
+end such a file with C<1;> unless you're sure it'll return true
+otherwise. But it's better just to put the C<1;>, in case you add more
statements.
-If EXPR is a bare word, the require assumes a "F<.pm>" extension for you,
-to make it easy to load standard modules. This form of loading of
+If EXPR is a bareword, the require assumes a "F<.pm>" extension and
+replaces "F<::>" with "F</>" in the filename for you,
+to make it easy to load standard modules. This form of loading of
modules does not risk altering your namespace.
-For a yet more powerful import facility, see the L</use()> below, and
-also L<perlmod>.
+In other words, if you try this:
+
+ require Foo::Bar; # a splendid bareword
+
+The require function will actually look for the "F<Foo/Bar.pm>" file in the
+directories specified in the C<@INC> array.
+
+But if you try this:
+
+ $class = 'Foo::Bar';
+ require $class; # $class is not a bareword
+ #or
+ require "Foo::Bar"; # not a bareword because of the ""
+
+The require function will look for the "F<Foo::Bar>" file in the @INC array and
+will complain about not finding "F<Foo::Bar>" there. In this case you can do:
+
+ eval "require $class";
+
+For a yet-more-powerful import facility, see L</use> and L<perlmod>.
=item reset EXPR
=item reset
Generally used in a C<continue> block at the end of a loop to clear
-variables and reset ?? searches so that they work again. The
+variables and reset C<??> searches so that they work again. The
expression is interpreted as a list of single characters (hyphens
allowed for ranges). All variables and arrays beginning with one of
those letters are reset to their pristine state. If the expression is
-omitted, one-match searches (?pattern?) are reset to match again. Only
-resets variables or searches in the current package. Always returns
+omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
+only variables or searches in the current package. Always returns
1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
- reset; # just reset ?? searches
+ reset; # just reset ?one-time? searches
+
+Resetting C<"A-Z"> is not recommended because you'll wipe out your
+C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
+variables--lexical variables are unaffected, but they clean themselves
+up on scope exit anyway, so you'll probably want to use them instead.
+See L</my>.
+
+=item return EXPR
-Resetting "A-Z" is not recommended since you'll wipe out your
-ARGV and ENV arrays. Only resets package variables--lexical variables
-are unaffected, but they clean themselves up on scope exit anyway,
-so anymore you probably want to use them instead. See L</my>.
+=item return
-=item return LIST
+Returns from a subroutine, C<eval>, or C<do FILE> with the value
+given in EXPR. Evaluation of EXPR may be in list, scalar, or void
+context, depending on how the return value will be used, and the context
+may vary from one execution to the next (see C<wantarray>). If no EXPR
+is given, returns an empty list in list context, the undefined value in
+scalar context, and (of course) nothing at all in a void context.
-Returns from a subroutine or eval with the value specified. (Note that
-in the absence of a return a subroutine or eval will automatically
-return the value of the last expression evaluated.)
+(Note that in the absence of a explicit C<return>, a subroutine, eval,
+or do FILE will automatically return the value of the last expression
+evaluated.)
=item reverse LIST
-In a list context, returns a list value consisting of the elements
-of LIST in the opposite order. In a scalar context, returns a string
-value consisting of the bytes of the first element of LIST in the
-opposite order.
+In list context, returns a list value consisting of the elements
+of LIST in the opposite order. In scalar context, concatenates the
+elements of LIST and returns a string value with all characters
+in the opposite order.
+
+ print reverse <>; # line tac, last line first
+
+ undef $/; # for efficiency of <>
+ print scalar reverse <>; # character tac, last line tsrif
+
+This operator is also handy for inverting a hash, although there are some
+caveats. If a value is duplicated in the original hash, only one of those
+can be represented as a key in the inverted hash. Also, this has to
+unwind one hash and build a whole new one, which may take some time
+on a large hash, such as from a DBM file.
+
+ %by_name = reverse %by_address; # Invert the hash
=item rewinddir DIRHANDLE
Sets the current position to the beginning of the directory for the
-readdir() routine on DIRHANDLE.
+C<readdir> routine on DIRHANDLE.
=item rindex STR,SUBSTR,POSITION
=item rindex STR,SUBSTR
-Works just like index except that it returns the position of the LAST
+Works just like index() except that it returns the position of the LAST
occurrence of SUBSTR in STR. If POSITION is specified, returns the
last occurrence at or before that position.
=item rmdir FILENAME
-Deletes the directory specified by FILENAME if it is empty. If it
-succeeds it returns 1, otherwise it returns 0 and sets $! (errno). If
-FILENAME is omitted, uses $_.
+=item rmdir
+
+Deletes the directory specified by FILENAME if that directory is empty. If it
+succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
+FILENAME is omitted, uses C<$_>.
=item s///
=item scalar EXPR
-Forces EXPR to be interpreted in a scalar context and returns the value
+Forces EXPR to be interpreted in scalar context and returns the value
of EXPR.
+ @counts = ( scalar @a, scalar @b, scalar @c );
+
+There is no equivalent operator to force an expression to
+be interpolated in list context because in practice, this is never
+needed. If you really wanted to do so, however, you could use
+the construction C<@{[ (some expression) ]}>, but usually a simple
+C<(some expression)> suffices.
+
+Because C<scalar> is unary operator, if you accidentally use for EXPR a
+parenthesized list, this behaves as a scalar comma expression, evaluating
+all but the last element in void context and returning the final element
+evaluated in scalar context. This is seldom what you want.
+
+The following single statement:
+
+ print uc(scalar(&foo,$bar)),$baz;
+
+is the moral equivalent of these two:
+
+ &foo;
+ print(uc($bar),$baz);
+
+See L<perlop> for more details on unary operators and the comma operator.
+
=item seek FILEHANDLE,POSITION,WHENCE
-Randomly positions the file pointer for FILEHANDLE, just like the fseek()
-call of stdio. FILEHANDLE may be an expression whose value gives the name
-of the filehandle. The values for WHENCE are 0 to set the file pointer to
-POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
-plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
-this is usin the POSIX module. Returns 1 upon success, 0 otherwise.
+Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
+FILEHANDLE may be an expression whose value gives the name of the
+filehandle. The values for WHENCE are C<0> to set the new position to
+POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
+set it to EOF plus POSITION (typically negative). For WHENCE you may
+use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
+C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
+
+If you want to position file for C<sysread> or C<syswrite>, don't use
+C<seek>--buffering makes its effect on the file's system position
+unpredictable and non-portable. Use C<sysseek> instead.
+
+Due to the rules and rigors of ANSI C, on some systems you have to do a
+seek whenever you switch between reading and writing. Amongst other
+things, this may have the effect of calling stdio's clearerr(3).
+A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
+
+ seek(TEST,0,1);
+
+This is also useful for applications emulating C<tail -f>. Once you hit
+EOF on your read, and then sleep for a while, you might have to stick in a
+seek() to reset things. The C<seek> doesn't change the current position,
+but it I<does> clear the end-of-file condition on the handle, so that the
+next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
+
+If that doesn't work (some stdios are particularly cantankerous), then
+you may need something more like this:
+
+ for (;;) {
+ for ($curpos = tell(FILE); $_ = <FILE>;
+ $curpos = tell(FILE)) {
+ # search for some stuff and put it into files
+ }
+ sleep($for_a_while);
+ seek(FILE, $curpos, 0);
+ }
=item seekdir DIRHANDLE,POS
-Sets the current position for the readdir() routine on DIRHANDLE. POS
-must be a value returned by telldir(). Has the same caveats about
+Sets the current position for the C<readdir> routine on DIRHANDLE. POS
+must be a value returned by C<telldir>. Has the same caveats about
possible directory compaction as the corresponding system library
routine.
$oldfh = select(STDERR); $| = 1; select($oldfh);
-With Perl 5, filehandles are objects with methods, and the last example
-is preferably written
+Some programmers may prefer to think of filehandles as objects with
+methods, preferring to write the last example as:
- use FileHandle;
+ use IO::Handle;
STDERR->autoflush(1);
=item select RBITS,WBITS,EBITS,TIMEOUT
-This calls the select system(2) call with the bitmasks specified, which
-can be constructed using fileno() and vec(), along these lines:
+This calls the select(2) system call with the bit masks specified, which
+can be constructed using C<fileno> and C<vec>, along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
subroutine:
sub fhbits {
- local(@fhlist) = split(' ',$_[0]);
- local($bits);
+ my(@fhlist) = split(' ',$_[0]);
+ my($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
- $rin = &fhbits('STDIN TTY SOCK');
+ $rin = fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
-or to block until something becomes ready:
+or to block until something becomes ready just do this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
-Any of the bitmasks can also be undef. The timeout, if specified, is
+Most systems do not bother to return anything useful in $timeleft, so
+calling select() in scalar context just returns $nfound.
+
+Any of the bit masks can also be undef. The timeout, if specified, is
in seconds, which may be fractional. Note: not all implementations are
-capable of returning the $timeleft. If not, they always return
+capable of returning the$timeleft. If not, they always return
$timeleft equal to the supplied $timeout.
-You can effect a 250 microsecond sleep this way:
+You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
+B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
+or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
+then only on POSIX systems. You have to use C<sysread> instead.
=item semctl ID,SEMNUM,CMD,ARG
-Calls the System V IPC function semctl. If CMD is &IPC_STAT or
-&GETALL, then ARG must be a variable which will hold the returned
-semid_ds structure or semaphore value array. Returns like ioctl: the
-undefined value for error, "0 but true" for zero, or the actual return
-value otherwise.
+Calls the System V IPC function C<semctl>. You'll probably have to say
+
+ use IPC::SysV;
+
+first to get the correct constant definitions. If CMD is IPC_STAT or
+GETALL, then ARG must be a variable which will hold the returned
+semid_ds structure or semaphore value array. Returns like C<ioctl>: the
+undefined value for error, "C<0 but true>" for zero, or the actual return
+value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
=item semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget. Returns the semaphore id, or
-the undefined value if there is an error.
+the undefined value if there is an error. See also C<IPC::SysV> and
+C<IPC::SysV::Semaphore> documentation.
=item semop KEY,OPSTRING
such as signaling and waiting. OPSTRING must be a packed array of
semop structures. Each semop structure can be generated with
C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
-operations is implied by the length of OPSTRING. Returns TRUE if
-successful, or FALSE if there is an error. As an example, the
+operations is implied by the length of OPSTRING. Returns true if
+successful, or false if there is an error. As an example, the
following code waits on semaphore $semnum of semaphore id $semid:
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
-To signal the semaphore, replace "-1" with "1".
+To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
+and C<IPC::SysV::Semaphore> documentation.
=item send SOCKET,MSG,FLAGS,TO
Sends a message on a socket. Takes the same flags as the system call
of the same name. On unconnected sockets you must specify a
-destination to send TO, in which case it does a C sendto(). Returns
+destination to send TO, in which case it does a C C<sendto>. Returns
the number of characters sent, or the undefined value if there is an
-error.
+error. The C system call sendmsg(2) is currently unimplemented.
+See L<perlipc/"UDP: Message Passing"> for examples.
=item setpgrp PID,PGRP
-Sets the current process group for the specified PID, 0 for the current
+Sets the current process group for the specified PID, C<0> for the current
process. Will produce a fatal error if used on a machine that doesn't
-implement setpgrp(2).
+implement setpgrp(2). If the arguments are omitted, it defaults to
+C<0,0>. Note that the POSIX version of C<setpgrp> does not accept any
+arguments, so only C<setpgrp(0,0)> is portable. See also C<POSIX::setsid()>.
=item setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process group, or a user.
-(See Lsetpriority(2)>.) Will produce a fatal error if used on a machine
+(See setpriority(2).) Will produce a fatal error if used on a machine
that doesn't implement setpriority(2).
=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns undefined if there is an
-error. OPTVAL may be specified as undef if you don't want to pass an
+error. OPTVAL may be specified as C<undef> if you don't want to pass an
argument.
=item shift ARRAY
Shifts the first value of the array off and returns it, shortening the
array by 1 and moving everything down. If there are no elements in the
array, returns the undefined value. If ARRAY is omitted, shifts the
-@ARGV array in the main program, and the @_ array in subroutines.
-(This is determined lexically.) See also unshift(), push(), and pop().
-Shift() and unshift() do the same thing to the left end of an array
-that push() and pop() do to the right end.
+C<@_> array within the lexical scope of subroutines and formats, and the
+C<@ARGV> array at file scopes or within the lexical scopes established by
+the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
+See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
+same thing to the left end of an array that C<pop> and C<push> do to the
+right end.
=item shmctl ID,CMD,ARG
-Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
-must be a variable which will hold the returned shmid_ds structure.
-Returns like ioctl: the undefined value for error, "0 but true" for
-zero, or the actual return value otherwise.
+Calls the System V IPC function shmctl. You'll probably have to say
+
+ use IPC::SysV;
+
+first to get the correct constant definitions. If CMD is C<IPC_STAT>,
+then ARG must be a variable which will hold the returned C<shmid_ds>
+structure. Returns like ioctl: the undefined value for error, "C<0> but
+true" for zero, or the actual return value otherwise.
+See also C<IPC::SysV> documentation.
=item shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the shared memory
segment id, or the undefined value if there is an error.
+See also C<IPC::SysV> documentation.
=item shmread ID,VAR,POS,SIZE
Reads or writes the System V shared memory segment ID starting at
position POS for size SIZE by attaching to it, copying in/out, and
-detaching from it. When reading, VAR must be a variable which will
+detaching from it. When reading, VAR must be a variable that will
hold the data read. When writing, if STRING is too long, only SIZE
bytes are used; if STRING is too short, nulls are written to fill out
-SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
+SIZE bytes. Return true if successful, or false if there is an error.
+See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
+from CPAN.
=item shutdown SOCKET,HOW
Shuts down a socket connection in the manner indicated by HOW, which
has the same interpretation as in the system call of the same name.
+ shutdown(SOCKET, 0); # I/we have stopped reading data
+ shutdown(SOCKET, 1); # I/we have stopped writing data
+ shutdown(SOCKET, 2); # I/we have stopped using this socket
+
+This is useful with sockets when you want to tell the other
+side you're done writing but not done reading, or vice versa.
+It's also a more insistent form of close because it also
+disables the file descriptor in any forked copies in other
+processes.
+
=item sin EXPR
+=item sin
+
Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
-returns sine of $_.
+returns sine of C<$_>.
+
+For the inverse sine operation, you may use the C<POSIX::asin>
+function, or use this relation:
+
+ sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
=item sleep EXPR
=item sleep
Causes the script to sleep for EXPR seconds, or forever if no EXPR.
-May be interrupted by sending the process a SIGALRM. Returns the
-number of seconds actually slept. You probably cannot mix alarm() and
-sleep() calls, since sleep() is often implemented using alarm().
+May be interrupted if the process receives a signal such as C<SIGALRM>.
+Returns the number of seconds actually slept. You probably cannot
+mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
+using C<alarm>.
On some older systems, it may sleep up to a full second less than what
you requested, depending on how it counts seconds. Most modern systems
-always sleep the full amount.
+always sleep the full amount. They may appear to sleep longer than that,
+however, because your process might not be scheduled right away in a
+busy multitasking system.
+
+For delays of finer granularity than one second, you may use Perl's
+C<syscall> interface to access setitimer(2) if your system supports it,
+or else see L</select> above.
+
+See also the POSIX module's C<sigpause> function.
=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it to filehandle
-SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
-system call of the same name. You should "use Socket;" first to get
-the proper definitions imported. See the example in L<perlipc>.
+SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
+the system call of the same name. You should C<use Socket> first
+to get the proper definitions imported. See the examples in
+L<perlipc/"Sockets: Client/Server Communication">.
=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified domain, of the
-specified type. DOMAIN, TYPE and PROTOCOL are specified the same as
+specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
for the system call of the same name. If unimplemented, yields a fatal
-error. Returns TRUE if successful.
+error. Returns true if successful.
+
+Some systems defined C<pipe> in terms of C<socketpair>, in which a call
+to C<pipe(Rdr, Wtr)> is essentially:
+
+ use Socket;
+ socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
+ shutdown(Rdr, 1); # no more writing for reader
+ shutdown(Wtr, 0); # no more reading for writer
+
+See L<perlipc> for an example of socketpair use.
=item sort SUBNAME LIST
=item sort LIST
-Sorts the LIST and returns the sorted list value. Nonexistent values
-of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
-in standard string comparison order. If SUBNAME is specified, it
-gives the name of a subroutine that returns an integer less than, equal
-to, or greater than 0, depending on how the elements of the array are
-to be ordered. (The <=> and cmp operators are extremely useful in such
-routines.) SUBNAME may be a scalar variable name, in which case the
-value provides the name of the subroutine to use. In place of a
-SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
+Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
+is omitted, C<sort>s in standard string comparison order. If SUBNAME is
+specified, it gives the name of a subroutine that returns an integer
+less than, equal to, or greater than C<0>, depending on how the elements
+of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
+operators are extremely useful in such routines.) SUBNAME may be a
+scalar variable name (unsubscripted), in which case the value provides
+the name of (or a reference to) the actual subroutine to use. In place
+of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
subroutine.
-In the interests of efficiency the normal calling code for subroutines
-is bypassed, with the following effects: the subroutine may not be a
-recursive subroutine, and the two elements to be compared are passed
-into the subroutine not via @_ but as $a and $b (see example below).
-They are passed by reference, so don't modify $a and $b.
+In the interests of efficiency the normal calling code for subroutines is
+bypassed, with the following effects: the subroutine may not be a
+recursive subroutine, and the two elements to be compared are passed into
+the subroutine not via C<@_> but as the package global variables $a and
+$b (see example below). They are passed by reference, so don't
+modify $a and $b. And don't try to declare them as lexicals either.
+
+You also cannot exit out of the sort block or subroutine using any of the
+loop control operators described in L<perlsyn> or with C<goto>.
+
+When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
+current collation locale. See L<perllocale>.
Examples:
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
+ # now case-insensitively
+ @articles = sort {uc($a) cmp uc($b)} @files;
+
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
+ # this sorts the %age hash by value instead of key
+ # using an in-line function
+ @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
+
# sort using explicit subroutine name
sub byage {
- $age{$a} <=> $age{$b}; # presuming integers
+ $age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
- sub backwards { $b cmp $a; }
- @harry = ('dog','cat','x','Cain','Abel');
- @george = ('gone','chased','yz','Punished','Axed');
+ sub backwards { $b cmp $a }
+ @harry = qw(dog cat x Cain Abel);
+ @george = qw(gone chased yz Punished Axed);
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
+ # inefficiently sort by descending numeric compare using
+ # the first integer after the first = sign, or the
+ # whole record case-insensitively otherwise
+
+ @new = sort {
+ ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
+ ||
+ uc($a) cmp uc($b)
+ } @old;
+
+ # same thing, but much more efficiently;
+ # we'll build auxiliary indices instead
+ # for speed
+ @nums = @caps = ();
+ for (@old) {
+ push @nums, /=(\d+)/;
+ push @caps, uc($_);
+ }
+
+ @new = @old[ sort {
+ $nums[$b] <=> $nums[$a]
+ ||
+ $caps[$a] cmp $caps[$b]
+ } 0..$#old
+ ];
+
+ # same thing, but without any temps
+ @new = map { $_->[0] }
+ sort { $b->[1] <=> $a->[1]
+ ||
+ $a->[2] cmp $b->[2]
+ } map { [$_, /=(\d+)/, uc($_)] } @old;
+
+If you're using strict, you I<must not> declare $a
+and $b as lexicals. They are package globals. That means
+if you're in the C<main> package, it's
+
+ @articles = sort {$main::b <=> $main::a} @files;
+
+or just
+
+ @articles = sort {$::b <=> $::a} @files;
+
+but if you're in the C<FooPack> package, it's
+
+ @articles = sort {$FooPack::b <=> $FooPack::a} @files;
+
+The comparison function is required to behave. If it returns
+inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
+sometimes saying the opposite, for example) the results are not
+well-defined.
+
=item splice ARRAY,OFFSET,LENGTH,LIST
=item splice ARRAY,OFFSET,LENGTH
=item splice ARRAY,OFFSET
Removes the elements designated by OFFSET and LENGTH from an array, and
-replaces them with the elements of LIST, if any. Returns the elements
-removed from the array. The array grows or shrinks as necessary. If
-LENGTH is omitted, removes everything from OFFSET onward. The
-following equivalencies hold (assuming $[ == 0):
-
- push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
+replaces them with the elements of LIST, if any. In list context,
+returns the elements removed from the array. In scalar context,
+returns the last element removed, or C<undef> if no elements are
+removed. The array grows or shrinks as necessary.
+If OFFSET is negative then it starts that far from the end of the array.
+If LENGTH is omitted, removes everything from OFFSET onward.
+If LENGTH is negative, leave that many elements off the end of the array.
+The following equivalences hold (assuming C<$[ == 0>):
+
+ push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
- $a[$x] = $y splice(@a,$x,1,$y);
+ $a[$x] = $y splice(@a,$x,1,$y)
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two list values
- local(@a) = splice(@_,0,shift);
- local(@b) = splice(@_,0,shift);
+ my(@a) = splice(@_,0,shift);
+ my(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
=item split
-Splits a string into an array of strings, and returns it.
+Splits a string into a list of strings and returns that list. By default,
+empty leading fields are preserved, and empty trailing ones are deleted.
+
+If not in list context, returns the number of fields found and splits into
+the C<@_> array. (In list context, you can force the split into C<@_> by
+using C<??> as the pattern delimiters, but it still returns the list
+value.) The use of implicit split to C<@_> is deprecated, however, because
+it clobbers your subroutine arguments.
-If not in a list context, returns the number of fields found and splits into
-the @_ array. (In a list context, you can force the split into @_ by
-using C<??> as the pattern delimiters, but it still returns the array
-value.) The use of implicit split to @_ is deprecated, however.
+If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
+splits on whitespace (after skipping any leading whitespace). Anything
+matching PATTERN is taken to be a delimiter separating the fields. (Note
+that the delimiter may be longer than one character.)
-If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
-splits on whitespace (C</[ \t\n]+/>). Anything matching PATTERN is taken
-to be a delimiter separating the fields. (Note that the delimiter may
-be longer than one character.) If LIMIT is specified and is not
-negative, splits into no more than that many fields (though it may
-split into fewer). If LIMIT is unspecified, trailing null fields are
-stripped (which potential users of pop() would do well to remember).
-If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
-had been specified.
+If LIMIT is specified and positive, splits into no more than that
+many fields (though it may split into fewer). If LIMIT is unspecified
+or zero, trailing null fields are stripped (which potential users
+of C<pop> would do well to remember). If LIMIT is negative, it is
+treated as if an arbitrarily large LIMIT had been specified.
A pattern matching the null string (not to be confused with
-a null pattern C<//., which is just one member of the set of patterns
+a null pattern C<//>, which is just one member of the set of patterns
matching a null string) will split the value of EXPR into separate
characters at each point it matches that way. For example:
produces the output 'h:i:t:h:e:r:e'.
-The LIMIT parameter can be used to partially split a line
+The LIMIT parameter can be used to split a line partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
default. In time critical applications it behooves you not to split
into more fields than you really need.
-If the PATTERN contains parentheses, additional array elements are
+If the PATTERN contains parentheses, additional list elements are
created from each matching substring in the delimiter.
- split(/([,-])/, "1-10,20");
+ split(/([,-])/, "1-10,20", 3);
produces the list value
(1, '-', 10, ',', 20)
+If you had the entire header of a normal Unix email message in $header,
+you could split it up into fields and their values this way:
+
+ $header =~ s/\n\s+/ /g; # fix continuation lines
+ %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
+
The pattern C</PATTERN/> may be replaced with an expression to specify
patterns that vary at runtime. (To do runtime compilation only once,
-use C</$variable/o>.) As a special case, specifying a space S<(' ')> will
-split on white space just as split with no arguments does, but leading
-white space does I<NOT> produce a null first field. Thus, split(' ') can
-be used to emulate B<awk>'s default behavior, whereas C<split(/ /)> will
-give you as many null initial fields as there are leading spaces.
+use C</$variable/o>.)
+
+As a special case, specifying a PATTERN of space (C<' '>) will split on
+white space just as C<split> with no arguments does. Thus, C<split(' ')> can
+be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
+will give you as many null initial fields as there are leading spaces.
+A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
+whitespace produces a null first field. A C<split> with no arguments
+really does a C<split(' ', $_)> internally.
Example:
- open(passwd, '/etc/passwd');
- while (<passwd>) {
- ($login, $passwd, $uid, $gid, $gcos, $home, $shell) = split(/:/);
- ...
+ open(PASSWD, '/etc/passwd');
+ while (<PASSWD>) {
+ ($login, $passwd, $uid, $gid,
+ $gcos, $home, $shell) = split(/:/);
+ #...
}
-(Note that $shell above will still have a newline on it. See L</chop>,
+(Note that $shell above will still have a newline on it. See L</chop>,
L</chomp>, and L</join>.)
-=item sprintf FORMAT,LIST
-
-Returns a string formatted by the usual printf conventions of the C
-language. (The * character for an indirectly specified length is not
-supported, but you can get the same effect by interpolating a variable
-into the pattern.)
+=item sprintf FORMAT, LIST
+
+Returns a string formatted by the usual C<printf> conventions of the
+C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
+on your system for an explanation of the general principles.
+
+Perl does its own C<sprintf> formatting--it emulates the C
+function C<sprintf>, but it doesn't use it (except for floating-point
+numbers, and even then only the standard modifiers are allowed). As a
+result, any non-standard extensions in your local C<sprintf> are not
+available from Perl.
+
+Perl's C<sprintf> permits the following universally-known conversions:
+
+ %% a percent sign
+ %c a character with the given number
+ %s a string
+ %d a signed integer, in decimal
+ %u an unsigned integer, in decimal
+ %o an unsigned integer, in octal
+ %x an unsigned integer, in hexadecimal
+ %e a floating-point number, in scientific notation
+ %f a floating-point number, in fixed decimal notation
+ %g a floating-point number, in %e or %f notation
+
+In addition, Perl permits the following widely-supported conversions:
+
+ %X like %x, but using upper-case letters
+ %E like %e, but using an upper-case "E"
+ %G like %g, but with an upper-case "E" (if applicable)
+ %b an unsigned integer, in binary
+ %p a pointer (outputs the Perl value's address in hexadecimal)
+ %n special: *stores* the number of characters output so far
+ into the next variable in the parameter list
+
+Finally, for backward (and we do mean "backward") compatibility, Perl
+permits these unnecessary but widely-supported conversions:
+
+ %i a synonym for %d
+ %D a synonym for %ld
+ %U a synonym for %lu
+ %O a synonym for %lo
+ %F a synonym for %f
+
+Perl permits the following universally-known flags between the C<%>
+and the conversion letter:
+
+ space prefix positive number with a space
+ + prefix positive number with a plus sign
+ - left-justify within the field
+ 0 use zeros, not spaces, to right-justify
+ # prefix non-zero octal with "0", non-zero hex with "0x"
+ number minimum field width
+ .number "precision": digits after decimal point for
+ floating-point, max length for string, minimum length
+ for integer
+ l interpret integer as C type "long" or "unsigned long"
+ h interpret integer as C type "short" or "unsigned short"
+
+There is also one Perl-specific flag:
+
+ V interpret integer as Perl's standard integer type
+
+Where a number would appear in the flags, an asterisk (C<*>) may be
+used instead, in which case Perl uses the next item in the parameter
+list as the given number (that is, as the field width or precision).
+If a field width obtained through C<*> is negative, it has the same
+effect as the C<-> flag: left-justification.
+
+If C<use locale> is in effect, the character used for the decimal
+point in formatted real numbers is affected by the LC_NUMERIC locale.
+See L<perllocale>.
=item sqrt EXPR
+=item sqrt
+
Return the square root of EXPR. If EXPR is omitted, returns square
-root of $_.
+root of C<$_>. Only works on non-negative operands, unless you've
+loaded the standard Math::Complex module.
+
+ use Math::Complex;
+ print sqrt(-2); # prints 1.4142135623731i
=item srand EXPR
+=item srand
+
Sets the random number seed for the C<rand> operator. If EXPR is
-omitted, does C<srand(time)>. Of course, you'd need something much more
-random than that for cryptographic purposes, since it's easy to guess
-the current time. Checksumming the compressed output of rapidly
-changing operating system status programs is the usual method.
-Examples are posted regularly to comp.security.unix.
+omitted, uses a semi-random value supplied by the kernel (if it supports
+the F</dev/urandom> device) or based on the current time and process
+ID, among other things. In versions of Perl prior to 5.004 the default
+seed was just the current C<time>. This isn't a particularly good seed,
+so many old programs supply their own seed value (often C<time ^ $$> or
+C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
+
+In fact, it's usually not necessary to call C<srand> at all, because if
+it is not called explicitly, it is called implicitly at the first use of
+the C<rand> operator. However, this was not the case in version of Perl
+before 5.004, so if your script will run under older Perl versions, it
+should call C<srand>.
+
+Note that you need something much more random than the default seed for
+cryptographic purposes. Checksumming the compressed output of one or more
+rapidly changing operating system status programs is the usual method. For
+example:
+
+ srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
+
+If you're particularly concerned with this, see the C<Math::TrulyRandom>
+module in CPAN.
+
+Do I<not> call C<srand> multiple times in your program unless you know
+exactly what you're doing and why you're doing it. The point of the
+function is to "seed" the C<rand> function so that C<rand> can produce
+a different sequence each time you run your program. Just do it once at the
+top of your program, or you I<won't> get random numbers out of C<rand>!
+
+Frequently called programs (like CGI scripts) that simply use
+
+ time ^ $$
+
+for a seed can fall prey to the mathematical property that
+
+ a^b == (a+1)^(b+1)
+
+one-third of the time. So don't do that.
=item stat FILEHANDLE
=item stat EXPR
-Returns a 13-element array giving the status info for a file, either the
-file opened via FILEHANDLE, or named by EXPR. Returns a null list if
-the stat fails. Typically used as follows:
+=item stat
+
+Returns a 13-element list giving the status info for a file, either
+the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
+it stats C<$_>. Returns a null list if the stat fails. Typically used
+as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
+Not all fields are supported on all filesystem types. Here are the
+meaning of the fields:
+
+ 0 dev device number of filesystem
+ 1 ino inode number
+ 2 mode file mode (type and permissions)
+ 3 nlink number of (hard) links to the file
+ 4 uid numeric user ID of file's owner
+ 5 gid numeric group ID of file's owner
+ 6 rdev the device identifier (special files only)
+ 7 size total size of file, in bytes
+ 8 atime last access time since the epoch
+ 9 mtime last modify time since the epoch
+ 10 ctime inode change time (NOT creation time!) since the epoch
+ 11 blksize preferred block size for file system I/O
+ 12 blocks actual number of blocks allocated
+
+(The epoch was at 00:00 January 1, 1970 GMT.)
+
If stat is passed the special filehandle consisting of an underline, no
stat is done, but the current contents of the stat structure from the
last stat or filetest are returned. Example:
print "$file is executable NFS file\n";
}
-(This only works on machines for which the device number is negative under NFS.)
+(This works on machines only for which the device number is negative under NFS.)
+
+Because the mode contains both the file type and its permissions, you
+should mask off the file type portion and (s)printf using a C<"%o">
+if you want to see the real permissions.
+
+ $mode = (stat($filename))[2];
+ printf "Permissions are %04o\n", $mode & 07777;
+
+In scalar context, C<stat> returns a boolean value indicating success
+or failure, and, if successful, sets the information associated with
+the special filehandle C<_>.
+
+The File::stat module provides a convenient, by-name access mechanism:
+
+ use File::stat;
+ $sb = stat($filename);
+ printf "File is %s, size is %s, perm %04o, mtime %s\n",
+ $filename, $sb->size, $sb->mode & 07777,
+ scalar localtime $sb->mtime;
=item study SCALAR
=item study
-Takes extra time to study SCALAR ($_ if unspecified) in anticipation of
+Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
doing many pattern matches on the string before it is next modified.
This may or may not save time, depending on the nature and number of
patterns you are searching on, and on the distribution of character
frequencies in the string to be searched--you probably want to compare
-runtimes with and without it to see which runs faster. Those loops
+run times with and without it to see which runs faster. Those loops
which scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most. You may have only
-one study active at a time--if you study a different scalar the first
-is "unstudied". (The way study works is this: a linked list of every
+one C<study> active at a time--if you study a different scalar the first
+is "unstudied". (The way C<study> works is this: a linked list of every
character in the string to be searched is made, so we know, for
-example, where all the 'k' characters are. From each search string,
+example, where all the C<'k'> characters are. From each search string,
the rarest character is selected, based on some static frequency tables
constructed from some C programs and English text. Only those places
that contain this "rarest" character are examined.)
-For example, here is a loop which inserts index producing entries
+For example, here is a loop that inserts index producing entries
before any line containing a certain pattern:
while (<>) {
study;
- print ".IX foo\n" if /\bfoo\b/;
- print ".IX bar\n" if /\bbar\b/;
- print ".IX blurfl\n" if /\bblurfl\b/;
- ...
+ print ".IX foo\n" if /\bfoo\b/;
+ print ".IX bar\n" if /\bbar\b/;
+ print ".IX blurfl\n" if /\bblurfl\b/;
+ # ...
print;
}
-In searching for /\bfoo\b/, only those locations in $_ that contain "f"
-will be looked at, because "f" is rarer than "o". In general, this is
+In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
+will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
a big win except in pathological cases. The only question is whether
it saves you more time than it took to build the linked list in the
first place.
Note that if you have to look for strings that you don't know till
-runtime, you can build an entire loop as a string and eval that to
+runtime, you can build an entire loop as a string and C<eval> that to
avoid recompiling all your patterns all the time. Together with
-undefining $/ to input entire files as one record, this can be very
+undefining C<$/> to input entire files as one record, this can be very
fast, often faster than specialized programs like fgrep(1). The following
-scans a list of files (@files) for a list of words (@words), and prints
+scans a list of files (C<@files>) for a list of words (C<@words>), and prints
out the names of those files that contain a match:
$search = 'while (<>) { study;';
@ARGV = @files;
undef $/;
eval $search; # this screams
- $/ = "\n"; # put back to normal input delim
+ $/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
+=item sub BLOCK
+
+=item sub NAME
+
+=item sub NAME BLOCK
+
+This is subroutine definition, not a real function I<per se>. With just a
+NAME (and possibly prototypes), it's just a forward declaration. Without
+a NAME, it's an anonymous function declaration, and does actually return a
+value: the CODE ref of the closure you just created. See L<perlsub> and
+L<perlref> for details.
+
+=item substr EXPR,OFFSET,LEN,REPLACEMENT
+
=item substr EXPR,OFFSET,LEN
=item substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First character is at
-offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
+offset C<0>, or whatever you've set C<$[> to (but don't do that).
+If OFFSET is negative (or more precisely, less than C<$[>), starts
that far from the end of the string. If LEN is omitted, returns
-everything to the end of the string. You can use the substr() function
-as an lvalue, in which case EXPR must be an lvalue. If you assign
-something shorter than LEN, the string will shrink, and if you assign
-something longer than LEN, the string will grow to accommodate it. To
-keep the string the same length you may need to pad or chop your value
-using sprintf().
+everything to the end of the string. If LEN is negative, leaves that
+many characters off the end of the string.
+
+If you specify a substring that is partly outside the string, the part
+within the string is returned. If the substring is totally outside
+the string a warning is produced.
+
+You can use the substr() function as an lvalue, in which case EXPR
+must itself be an lvalue. If you assign something shorter than LEN,
+the string will shrink, and if you assign something longer than LEN,
+the string will grow to accommodate it. To keep the string the same
+length you may need to pad or chop your value using C<sprintf>.
+
+An alternative to using substr() as an lvalue is to specify the
+replacement string as the 4th argument. This allows you to replace
+parts of the EXPR and return what was there before in one operation,
+just as you can with splice().
=item symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the old filename.
-Returns 1 for success, 0 otherwise. On systems that don't support
+Returns C<1> for success, C<0> otherwise. On systems that don't support
symbolic links, produces a fatal error at run time. To check for that,
use eval:
- $symlink_exists = (eval 'symlink("","");', $@ eq '');
+ $symlink_exists = eval { symlink("",""); 1 };
=item syscall LIST
as follows: if a given argument is numeric, the argument is passed as
an int. If not, the pointer to the string value is passed. You are
responsible to make sure a string is pre-extended long enough to
-receive any result that might be written into a string. If your
+receive any result that might be written into a string. You can't use a
+string literal (or other read-only string) as an argument to C<syscall>
+because Perl has to assume that any string pointer might be written
+through. If your
integer arguments are not literals and have never been interpreted in a
-numeric context, you may need to add 0 to them to force them to look
-like numbers.
+numeric context, you may need to add C<0> to them to force them to look
+like numbers. This emulates the C<syswrite> function (or vice versa):
require 'syscall.ph'; # may need to run h2ph
- syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
+ $s = "hi there\n";
+ syscall(&SYS_write, fileno(STDOUT), $s, length $s);
-Note that Perl only supports passing of up to 14 arguments to your system call,
+Note that Perl supports passing of up to only 14 arguments to your system call,
which in practice should usually suffice.
+Syscall returns whatever value returned by the system call it calls.
+If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
+Note that some system calls can legitimately return C<-1>. The proper
+way to handle such calls is to assign C<$!=0;> before the call and
+check the value of C<$!> if syscall returns C<-1>.
+
+There's a problem with C<syscall(&SYS_pipe)>: it returns the file
+number of the read end of the pipe it creates. There is no way
+to retrieve the file number of the other end. You can avoid this
+problem by using C<pipe> instead.
+
+=item sysopen FILEHANDLE,FILENAME,MODE
+
+=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
+
+Opens the file whose filename is given by FILENAME, and associates it
+with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
+the name of the real filehandle wanted. This function calls the
+underlying operating system's C<open> function with the parameters
+FILENAME, MODE, PERMS.
+
+The possible values and flag bits of the MODE parameter are
+system-dependent; they are available via the standard module C<Fcntl>.
+For historical reasons, some values work on almost every system
+supported by perl: zero means read-only, one means write-only, and two
+means read/write. We know that these values do I<not> work under
+OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
+use them in new code.
+
+If the file named by FILENAME does not exist and the C<open> call creates
+it (typically because MODE includes the C<O_CREAT> flag), then the value of
+PERMS specifies the permissions of the newly created file. If you omit
+the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
+These permission values need to be in octal, and are modified by your
+process's current C<umask>.
+
+You should seldom if ever use C<0644> as argument to C<sysopen>, because
+that takes away the user's option to have a more permissive umask.
+Better to omit it. See the perlfunc(1) entry on C<umask> for more
+on this.
+
+See L<perlopentut> for a kinder, gentler explanation of opening files.
+
=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
=item sysread FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable SCALAR from the
-specified FILEHANDLE, using the system call read(2). It bypasses
-stdio, so mixing this with other kinds of reads may cause confusion.
-Returns the number of bytes actually read, or undef if there was an
-error. SCALAR will be grown or shrunk to the length actually read. An
-OFFSET may be specified to place the read data at some other place than
-the beginning of the string.
+specified FILEHANDLE, using the system call read(2). It bypasses stdio,
+so mixing this with other kinds of reads, C<print>, C<write>,
+C<seek>, C<tell>, or C<eof> can cause confusion because stdio
+usually buffers data. Returns the number of bytes actually read, C<0>
+at end of file, or undef if there was an error. SCALAR will be grown or
+shrunk so that the last byte actually read is the last byte of the
+scalar after the read.
+
+An OFFSET may be specified to place the read data at some place in the
+string other than the beginning. A negative OFFSET specifies
+placement at that many bytes counting backwards from the end of the
+string. A positive OFFSET greater than the length of SCALAR results
+in the string being padded to the required size with C<"\0"> bytes before
+the result of the read is appended.
+
+There is no syseof() function, which is ok, since eof() doesn't work
+very well on device files (like ttys) anyway. Use sysread() and check
+for a return value for 0 to decide whether you're done.
+
+=item sysseek FILEHANDLE,POSITION,WHENCE
+
+Sets FILEHANDLE's system position using the system call lseek(2). It
+bypasses stdio, so mixing this with reads (other than C<sysread>),
+C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause
+confusion. FILEHANDLE may be an expression whose value gives the name
+of the filehandle. The values for WHENCE are C<0> to set the new
+position to POSITION, C<1> to set the it to the current position plus
+POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
+For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
+C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
+
+Returns the new position, or the undefined value on failure. A position
+of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
+true on success and false on failure, yet you can still easily determine
+the new position.
=item system LIST
-Does exactly the same thing as "exec LIST" except that a fork is done
-first, and the parent process waits for the child process to complete.
-Note that argument processing varies depending on the number of
-arguments. The return value is the exit status of the program as
-returned by the wait() call. To get the actual exit value divide by
-256. See also L</exec>.
+=item system PROGRAM LIST
+
+Does exactly the same thing as C<exec LIST>, except that a fork is
+done first, and the parent process waits for the child process to
+complete. Note that argument processing varies depending on the
+number of arguments. If there is more than one argument in LIST,
+or if LIST is an array with more than one value, starts the program
+given by the first element of the list with arguments given by the
+rest of the list. If there is only one scalar argument, the argument
+is checked for shell metacharacters, and if there are any, the
+entire argument is passed to the system's command shell for parsing
+(this is C</bin/sh -c> on Unix platforms, but varies on other
+platforms). If there are no shell metacharacters in the argument,
+it is split into words and passed directly to C<execvp>, which is
+more efficient.
+
+All files opened for output are flushed before attempting the exec().
+
+The return value is the exit status of the program as
+returned by the C<wait> call. To get the actual exit value divide by
+256. See also L</exec>. This is I<not> what you want to use to capture
+the output from a command, for that you should use merely backticks or
+C<qx//>, as described in L<perlop/"`STRING`">.
+
+Like C<exec>, C<system> allows you to lie to a program about its name if
+you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
+
+Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
+program they're running doesn't actually interrupt your program.
+
+ @args = ("command", "arg1", "arg2");
+ system(@args) == 0
+ or die "system @args failed: $?"
+
+You can check all the failure possibilities by inspecting
+C<$?> like this:
+
+ $exit_value = $? >> 8;
+ $signal_num = $? & 127;
+ $dumped_core = $? & 128;
+
+When the arguments get executed via the system shell, results
+and return codes will be subject to its quirks and capabilities.
+See L<perlop/"`STRING`"> and L</exec> for details.
=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
=item syswrite FILEHANDLE,SCALAR,LENGTH
+=item syswrite FILEHANDLE,SCALAR
+
Attempts to write LENGTH bytes of data from variable SCALAR to the
-specified FILEHANDLE, using the system call write(2). It bypasses
-stdio, so mixing this with prints may cause confusion. Returns the
-number of bytes actually written, or undef if there was an error. An
-OFFSET may be specified to place the read data at some other place than
-the beginning of the string.
+specified FILEHANDLE, using the system call write(2). If LENGTH
+is not specified, writes whole SCALAR. It bypasses stdio, so mixing
+this with reads (other than C<sysread())>, C<print>, C<write>,
+C<seek>, C<tell>, or C<eof> may cause confusion because stdio
+usually buffers data. Returns the number of bytes actually written,
+or C<undef> if there was an error. If the LENGTH is greater than
+the available data in the SCALAR after the OFFSET, only as much
+data as is available will be written.
+
+An OFFSET may be specified to write the data from some part of the
+string other than the beginning. A negative OFFSET specifies writing
+that many bytes counting backwards from the end of the string. In the
+case the SCALAR is empty you can use OFFSET but only zero offset.
=item tell FILEHANDLE
=item tell
-Returns the current file position for FILEHANDLE. FILEHANDLE may be an
+Returns the current position for FILEHANDLE. FILEHANDLE may be an
expression whose value gives the name of the actual filehandle. If
-FILEHANDLE is omitted, assumes the file last read.
+FILEHANDLE is omitted, assumes the file last read.
+
+There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
=item telldir DIRHANDLE
-Returns the current position of the readdir() routines on DIRHANDLE.
-Value may be given to seekdir() to access a particular location in a
+Returns the current position of the C<readdir> routines on DIRHANDLE.
+Value may be given to C<seekdir> to access a particular location in a
directory. Has the same caveats about possible directory compaction as
the corresponding system library routine.
-=item tie VARIABLE,PACKAGENAME,LIST
+=item tie VARIABLE,CLASSNAME,LIST
-This function binds a variable to a package that will provide the
+This function binds a variable to a package class that will provide the
implementation for the variable. VARIABLE is the name of the variable
-to be enchanted. PACKAGENAME is the name of a package implementing
-objects of correct type. Any additional arguments are passed to the
-"new" method of the package. Typically these are arguments such as
-might be passed to the dbm_open() function of C.
-
-Note that functions such as keys() and values() may return huge array
-values when used on large DBM files. You may prefer to use the each()
-function to iterate over large DBM files. Example:
+to be enchanted. CLASSNAME is the name of a class implementing objects
+of correct type. Any additional arguments are passed to the C<new>
+method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
+or C<TIEHASH>). Typically these are arguments such as might be passed
+to the C<dbm_open()> function of C. The object returned by the C<new>
+method is also returned by the C<tie> function, which would be useful
+if you want to access other methods in CLASSNAME.
+
+Note that functions such as C<keys> and C<values> may return huge lists
+when used on large objects, like DBM files. You may prefer to use the
+C<each> function to iterate over such. Example:
# print out history file offsets
- tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0);
+ use NDBM_File;
+ tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
-A package implementing an associative array should have the following
-methods:
+A class implementing a hash should have the following methods:
- TIEHASH objectname, LIST
- DESTROY this
+ TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
+ CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
+ DESTROY this
-A package implementing an ordinary array should have the following methods:
+A class implementing an ordinary array should have the following methods:
- TIEARRAY objectname, LIST
- DESTROY this
+ TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
- [others TBD]
+ FETCHSIZE this
+ STORESIZE this, count
+ CLEAR this
+ PUSH this, LIST
+ POP this
+ SHIFT this
+ UNSHIFT this, LIST
+ SPLICE this, offset, length, LIST
+ EXTEND this, count
+ DESTROY this
-A package implementing a scalar should have the following methods:
+A class implementing a file handle should have the following methods:
- TIESCALAR objectname, LIST
+ TIEHANDLE classname, LIST
+ READ this, scalar, length, offset
+ READLINE this
+ GETC this
+ WRITE this, scalar, length, offset
+ PRINT this, LIST
+ PRINTF this, format, LIST
+ CLOSE this
DESTROY this
- FETCH this,
+
+A class implementing a scalar should have the following methods:
+
+ TIESCALAR classname, LIST
+ FETCH this,
STORE this, value
+ DESTROY this
+
+Not all methods indicated above need be implemented. See L<perltie>,
+L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
+
+Unlike C<dbmopen>, the C<tie> function will not use or require a module
+for you--you need to do that explicitly yourself. See L<DB_File>
+or the F<Config> module for interesting C<tie> implementations.
+
+For further details see L<perltie>, L<"tied VARIABLE">.
+
+=item tied VARIABLE
+
+Returns a reference to the object underlying VARIABLE (the same value
+that was originally returned by the C<tie> call that bound the variable
+to a package.) Returns the undefined value if VARIABLE isn't tied to a
+package.
=item time
-Returns the number of non-leap seconds since 00:00:00 UTC, January 1,
-1970. Suitable for feeding to gmtime() and localtime().
+Returns the number of non-leap seconds since whatever time the system
+considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
+and 00:00:00 UTC, January 1, 1970 for most other systems).
+Suitable for feeding to C<gmtime> and C<localtime>.
=item times
-Returns a four-element array giving the user and system times, in
+Returns a four-element list giving the user and system times, in
seconds, for this process and the children of this process.
($user,$system,$cuser,$csystem) = times;
=item tr///
-The translation operator. See L<perlop>.
+The transliteration operator. Same as C<y///>. See L<perlop>.
=item truncate FILEHANDLE,LENGTH
Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length. Produces a fatal error if truncate isn't implemented
-on your system.
+on your system. Returns true if successful, the undefined value
+otherwise.
=item uc EXPR
+=item uc
+
Returns an uppercased version of EXPR. This is the internal function
-implementing the \U escape in double-quoted strings.
+implementing the C<\U> escape in double-quoted strings.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
+Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
+does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
+
+If EXPR is omitted, uses C<$_>.
=item ucfirst EXPR
-Returns the value of EXPR with the first character uppercased. This is
-the internal function implementing the \u escape in double-quoted strings.
+=item ucfirst
+
+Returns the value of EXPR with the first character
+in uppercase (titlecase in Unicode). This is
+the internal function implementing the C<\u> escape in double-quoted strings.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
+and L<utf8>.
+
+If EXPR is omitted, uses C<$_>.
=item umask EXPR
=item umask
-Sets the umask for the process and returns the old one. If EXPR is
-omitted, merely returns current umask.
+Sets the umask for the process to EXPR and returns the previous value.
+If EXPR is omitted, merely returns the current umask.
+
+The Unix permission C<rwxr-x---> is represented as three sets of three
+bits, or three octal digits: C<0750> (the leading 0 indicates octal
+and isn't one of the digits). The C<umask> value is such a number
+representing disabled permissions bits. The permission (or "mode")
+values you pass C<mkdir> or C<sysopen> are modified by your umask, so
+even if you tell C<sysopen> to create a file with permissions C<0777>,
+if your umask is C<0022> then the file will actually be created with
+permissions C<0755>. If your C<umask> were C<0027> (group can't
+write; others can't read, write, or execute), then passing
+C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
+027> is C<0640>).
+
+Here's some advice: supply a creation mode of C<0666> for regular
+files (in C<sysopen>) and one of C<0777> for directories (in
+C<mkdir>) and executable files. This gives users the freedom of
+choice: if they want protected files, they might choose process umasks
+of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
+Programs should rarely if ever make policy decisions better left to
+the user. The exception to this is when writing files that should be
+kept private: mail files, web browser cookies, I<.rhosts> files, and
+so on.
+
+If umask(2) is not implemented on your system and you are trying to
+restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
+fatal error at run time. If umask(2) is not implemented and you are
+not trying to restrict access for yourself, returns C<undef>.
+
+Remember that a umask is a number, usually given in octal; it is I<not> a
+string of octal digits. See also L</oct>, if all you have is a string.
=item undef EXPR
=item undef
Undefines the value of EXPR, which must be an lvalue. Use only on a
-scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
+scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
+(using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
will probably not do what you expect on most predefined variables or
-DBM list values, so don't do that.) Always returns the undefined value. You can omit
-the EXPR, in which case nothing is undefined, but you still get an
-undefined value that you could, for instance, return from a
-subroutine. Examples:
+DBM list values, so don't do that; see L<delete>.) Always returns the
+undefined value. You can omit the EXPR, in which case nothing is
+undefined, but you still get an undefined value that you could, for
+instance, return from a subroutine, assign to a variable or pass as a
+parameter. Examples:
undef $foo;
- undef $bar{'blurfl'};
+ undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
- undef %assoc;
+ undef %hash;
undef &mysub;
- return (wantarray ? () : undef) if $they_blew_it;
+ undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
+ return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
+ select undef, undef, undef, 0.25;
+ ($a, $b, undef, $c) = &foo; # Ignore third value returned
+
+Note that this is a unary operator, not a list operator.
=item unlink LIST
+=item unlink
+
Deletes a list of files. Returns the number of files successfully
deleted.
unlink @goners;
unlink <*.bak>;
-Note: unlink will not delete directories unless you are superuser and
+Note: C<unlink> will not delete directories unless you are superuser and
the B<-U> flag is supplied to Perl. Even if these conditions are
met, be warned that unlinking a directory can inflict damage on your
-filesystem. Use rmdir instead.
+filesystem. Use C<rmdir> instead.
+
+If LIST is omitted, uses C<$_>.
=item unpack TEMPLATE,EXPR
-Unpack does the reverse of pack: it takes a string representing a
-structure and expands it out into a list value, returning the array
-value. (In a scalar context, it merely returns the first value
-produced.) The TEMPLATE has the same format as in the pack function.
+C<unpack> does the reverse of C<pack>: it takes a string
+representing a structure and expands it out into a list of values.
+(In scalar context, it returns merely the first value produced.)
+The TEMPLATE has the same format as in the C<pack> function.
Here's a subroutine that does substring:
sub substr {
- local($what,$where,$howmuch) = @_;
+ my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
sub ordinal { unpack("c",$_[0]); } # same as ord()
-In addition, you may prefix a field with a %<number> to indicate that
-you want a <number>-bit checksum of the items instead of the items
+In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
+you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
themselves. Default is a 16-bit checksum. For example, the following
computes the same number as the System V sum program:
- while (<>) {
- $checksum += unpack("%16C*", $_);
- }
- $checksum %= 65536;
+ $checksum = do {
+ local $/; # slurp!
+ unpack("%32C*",<>) % 65535;
+ };
The following efficiently counts the number of set bits in a bit vector:
$setbits = unpack("%32b*", $selectmask);
+The C<"p"> and C<"P"> formats should be used with care. Since Perl
+has no way of checking whether the value passed to C<unpack()>
+corresponds to a valid memory location, passing a pointer value that's
+not known to be valid is likely to have disastrous consequences.
+
+See L</pack> for more examples.
+
=item untie VARIABLE
-Breaks the binding between a variable and a package. (See tie().)
+Breaks the binding between a variable and a package. (See C<tie>.)
=item unshift ARRAY,LIST
unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at a time, so the
-prepended elements stay in the same order. Use reverse to do the
+prepended elements stay in the same order. Use C<reverse> to do the
reverse.
=item use Module LIST
=item use Module
+=item use Module VERSION LIST
+
+=item use VERSION
+
Imports some semantics into the current package from the named module,
generally by aliasing certain subroutine or variable names into your
package. It is exactly equivalent to
BEGIN { require Module; import Module LIST; }
-If you don't want your namespace altered, use require instead.
+except that Module I<must> be a bareword.
+
+If the first argument to C<use> is a number, it is treated as a version
+number instead of a module name. If the version of the Perl interpreter
+is less than VERSION, then an error message is printed and Perl exits
+immediately. This is often useful if you need to check the current
+Perl version before C<use>ing library modules that have changed in
+incompatible ways from older versions of Perl. (We try not to do
+this more than we have to.)
-The BEGIN forces the require and import to happen at compile time. The
-require makes sure the module is loaded into memory if it hasn't been
-yet. The import is not a builtin--it's just an ordinary static method
-call into the "Module" package to tell the module to import the list of
+The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
+C<require> makes sure the module is loaded into memory if it hasn't been
+yet. The C<import> is not a builtin--it's just an ordinary static method
+call into the C<Module> package to tell the module to import the list of
features back into the current package. The module can implement its
-import method any way it likes, though most modules just choose to
-derive their import method via inheritance from the Exporter class that
-is defined in the Exporter module.
+C<import> method any way it likes, though most modules just choose to
+derive their C<import> method via inheritance from the C<Exporter> class that
+is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
+method can be found then the error is currently silently ignored. This
+may change to a fatal error in a future version.
+
+If you don't want your namespace altered, explicitly supply an empty list:
+
+ use Module ();
+
+That is exactly equivalent to
+
+ BEGIN { require Module }
+
+If the VERSION argument is present between Module and LIST, then the
+C<use> will call the VERSION method in class Module with the given
+version as an argument. The default VERSION method, inherited from
+the Universal class, croaks if the given version is larger than the
+value of the variable C<$Module::VERSION>. (Note that there is not a
+comma after VERSION!)
Because this is a wide-open interface, pragmas (compiler directives)
are also implemented this way. Currently implemented pragmas are:
use integer;
+ use diagnostics;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
-These pseudomodules import semantics into the current block scope, unlike
-ordinary modules, which import symbols into the current package (which are
-effective through the end of the file).
+Some of these pseudo-modules import semantics into the current
+block scope (like C<strict> or C<integer>, unlike ordinary modules,
+which import symbols into the current package (which are effective
+through the end of the file).
-There's a corresponding "no" command that unimports meanings imported
-by use.
+There's a corresponding C<no> command that unimports meanings imported
+by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
no integer;
no strict 'refs';
+If no C<unimport> method can be found the call fails with a fatal error.
+
See L<perlmod> for a list of standard modules and pragmas.
=item utime LIST
files. The first two elements of the list must be the NUMERICAL access
and modification times, in that order. Returns the number of files
successfully changed. The inode modification time of each file is set
-to the current time. Example of a "touch" command:
+to the current time. This code has the same effect as the C<touch>
+command if the files already exist:
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
-=item values ASSOC_ARRAY
+=item values HASH
-Returns a normal array consisting of all the values of the named
-associative array. (In a scalar context, returns the number of
-values.) The values are returned in an apparently random order, but it
-is the same order as either the keys() or each() function would produce
-on the same array. See also keys() and each().
+Returns a list consisting of all the values of the named hash. (In a
+scalar context, returns the number of values.) The values are
+returned in an apparently random order. The actual random order is
+subject to change in future versions of perl, but it is guaranteed to
+be the same order as either the C<keys> or C<each> function would
+produce on the same (unmodified) hash.
-=item vec EXPR,OFFSET,BITS
+Note that you cannot modify the values of a hash this way, because the
+returned list is just a copy. You need to use a hash slice for that,
+since it's lvaluable in a way that values() is not.
+
+ for (values %hash) { s/foo/bar/g } # FAILS!
+ for (@hash{keys %hash}) { s/foo/bar/g } # ok
-Treats a string as a vector of unsigned integers, and returns the value
-of the bitfield specified. May also be assigned to. BITS must be a
-power of two from 1 to 32.
+As a side effect, calling values() resets the HASH's internal iterator.
+See also C<keys>, C<each>, and C<sort>.
-Vectors created with vec() can also be manipulated with the logical
-operators |, & and ^, which will assume a bit vector operation is
-desired when both operands are strings.
+=item vec EXPR,OFFSET,BITS
-To transform a bit vector into a string or array of 0's and 1's, use these:
+Treats the string in EXPR as a vector of unsigned integers, and
+returns the value of the bit field specified by OFFSET. BITS specifies
+the number of bits that are reserved for each entry in the bit
+vector. This must be a power of two from 1 to 32. C<vec> may also be
+assigned to, in which case parentheses are needed to give the expression
+the correct precedence as in
+
+ vec($image, $max_x * $x + $y, 8) = 3;
+
+Vectors created with C<vec> can also be manipulated with the logical
+operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
+desired when both operands are strings. See L<perlop/"Bitwise String Operators">.
+
+The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
+The comments show the string after each step. Note that this code works
+in the same way on big-endian or little-endian machines.
+
+ my $foo = '';
+ vec($foo, 0, 32) = 0x5065726C; # 'Perl'
+ vec($foo, 2, 16) = 0x5065; # 'PerlPe'
+ vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
+ vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
+ vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
+ vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
+ vec($foo, 21, 4) = 7; # 'PerlPerlPer'
+ # 'r' is "\x72"
+ vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
+ vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
+ vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
+ # 'l' is "\x6c"
+
+To transform a bit vector into a string or list of 0's and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
-If you know the exact length in bits, it can be used in place of the *.
+If you know the exact length in bits, it can be used in place of the C<*>.
=item wait
-Waits for a child process to terminate and returns the pid of the
-deceased process, or -1 if there are no child processes. The status is
-returned in $?.
+Behaves like the wait(2) system call on your system: it waits for a child
+process to terminate and returns the pid of the deceased process, or
+C<-1> if there are no child processes. The status is returned in C<$?>.
+Note that a return value of C<-1> could mean that child processes are
+being automatically reaped, as described in L<perlipc>.
=item waitpid PID,FLAGS
-Waits for a particular child process to terminate and returns the pid
-of the deceased process, or -1 if there is no such child process. The
-status is returned in $?. If you say
+Waits for a particular child process to terminate and returns the pid of
+the deceased process, or C<-1> if there is no such child process. On some
+systems, a value of 0 indicates that there are processes still running.
+The status is returned in C<$?>. If you say
+
+ use POSIX ":sys_wait_h";
+ #...
+ do {
+ $kid = waitpid(-1,&WNOHANG);
+ } until $kid == -1;
- use POSIX "wait_h";
- ...
- waitpid(-1,&WNOHANG);
+then you can do a non-blocking wait for all pending zombie processes.
+Non-blocking wait is available on machines supporting either the
+waitpid(2) or wait4(2) system calls. However, waiting for a particular
+pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
+system call by remembering the status values of processes that have
+exited but have not been harvested by the Perl script yet.)
-then you can do a non-blocking wait for any process. Non-blocking wait
-is only available on machines supporting either the waitpid(2) or
-wait4(2) system calls. However, waiting for a particular pid with
-FLAGS of 0 is implemented everywhere. (Perl emulates the system call
-by remembering the status values of processes that have exited but have
-not been harvested by the Perl script yet.)
+Note that on some systems, a return value of C<-1> could mean that child
+processes are being automatically reaped. See L<perlipc> for details,
+and for other examples.
=item wantarray
-Returns TRUE if the context of the currently executing subroutine is
-looking for a list value. Returns FALSE if the context is looking
-for a scalar.
+Returns true if the context of the currently executing subroutine is
+looking for a list value. Returns false if the context is looking
+for a scalar. Returns the undefined value if the context is looking
+for no value (void context).
- return wantarray ? () : undef;
+ return unless defined wantarray; # don't bother doing more
+ my @a = complex_calculation();
+ return wantarray ? @a : "@a";
+
+This function should have been named wantlist() instead.
=item warn LIST
-Produces a message on STDERR just like die(), but doesn't exit or
-throw an exception.
+Produces a message on STDERR just like C<die>, but doesn't exit or throw
+an exception.
+
+If LIST is empty and C<$@> already contains a value (typically from a
+previous eval) that value is used after appending C<"\t...caught">
+to C<$@>. This is useful for staying almost, but not entirely similar to
+C<die>.
+
+If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
+
+No message is printed if there is a C<$SIG{__WARN__}> handler
+installed. It is the handler's responsibility to deal with the message
+as it sees fit (like, for instance, converting it into a C<die>). Most
+handlers must therefore make arrangements to actually display the
+warnings that they are not prepared to deal with, by calling C<warn>
+again in the handler. Note that this is quite safe and will not
+produce an endless loop, since C<__WARN__> hooks are not called from
+inside one.
+
+You will find this behavior is slightly different from that of
+C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
+instead call C<die> again to change it).
+
+Using a C<__WARN__> handler provides a powerful way to silence all
+warnings (even the so-called mandatory ones). An example:
+
+ # wipe out *all* compile-time warnings
+ BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
+ my $foo = 10;
+ my $foo = 20; # no warning about duplicate my $foo,
+ # but hey, you asked for it!
+ # no compile-time or run-time warnings before here
+ $DOWARN = 1;
+
+ # run-time warnings enabled after here
+ warn "\$foo is alive and $foo!"; # does show up
+
+See L<perlvar> for details on setting C<%SIG> entries, and for more
+examples. See the Carp module for other kinds of warnings using its
+carp() and cluck() functions.
=item write FILEHANDLE
=item write
-Writes a formatted record (possibly multi-line) to the specified file,
+Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
using the format associated with that file. By default the format for
-a file is the one having the same name is the filehandle, but the
-format for the current output channel (see the select() function) may be set
-explicitly by assigning the name of the format to the $~ variable.
+a file is the one having the same name as the filehandle, but the
+format for the current output channel (see the C<select> function) may be set
+explicitly by assigning the name of the format to the C<$~> variable.
Top of form processing is handled automatically: if there is
insufficient room on the current page for the formatted record, the
is used to format the new page header, and then the record is written.
By default the top-of-page format is the name of the filehandle with
"_TOP" appended, but it may be dynamically set to the format of your
-choice by assigning the name to the $^ variable while the filehandle is
+choice by assigning the name to the C<$^> variable while the filehandle is
selected. The number of lines remaining on the current page is in
-variable $-, which can be set to 0 to force a new page.
+variable C<$->, which can be set to C<0> to force a new page.
If FILEHANDLE is unspecified, output goes to the current default output
channel, which starts out as STDOUT but may be changed by the
is evaluated and the resulting string is used to look up the name of
the FILEHANDLE at run time. For more on formats, see L<perlform>.
-Note that write is I<NOT> the opposite of read. Unfortunately.
+Note that write is I<not> the opposite of C<read>. Unfortunately.
=item y///
-The translation operator. See L<perlop/tr///>.
+The transliteration operator. Same as C<tr///>. See L<perlop>.
=back