+
=head1 NAME
perlfunc - Perl builtin functions
operator. A unary operator generally provides a scalar context to its
argument, while a list operator may provide either scalar and 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
+be first, and the list argument will follow. (Note that there can ever
+be only one list argument.) For instance, splice() has three scalar
arguments followed by a list.
In the syntax descriptions that follow, list operators that expect a
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
+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:
Useless use of integer addition in void context at - line 1.
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:
-=over 5
-
-=item *
+=over 8
-I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
+=item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
=back
operations. In general, they do what you want, unless you want
consistency.
+=head2 Perl Functions by Category
+
+Here are Perl's functions (including things that look like
+functions, like some of the 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>
+
+=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>
+
+=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
+
+=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
-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.
-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
+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
+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
+thus need to do a stat() to determine the actual mode of the
file, or temporarily set the uid to something else.
Example:
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 odd 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.
+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 stat() or 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 $_.
=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>.
+See 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 $_ 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
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
+syscall() interface to access setitimer(2) if your system supports it,
+or else see L</select()>. It is usually a mistake to intermix alarm()
+and sleep() calls.
+
+If you want to use alarm() to time out a system call you need to use an
+eval/die pair. You can't rely on the alarm causing the system call to
+fail with $! set to EINTR because Perl sets up signal handlers to
+restart system calls on some systems. Using eval/die always works.
+
+ eval {
+ local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
+ alarm $timeout;
+ $nread = sysread SOCKET, $buffer, $size;
+ alarm 0;
+ };
+ die if $@ && $@ ne "alarm\n"; # propagate errors
+ if ($@) {
+ # 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 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>.
+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.
+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 MS-DOS
+and similarly archaic systems, it may be imperative--otherwise your
+MS-DOS-damaged C library may mangle your file. The key distinction between
+systems that need binmode and those that don't is their text file
+formats. Systems like Unix and Plan9 that delimit lines with a single
+character, and that encode that character in C as '\n', do not need
+C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
+is taken as the name of the filehandle.
-=item bless REF,PACKAGE
+=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--or the current package if no CLASSNAME
+is specified, which is often the case. It returns the reference for
+convenience, because a bless() is often the last thing in a constructor.
+Always use the two-argument version if the function doing the blessing
+might be inherited by a derived class. See L<perlobj> for more about the
+blessing (and blessings) of objects.
=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 caller's package name if there is a caller, that is, if
+we're in a subroutine or eval() or require(), and the undefined value
+otherwise. In a list context, returns
($package, $filename, $line) = caller;
print a stack trace. The value of EXPR indicates how many call frames
to go back before the current one.
- ($package, $filename, $line,
- $subroutine, $hasargs, $wantargs) = caller($i);
+ ($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 $evaltext and
+$is_require are set: $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 sets the list variable @DB:args to be the
-arguments with which that subroutine was invoked.
+detailed information: it sets the list variable @DB::args to be the
+arguments with which the subroutine was invoked.
=item chdir EXPR
=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 $mode, 'foo'; # this is best
=item chomp VARIABLE
=item chomp
-This is a slightly safer version of chop (see below). It removes any
+This is a slightly safer version of L</chop>. It removes any
line ending that corresponds to the current value of C<$/> (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
-$_. Example:
+$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. If
+VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
$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>);
@ary = <${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.
+
=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 "A" in ASCII. For the reverse, use L</ord>.
+
+If NUMBER is omitted, uses $_.
=item chroot FILENAME
-Does the same as the system call of that name. If you don't know what
-it does, don't worry about it. If FILENAME is omitted, does chroot to
-$_.
+=item chroot
+
+This function works as the system call by the same name: it makes the
+named directory the new root directory for all further pathnames that
+begin with a "/" by your process and all of 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 chroot to $_.
=item close FILEHANDLE
Closes the file or pipe associated with the file handle, returning TRUE
only if stdio successfully flushes buffers and closes the system file
-descriptor. You don't have to close FILEHANDLE if you are immediately
-going to do another open on it, since open will close it for you. (See
+descriptor.
+
+You don't have to close FILEHANDLE if you are immediately going to do
+another open() on it, because open() will close it for you. (See
open().) However, an explicit close on an input file resets the line
-counter ($.), 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<$?>. Example:
+counter ($.), while the implicit close done by 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 $! will be set to 0.) Also, closing a pipe will
+wait for the process executing on the pipe to complete, in case you
+want to look at the output of the pipe afterwards. Closing a pipe
+explicitly also puts the exit status value of the command into C<$?>.
+Example:
- open(OUTPUT, '|sort >foo'); # pipe to sort
+ open(OUTPUT, '|sort >foo') # pipe to sort
+ or die "Can't start sort: $!";
... # print stuff to output
- close OUTPUT; # wait for sort to finish
- open(INPUT, 'foo'); # get sort's results
+ 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 gives the real filehandle 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
-packed address of the appropriate type for the socket. See example in
-L<perlipc>.
+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).
=item cos EXPR
Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
takes cosine of $_.
+For the inverse cosine operation, you may use the 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 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.)
Here's an example that makes sure that whoever runs this program knows
their own password:
die "Sorry...\n";
} else {
print "ok\n";
- }
+ }
-Of course, typing in your own password to whoever asks you
+Of course, typing in your own password to whoever asks you
for it is unwise.
-=item dbmclose ASSOC_ARRAY
+=item dbmclose HASH
[This function has been superseded by the untie() function.]
-Breaks the binding between a DBM file and an associative array.
+Breaks the binding between a DBM file and a hash.
-=item dbmopen ASSOC,DBNAME,MODE
+=item dbmopen HASH,DBNAME,MODE
[This function has been superseded by the tie() function.]
-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.
-
-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 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 MODE (as modified by the umask()). If your system supports
+only the older DBM functions, you may perform only one dbmopen() in your
+program. In older versions of Perl, if your system had neither DBM nor
+ndbm, calling dbmopen() produced a fatal error; it now falls back to
+sdbm(3).
+
+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 eval(),
+which will trap the error.
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()
}
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.
+
=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
+
+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.
-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.
+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 "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: 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 defined() to check whether a subroutine exists. On
+the other hand, use of 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 defined(), and then are surprised to
+discover that the number 0 and "" (the zero-length string) are, in fact,
+defined values. For example, if you say
+
+ "ab" =~ /a(.*)b/;
+
+the pattern match succeeds, and $1 is defined, despite the fact that it
+matched "nothing". But it didn't really match nothing--rather, it
+matched something that happened to be 0 characters long. This is all
+very above-board and honest. When a function returns an undefined value,
+it's an admission that it couldn't give you an honest answer. So you
+should use defined() only when you're questioning the integrity of what
+you're trying to do. At other times, a simple comparison to 0 or "" is
+what you want.
+
+Currently, using defined() on an entire array or hash reports whether
+memory for that aggregate has ever been allocated. So an array you set
+to the empty list appears undefined initially, and one that once was full
+and that you then set to the empty list still appears defined. You
+should instead use a simple test for size:
-See also undef().
+ if (@an_array) { print "has array elements\n" }
+ if (%a_hash) { print "has hash members\n" }
+
+Using undef() on these, however, does clear their memory and then report
+them as not defined anymore, but you shoudln't do that unless you don't
+plan to use them again, because it saves time when you load them up
+again to have memory already ready to be filled.
+
+This counterintuitive behaviour of defined() on aggregates may be
+changed, fixed, or broken in a future release of Perl.
+
+See also L</undef>, L</exists>, L</ref>.
=item delete EXPR
-Deletes the specified value from its hash array. Returns the deleted
-value, or the undefined value if nothing was deleted. Deleting from
-C<$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 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 the undef() command.) 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)> (backtick `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.
+the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
+C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? 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; this makes
+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
See also exit() and warn().
+If LIST is empty and $@ already contains a value (typically from a
+previous eval) that value is reused after appending "\t...propagated".
+This is useful for propagating exceptions:
+
+ eval { ... };
+ die unless $@ =~ /Expected exception/;
+
+If $@ is empty then the string "Died" is used.
+
+You can arrange for a callback to be called just before the die() does
+its deed, by setting the C<$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 die() again. See L<perlvar/$SIG{expr}> for details on
+setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
+
+Note that the C<$SIG{__DIE__}> hook is 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>).
+
=item do BLOCK
Not really a function. Returns the value of the last command in the
is just like
- eval `cat stat.pl`;
+ scalar eval `cat stat.pl`;
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
+array in L<perlvar/Predefined Names>). It is also different in how
+code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
+scope like 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.
Note that inclusion of library modules is better done with the
-use() and require() operators.
+use() and require() operators, which also do error checking
+and raise an exception if there's a problem.
=item dump LABEL
QUICKSTART:
Getopt('f');
-=item each ASSOC_ARRAY
+=item each HASH
+
+When called in a list context, returns a 2-element array consisting of the
+key and value for the next element of a hash, so that you can iterate over
+it. When called in a scalar context, returns the key for only the next
+element in the hash. (Note: Keys may be "0" or "", which are logically
+false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
+for this reason.)
+
+Entries are returned in an apparently random order. When the hash is
+entirely read, a null array is returned in list context (which when
+assigned produces a FALSE (0) value), and C<undef> is returned in a
+scalar context. The next call to each() after that will start iterating
+again. There is a single iterator for each hash, shared by all each(),
+keys(), and values() function calls in the program; it can be reset by
+reading all the elements from the hash, or by evaluating C<keys HASH> or
+C<values HASH>. If you add or delete elements of a hash while you're
+iterating over it, you may get entries skipped or duplicated, so don't.
-Returns a 2 element array consisting of the key and value for the next
-value of an associative array, so that you can iterate over it.
-Entries are returned in an apparently random order. When the array is
-entirely read, a null array is returned (which when assigned produces a
-FALSE (0) value). The next call to each() after that will start
-iterating again. The iterator can be reset only by reading all the
-elements from the array. You should not add elements to an array while
-you're iterating over it. There is a single iterator for each
-associative array, shared by all each(), keys() and values() function
-calls in the program. The following prints out your environment like
-the printenv(1) program, only in a different order:
+The following prints out your environment like the printenv(1) program,
+only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
=item eof FILEHANDLE
+=item eof ()
+
=item eof
Returns 1 if the next read on FILEHANDLE will return end of file, or if
as terminals may lose the end-of-file condition if you do.
An C<eof> without an argument uses the last file read as argument.
-Empty parentheses () may be used to indicate
-the pseudo file formed of the files listed on the command line, i.e.
-C<eof()> is reasonable to use inside a while (<>) loop to detect the end
-of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
-test I<EACH> file in a while (<>) loop. Examples:
+Empty parentheses () may be used to indicate the pseudo file formed of
+the files listed on the command line, i.e., C<eof()> is reasonable to
+use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
+last file. Use C<eof(ARGV)> or eof without the parentheses to test
+I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
# reset line numbering on each input file
while (<>) {
=item eval BLOCK
-EXPR is parsed and executed as if it were a little Perl program. It
-is executed in the context of the current Perl program, so that any
-variable settings, subroutine or format definitions remain afterwards.
-The value returned is the value of the last expression evaluated, or a
-return statement may be used, just as with subroutines.
+In the first form, the return value of EXPR is parsed and executed as if it
+were a little Perl program. The value of the expression (which is itself
+determined within a scalar context) is first parsed, and if there are no
+errors, executed in the context of the current Perl program, so that any
+variable settings or subroutine and format definitions remain afterwards.
+Note that the value is parsed every time the eval executes. If EXPR is
+omitted, evaluates C<$_>. 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, or a return statement may be used, just
+as with subroutines. The expression providing the return value is evaluated
+in void, scalar or array 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 die() statement is
executed, an undefined value is returned by 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 $_. The final semicolon, if
-any, may be omitted from the expression.
+string. Beware that using eval() neither silences perl from printing
+warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
+To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
+L</warn> and L<perlvar>.
-Note that, since eval() traps otherwise-fatal errors, it is useful for
-determining whether a particular feature (such as dbmopen() or symlink())
+Note that, because eval() traps otherwise-fatal errors, it is useful for
+determining whether a particular feature (such as socket() or symlink())
is implemented. It is also Perl's exception trapping mechanism, where
the die operator is used to raise exceptions.
recompiling each time. The error, if any, is still returned in C<$@>.
Examples:
- # make divide-by-zero non-fatal
+ # make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
# a run-time error
eval '$answer ='; # sets $@
-With an eval(), you should be especially careful to remember what's
+When using the 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
+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"
+ }
+
+With an eval(), you should be especially careful to remember what's
being looked at when:
eval $x; # CASE 1
eval "\$$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
-in that 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 '$x', which
+does nothing but return the value of C<$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
-The exec() function executes a system command I<AND NEVER RETURNS>. Use
-the system() function if you want it to return.
+=item exec PROGRAM LIST
+
+The exec() function executes a system command I<AND NEVER RETURNS> -
+use system() instead of 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 system() instead of exec(), Perl
+warns you if there is a following statement which isn't die(), warn()
+or exit() (if C<-w> is set - but you always do that). If you
+I<really> want to follow an 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, 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:
+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 execvp(), which is more efficient. Note: exec() and
+system() do not flush your output buffer, so you may need to set C<$|>
+to avoid lost output. 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.
+
=item exists EXPR
Returns TRUE if the specified hash key exists in its hash array, even
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
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
-See also die(). If EXPR is omitted, exits with 0 status.
+See also die(). If EXPR is omitted, exits with 0 status. The only
+universally portable values for EXPR are 0 for success and 1 for error;
+all other values are subject to unpredictable interpretation depending
+on the environment in which the Perl program is running.
+
+You shouldn't use exit() to abort a subroutine if there's any chance that
+someone might want to trap whatever error happened. Use die() instead,
+which can be trapped by an eval().
=item exp EXPR
-Returns I<e> (the natural logarithm base) to the power of EXPR.
+=item exp
+
+Returns I<e> (the natural logarithm base) to the power of EXPR.
If EXPR is omitted, gives C<exp($_)>.
=item fcntl FILEHANDLE,FUNCTION,SCALAR
=item flock FILEHANDLE,OPERATION
-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.
-
- $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). flock()
+is Perl's portable file locking interface, although it locks only entire
+files, not records.
+
+On many platforms (including most versions or clones of Unix), locks
+established by flock() are B<merely advisory>. This means that files
+locked with flock() may be modified by programs which do not also use
+flock(). Windows NT and OS/2, however, are among the platforms which
+supply mandatory locking. See your local documentation for details.
+
+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 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 mis-coordination, Perl flushes FILEHANDLE
+before (un)locking 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 (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 flock() cannot lock things over the
+network; you would need to use the more system-specific 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.
+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.
+and 0 to the child process, or C<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.
+you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
+method of IO::Handle to avoid duplicate output.
If you fork() without ever waiting on your children, you will accumulate
zombies:
- $SIG{'CHLD'} = sub { wait };
+ $SIG{CHLD} = sub { wait };
-There's also the double-fork trick (error checking on
+There's also the double-fork trick (error checking on
fork() returns omitted);
unless ($pid = fork) {
exec "what you really wanna do";
die "no exec";
# ... or ...
- some_perl_code_here;
+ ## (some_perl_code_here)
exit 0;
}
exit 0;
}
waitpid($pid,0);
+See also L<perlipc> for more examples of forking and reaping
+moribund children.
+
+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, the remote server (such as, say, httpd or rsh) won't think
+you're done. You should reopen those to /dev/null if it's any issue.
+
+=item format
+
+Declare a picture format for use by the write() function. For
+example:
+
+ format Something =
+ Test: @<<<<<<<< @||||| @>>>>>
+ $str, $%, '$' . int($num)
+ .
+
+ $str = "widget";
+ $num = $cost/$quantity;
+ $~ = 'Something';
+ write;
+
+See L<perlform> for many details and examples.
+
-=item formline PICTURE, LIST
+=item formline PICTURE,LIST
-This is an internal function used by formats, though you may call it
+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 $ACCUMULATOR in English).
+Eventually, when a 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. This means
-that the ~ and ~~ tokens will treat the entire PICTURE as a single line.
+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, since an "C<@>"
+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.
-formline() always returns TRUE.
+formline() always returns TRUE. See L<perlform> for other examples.
=item getc FILEHANDLE
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.
+This is not particularly efficient. It cannot be used to get unbuffered
+single-characters, however. 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 POSIX::getattr() function can do this more portably on systems
+alleging POSIX compliance.
+See also the C<Term::ReadKey> module from your nearest CPAN site;
+details on CPAN can be found on L<perlmod/CPAN>.
=item getlogin
Returns the current login from F</etc/utmp>, if any. If null, use
getpwuid().
- $login = getlogin || (getpwuid($<))[0] || "Kilroy";
+ $login = getlogin || getpwuid($<) || "Kilroy";
+
+Do not consider getlogin() for authentication: it is not as
+secure as 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) = unpack_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 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 getpgrp()
+does not accept a PID argument, so only 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
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*
$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
+<pwd.h> file. You can also find out from within Perl which meaning
+your $quota and $comment fields have and whether you have the $expire
+field by using the Config module and the values d_pwquota, d_pwage,
+d_pwchange, d_pwcomment, and d_pwexpire.
+
The $members value returned by I<getgr*()> is a space separated list of
the login names of the members of the group.
Returns the packed sockaddr address of this end of the SOCKET connection.
- # An internet sockaddr
- $sockaddr = 'S n a4 x8';
- $mysockaddr = getsockname(S);
- ($family, $port, $myaddr) =
- unpack($sockaddr,$mysockaddr);
+ use Socket;
+ $mysockaddr = getsockname(SOCK);
+ ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
=item getsockopt SOCKET,LEVEL,OPTNAME
=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
+
+Returns the value of EXPR with filename expansions such as a shell 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, $_ is used.
+The C<E<lt>*.cE<gt>> operator is discussed in more detail in
+L<perlop/"I/O Operators">.
=item gmtime EXPR
Converts a time as returned by the time function to a 9-element array
-with the time localized for the Greenwich timezone. Typically used as
-follows:
-
+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())>.
+the range 0..6 with sunday as day 0. Also, $year is the number of
+years since 1900, I<not> simply the last two digits of the year.
+
+If EXPR is omitted, does C<gmtime(time())>.
+
+In a scalar context, returns the ctime(3) value:
+
+ $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
+
+Also see the timegm() function provided by the Time::Local module,
+and the strftime(3) function available via the POSIX module.
=item goto LABEL
The 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,
+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 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).
=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
@foo = grep {!/^#/} @bar; # weed out comments
-Note that, since $_ is a reference into the list value, it can be used
+Note that, because $_ 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.
+array. Similarly, grep returns aliases into the original list,
+much like the way that a for loops'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.
+
+See also L</map> for an array 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 or 0x
+see L</oct>.) If EXPR is omitted, uses $_.
+
+ print hex '0xAf'; # prints '175'
+ print hex 'aF'; # same
=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
+There is no builtin 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> and L<perlmod>.
+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
+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 C<$[>
+variable to--but don't do that). If the substring is not found, returns
one less than the base, ordinarily -1.
=item int EXPR
+=item int
+
Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
=item ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably 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
+first 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.
+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 which
+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 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';
+ $getp = &TIOCGETP;
+ die "NO TIOCGETP" if $@ || !$getp;
$sgttyb_t = "ccccs"; # 4 chars and a short
- if (ioctl(STDIN,$TIOCGETP,$sgttyb)) {
+ if (ioctl(STDIN,$getp,$sgttyb)) {
@ary = unpack($sgttyb_t,$sgttyb);
$ary[2] = 127;
$sgttyb = pack($sgttyb_t,@ary);
- ioctl(STDIN,$TIOCSETP,$sgttyb)
+ ioctl(STDIN,&TIOCSETP,$sgttyb)
|| die "Can't ioctl: $!";
}
=item join EXPR,LIST
-Joins the separate strings of LIST or ARRAY into a single string with
+Joins the separate strings of LIST into a single string with
fields separated by the value of EXPR, and returns the string.
Example:
See L<perlfunc/split>.
-=item keys ASSOC_ARRAY
+=item keys HASH
+
+Returns a normal array consisting of all the keys of the named hash. (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 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;
print $key, '=', $ENV{$key}, "\n";
}
+To sort an array 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. 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).
+
=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
...
}
=item lc EXPR
+=item lc
+
Returns an lowercased version of EXPR. This is the internal function
implementing the \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 $_.
=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.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
+
+If EXPR is omitted, uses $_.
=item length EXPR
+=item length
+
Returns the length in characters of the value of EXPR. If EXPR is
omitted, returns length of $_.
=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>.
+it succeeded, FALSE otherwise. See 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 often use "local" though, as
-do other variables whose current 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;
- }
+subroutine, C<eval{}>, or C<do>. 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.
- $result;
- }
-
-
- if ($sw eq '-v') {
- # init local array with global array
- local @ARGV = @ARGV;
- unshift(@ARGV,'echo');
- system @ARGV;
- }
- # @ARGV restored
-
-
- # temporarily add to digits associative array
- if ($base12) {
- # (NOTE: not claiming this is efficient!)
- local(%digits) = (%digits,'t',10,'e',11);
- parse_num();
- }
-
-Note that local() is a run-time command, and so gets executed every
-time through a loop. In Perl 4 it used 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.
-
-A local is simply a modifier on an lvalue expression.
-When you assign to a localized EXPR, the local doesn't change whether
-EXPR is viewed as a scalar or an array. So
-
- local($foo) = <STDIN>;
- local @FOO = <STDIN>;
-
-both supply a list context to the righthand side, while
-
- local $foo = <STDIN>;
-
-supplies a scalar context.
+You really probably want to be using my() instead, because local() isn't
+what most people think of as "local". See L<perlsub/"Private Variables
+via my()"> for details.
=item localtime EXPR
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
+with the time analyzed for the local time zone. Typically used as
follows:
+ # 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(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 localtime(time).
+the range 0..6 with sunday as day 0. Also, $year is the number of
+years since 1900, that is, $year is 123 in year 2023.
+
+If EXPR is omitted, uses the current time (C<localtime(time)>).
+
+In a scalar context, returns the ctime(3) value:
-In a scalar context, prints out the ctime(3) value:
+ $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
- $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
+This scalar value is B<not> locale dependent, see L<perllocale>, but
+instead a Perl builtin. Also see the 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
-See also L<perlmod/timelocal> and the strftime(3) function available
-via the POSIX modulie.
+ use POSIX qw(strftime)
+ $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
+
+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.
=item log EXPR
+=item log
+
Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
of $_.
=item lstat EXPR
+=item lstat
+
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.
+If EXPR is omitted, stats $_.
+
=item m//
The match operator. See L<perlop>.
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($_)} = $_;
}
+Note that, because $_ 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. 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,MODE
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).
+it returns 0 and sets C<$!> (errno).
=item msgctl ID,CMD,ARG
-Calls the System V IPC function msgctl. If CMD is &IPC_STAT, then ARG
+Calls the System V IPC function msgctl(2). 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.
=item msgget KEY,FLAGS
-Calls the System V IPC function msgget. Returns the message queue id,
+Calls the System V IPC function msgget(2). Returns the message queue id,
or the undefined value if there is an error.
=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
+which may be created with C<pack("l", $type)>. Returns TRUE if
successful, or FALSE if there is an error.
=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
=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
-
-The "my" is simply a modifier on something you might assign to.
-So when you do 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. But the following only declares one variable:
-
- my $foo, $bar = 1;
-
-That has the same effect as
-
- my $foo;
- $bar = 1;
-
-The declared variable is not introduced (is not visible) until after
-the current statement. Thus,
-
- my $x = $x;
-
-can be used to initialize the new $x with the value of the old $x, and
-the expression
-
- my $x = 123 and $x == 123
-
-is false unless the old $x happened to have the value 123.
-
-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'">.
+enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. 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
...
}
=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 0x, interprets it as
+a hex string instead.) The following will handle decimal, 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 $_. This function is commonly used when
+a string such as "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.)
+
+If the filename begins with '<' or nothing, the file is opened for input.
+If the filename begins with '>', the file is truncated and 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; thus '+<' is almost
+always preferred for read/write updates--the '+>' mode would clobber the
+file first. The prefix and the filename may be separated with spaces.
+These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
+'w+', 'a', and 'a+'.
+
+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 See L<perlipc/"Using open() for IPC"> for more
+examples of this. as command which pipes input to us. (You may not have
+a raw 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 '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
+nonzero upon success, the undefined value otherwise. If the 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 binmode
+and those that don't is their text file formats. Systems like Unix and
+Plan9 that delimit lines with a single character, and that encode that
+character in C as '\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 which 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
}
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
+with "E<gt>&", 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
+duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
++E<gt>E<gt>, and +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:
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 "E<lt>&=N", where N is a number, then Perl will do an
+equivalent of C's 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 "-", i.e., either "|-" or "-|", 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.)
+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
+See L<perlipc/"Safe Pipe Opens"> for more examples of this.
+
+NOTE: On any operation which may do a fork, unflushed buffers remain
+unflushed in both processes, which means you may need to set C<$|> to
avoid duplicate output.
+Closing any piped filehandle causes the parent process to wait for the
+child to finish, and returns the status value in C<$?>.
+
+Using the constructor from the IO::Handle package (or one of its
+subclasses, such as IO::File or IO::Socket),
+you can generate anonymous filehandles which 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.
+ }
+
The filename that is passed to open will have leading and trailing
-whitespace deleted. In order to open a file with arbitrary weird
+whitespace deleted. To open a file with arbitrary weird
characters in it, it's necessary to protect any leading and trailing
whitespace thusly:
- $file =~ s#^(\s)#./$1#;
- open(FOO, "< $file\0");
+ $file =~ s#^(\s)#./$1#;
+ open(FOO, "< $file\0");
+
+If you want a "real" C open() (see L<open(2)> on your system), then
+you should use the sysopen() function. This is another way to
+protect your filenames from interpretation. For example:
+
+ use IO::Handle;
+ sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
+ or die "sysopen $path: $!";
+ HANDLE->autoflush(1);
+ HANDLE->print("stuff $$\n");
+ seek(HANDLE, 0, 0);
+ print "File contains: ", <HANDLE>;
+
+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.
+seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.
=item ord EXPR
+=item ord
+
Returns the numeric ascii value of the first character of EXPR. If
-EXPR is omitted, uses $_.
+EXPR is omitted, uses $_. For the reverse, see L</chr>.
=item pack TEMPLATE,LIST
c A signed char value.
C An unsigned char value.
+
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.)
f A single-precision float in the native format.
d A double-precision float in the native format.
u A uuencoded string.
+ 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
+count. With all types except "a", "A", "b", "B", "h", "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
+string that many nybbles long. The "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 "P" packs a pointer to a structure
+of the size indicated by the length. A NULL pointer is created if the
+corresponding value for "p" or "P" is C<undef>.
+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
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.
+float and thence back to double again will lose precision (i.e.,
C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
Examples:
The same template may generally also be used in the unpack function.
+=item package NAMESPACE
+
+Declares the compilation unit as being in the given namespace. The scope
+of the package declaration is from the declaration itself through the end of
+the enclosing block (the same scope as the local() operator). All further
+unqualified dynamic identifiers will be in this namespace. A package
+statement affects only dynamic variables--including those you've used
+local() on--but I<not> lexical variables created with my(). Typically it
+would be the first declaration in a file to be included by the C<require>
+or C<use> operator. You can switch into a package in more than one place;
+it influences merely which symbol table is used by the compiler for the
+rest of that block. You can refer to variables and filehandles in other
+packages by prefixing the identifier with the package name and a double
+colon: C<$Package::Variable>. If the package name is null, the C<main>
+package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
+
+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.
+
=item pop ARRAY
+=item pop
+
Pops and returns the last value of the array, shortening the array by
1. Has a similar effect to
$tmp = $ARRAY[$#ARRAY--];
If there are no elements in the array, returns the undefined value.
+If ARRAY is omitted, pops the
+@ARGV array in the main program, and the @_ array in subroutines, just
+like 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 ($_ 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
+Prints a string or a comma-separated list of strings. Returns TRUE
if successful. FILEHANDLE may be a scalar variable name, in which case
-the variable contains the name of the filehandle, thus introducing one
+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 + or put parens around the arguments.) If FILEHANDLE is
+interpose a + or put parentheses 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
+output channel--see L</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
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.
+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 FILEHANDLE LIST
+=item printf FORMAT, LIST
-=item printf LIST
+Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that $\
+(the output record separator) is not appended. The first argument
+of the list will be interpreted as the 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>.
-Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
-of the list will be interpreted as the printf format.
+Don't fall into the trap of using a printf() when a simple
+print() would do. The 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 builtin is not I<overridable> (such as
+C<qw>) or its arguments cannot be expressed by a prototype (such as
+C<system>) - in other words, the builtin does not behave like a Perl
+function - returns C<undef>. Otherwise, the string describing the
+equivalent prototype is returned.
=item push ARRAY,LIST
=item quotemeta EXPR
-Returns the value of EXPR with with all regular expression
-metacharacters backslashed. This is the internal function implementing
+=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 \Q escape in double-quoted strings.
+If EXPR is omitted, uses $_.
+
=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 0 and less
+than the value of EXPR. (EXPR should be positive.) If EXPR is
+omitted, the value 1 is used. Automatically calls srand() unless
+srand() has already been called. See also 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
directory. If there are no more entries, returns an undefined value in
a scalar context or a null list in a list context.
+If you're planning to filetest the return values out of a readdir(), you'd
+better prepend the directory in question. Otherwise, because we didn't
+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 file handle EXPR. In scalar context, a single line
+is read and returned. In list context, reads until end-of-file is
+reached and returns a list of lines (however you've defined lines
+with $/ or $INPUT_RECORD_SEPARATOR).
+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">.
+
=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 $! (errno). If EXPR is
+error, returns the undefined value and sets C<$!> (errno). If EXPR is
omitted, uses $_.
+=item readpipe EXPR
+
+EXPR is interpolated and then 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 $/ or $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,LEN,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 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|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front{|;
- redo line;
+ redo LINE;
}
}
}
=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, $_ will be used. The value returned depends on the
+type of thing the reference is a reference to.
Builtin types include:
REF
CODE
GLOB
-If the referenced object has been blessed into a package, then that package
+If the referenced object has been blessed into a package, then that package
name is returned instead. You can think of ref() as a typeof() operator.
if (ref($r) eq "HASH") {
- print "r is a reference to an associative array.\n";
- }
+ print "r is a reference to a hash.\n";
+ }
if (!ref ($r) {
print "r is not a reference at all.\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.
+not work across file system boundaries.
=item require EXPR
Demands some semantics specified by EXPR, or by $_ if EXPR is not
supplied. If EXPR is numeric, demands that the current version of Perl
-($] 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
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()> and
-L<perlmod>.
+In other words, if you try this:
+
+ require Foo::Bar ; # a splendid bareword
+
+The require function will actually look for the "Foo/Bar.pm" file in the
+directories specified in the @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 "Foo::Bar" file in the @INC array and
+will complain about not finding "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
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 (?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
-Resetting "A-Z" is not recommended since you'll wipe out your
-ARGV and ENV arrays. Only resets package variables--lexical variables
+Resetting "A-Z" is not recommended because you'll wipe out your
+ARGV and ENV arrays. Resets only 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>.
+so you'll probably want to use them instead. See L</my>.
-=item return LIST
+=item return EXPR
-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.)
+=item return
+
+Returns from a subroutine, eval(), or do FILE with the value of the
+given EXPR. Evaluation of EXPR may be in a 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 wantarray()). If no EXPR
+is given, returns an empty list in a list context, an undefined value in
+a scalar context, or nothing in a void context.
+
+(Note that in the absence of a 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.
+of LIST in the opposite order. In a scalar context, concatenates the
+elements of LIST, and returns a string value consisting of those bytes,
+but in the opposite order.
+
+ print reverse <>; # line tac, last line first
+
+ undef $/; # for efficiency of <>
+ print scalar reverse <>; # byte 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.
+
+ %by_name = reverse %by_address; # Invert the hash
=item rewinddir DIRHANDLE
=item rmdir FILENAME
+=item rmdir
+
Deletes the directory specified by FILENAME if it is empty. If it
-succeeds it returns 1, otherwise it returns 0 and sets $! (errno). If
+succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
FILENAME is omitted, uses $_.
=item s///
Forces EXPR to be interpreted in a 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 a list context because it's in practice 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.
+
=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 using the POSIX module. Returns 1 upon success, 0 otherwise.
+Sets FILEHANDLE's position, 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 new position to
+POSITION, 1 to set it to the current position plus POSITION, and 2 to
+set it to EOF plus POSITION (typically negative). For WHENCE you may
+use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
+IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
+
+If you want to position file for sysread() or syswrite(), don't use
+seek() -- buffering makes its effect on the file's system position
+unpredictable and non-portable. Use sysseek() instead.
+
+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 1 (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 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
$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
+This calls the select(2) system call with the bit masks specified, which
can be constructed using fileno() and vec(), along these lines:
$rin = $win = $ein = '';
}
$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 a 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
$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>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
+with select(). You have to use sysread() instead.
=item semctl ID,SEMNUM,CMD,ARG
destination to send TO, in which case it does a C sendto(). Returns
the number of characters sent, or the undefined value if there is an
error.
+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
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
+0,0. Note that the POSIX version of setpgrp() does not accept any
+arguments, so only setpgrp 0,0 is portable.
=item setpriority WHICH,WHO,PRIORITY
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.
+@_ array within the lexical scope of subroutines and formats, and the
+@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 unshift(), push(), and pop(). Shift() and unshift() do the
+same thing to the left end of an array that pop() and push() do to the
+right end.
=item shmctl ID,CMD,ARG
=item sin EXPR
+=item sin
+
Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
returns sine of $_.
+For the inverse sine operation, you may use the POSIX::asin()
+function, or use this relation:
+
+ sub asin { atan2($_[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().
+sleep() calls, because sleep() is often implemented using 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.
+For delays 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()> above.
+
+See also the POSIX module's 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
+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>.
+the proper definitions imported. See the example 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.
=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
-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.
+Sorts the LIST and returns the sorted list value. 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 C<E<lt>=E<gt>> and C<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 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 the package global variables $a and
+$b (see example below). They are passed by reference, so don't
+modify $a and $b. And don't try to declare them as lexicals either.
+
+You also cannot exit out of the sort block or subroutine using any of the
+loop control operators described in L<perlsyn> or with goto().
+
+When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
+current collation locale. See L<perllocale>.
Examples:
# same thing, but with explicit sort routine
@articles = sort {$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 using explicit subroutine name
sub byage {
- $age{$a} <=> $age{$b}; # presuming integers
+ $age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
+ # this sorts the %age hash by value instead of key
+ # using an in-line function
+ @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
+
sub backwards { $b cmp $a; }
@harry = ('dog','cat','x','Cain','Abel');
@george = ('gone','chased','yz','Punished','Axed');
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
+ # inefficiently sort by descending numeric compare using
+ # the first integer after the first = sign, or the
+ # whole record case-insensitively otherwise
+
+ @new = sort {
+ ($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 using a Schwartzian Transform (no 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 $x[1] is less than $x[2] and
+sometimes saying the opposite, for example) the Perl interpreter will
+probably crash and dump core. This is entirely due to and dependent
+upon your system's qsort(3) library routine; this routine often avoids
+sanity checks in the interest of speed.
+
=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 $[ == 0):
+replaces them with the elements of LIST, if any. In a list context,
+returns the elements removed from the array. In a scalar context,
+returns the last element removed, or C<undef> if no elements are
+removed. The array grows or shrinks as necessary. If LENGTH is
+omitted, removes everything from OFFSET onward. The following
+equivalences hold (assuming C<$[ == 0>):
push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
pop(@a) splice(@a,-1)
value.) The use of implicit split to @_ is deprecated, however.
If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
-splits on whitespace (after skipping any leading whitespace).
-Anything matching PATTERN is taken
-to be a delimiter separating the fields. (Note that the delimiter may
-be longer than one character.) If LIMIT is specified and is not
-negative, splits into no more than that many fields (though it may
-split into fewer). If LIMIT is unspecified, trailing null fields are
-stripped (which potential users of pop() would do well to remember).
-If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
-had been specified.
+splits on whitespace (after skipping any leading whitespace). Anything
+matching PATTERN is taken to be a delimiter separating the fields. (Note
+that the delimiter may be longer than one character.)
+
+If LIMIT is specified and is positive, splits into no more than that
+many fields (though it may split into fewer). If LIMIT is unspecified
+or zero, trailing null fields are stripped (which potential users
+of pop() would do well to remember). If LIMIT is negative, it is
+treated as if an arbitrarily large LIMIT had been specified.
A pattern matching the null string (not to be confused with
a null pattern C<//>, which is just one member of the set of patterns
produces the output 'h:i:t:h:e:r:e'.
-The LIMIT parameter can be used to partially split a line
+The LIMIT parameter can be used to split a line partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
If the PATTERN contains parentheses, additional array 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>.)
open(passwd, '/etc/passwd');
while (<passwd>) {
- ($login, $passwd, $uid, $gid, $gcos,
+ ($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 printf conventions of the
+C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
+on your system for an explanation of the general principles.
+
+Perl does all of its own sprintf() formatting -- it emulates the C
+function 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 sprintf() are not
+available from Perl.
+
+Perl's 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)
+ %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 ("*") 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 "*" is negative, it has the same
+effect as the '-' 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 $_.
=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 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 time(). This isn't a particularly good seed,
+so many old programs supply their own seed value (often C<time ^ $$> or
+C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
+
+In fact, it's usually not necessary to call srand() at all, because if
+it is not called explicitly, it is called implicitly at the first use of
+the C<rand> operator. However, this was not the case in version of Perl
+before 5.004, so if your script will run under older Perl versions, it
+should call srand().
+
+Note that you need something much more random than the default seed for
+cryptographic purposes. Checksumming the compressed output of one or more
+rapidly changing operating system status programs is the usual method. For
+example:
+
+ srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
+
+If you're particularly concerned with this, see the Math::TrulyRandom
+module in CPAN.
+
+Do I<not> call 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 rand() function so that 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 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
+=item stat
+
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:
+file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
+stats $_. Returns a null list if the stat fails. Typically used as
+follows:
+
($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 "$file is executable NFS file\n";
}
-(This only works on machines for which the device number is negative under NFS.)
+(This works on machines only for which the device number is negative under NFS.)
=item study SCALAR
=item study
-Takes extra time to study SCALAR ($_ 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
+frequencies in the string to be searched -- you probably want to compare
+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
+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
character in the string to be searched is made, so we know, for
example, where all the 'k' characters are. From each search string,
avoid recompiling all your patterns all the time. Together with
undefining $/ 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 $[ to. If OFFSET is negative, starts
+offset 0, or whatever you've set C<$[> 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. If LEN is negative, leaves that
many characters off the end of the string.
+If you specify a substring which 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 be an lvalue. If you assign
something shorter than LEN, the string will shrink, and if you assign
keep the string the same length you may need to pad or chop your value
using 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.
+In this case LEN can be C<undef> if you want to affect everything to
+the end of the string.
+
=item symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the old filename.
symbolic links, produces a fatal error at run time. To check for that,
use eval:
- $symlink_exists = (eval 'symlink("","");', $@ eq '');
+ $symlink_exists = (eval {symlink("","")};, $@ eq '');
=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 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.
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 $s);
-Note that Perl only supports passing of up to 14 arguments to your system call,
+Note that Perl supports passing of up to only 14 arguments to your system call,
which in practice should usually suffice.
+Syscall returns whatever value returned by the system call it calls.
+If the system call fails, syscall returns -1 and sets C<$!> (errno).
+Note that some system calls can legitimately return -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 -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>.
+However, for historical reasons, some values are universal: zero means
+read-only, one means write-only, and two means read/write.
+
+If the file named by FILENAME does not exist and the C<open> call
+creates it (typically because MODE includes the O_CREAT flag), then
+the value of PERMS specifies the permissions of the newly created
+file. If PERMS is omitted, the default value is 0666, which allows
+read and write for all. This default is reasonable: see C<umask>.
+
+The IO::File module provides a more object-oriented approach, if you're
+into that kind of thing.
+
=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.
+stdio, so mixing this with other kinds of reads, print(), write(),
+seek(), or tell() can cause confusion because stdio usually buffers
+data. Returns the number of bytes actually read, 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 "\0" bytes before
+the result of the read is appended.
+
+=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 sysread()),
+print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may
+be an expression whose value gives the name of the filehandle. The
+values for WHENCE are 0 to set the new position to POSITION, 1 to set
+the it to the current position plus POSITION, and 2 to set it to EOF
+plus POSITION (typically negative). For WHENCE, you may use the
+constants SEEK_SET, SEEK_CUR, and SEEK_END from either the 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 "0 but true"; thus sysseek() returns
+TRUE on success and FALSE on failure, yet you can still easily determine
+the new position.
=item system LIST
+=item system PROGRAM 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>.
+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
+qx//, as described in L<perlop/"`STRING`">.
+
+Like exec(), system() allows you to lie to a program about its name if
+you use the "system PROGRAM LIST" syntax. Again, see L</exec>.
+
+Because system() and backticks block SIGINT and 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: $?"
+
+Here's a more elaborate example of analysing the return value from
+system() on a Unix system to check for all possibilities, including for
+signals and core dumps.
+
+ $! = 0;
+ $rc = system @args;
+ printf "system(%s) returned %#04x: ", "@args", $rc;
+ if ($rc == 0) {
+ print "ran with normal exit\n";
+ }
+ elsif ($rc == 0xff00) {
+ # Note that $! can be an empty string if the command that
+ # system() tried to execute was not found, not executable, etc.
+ # These errors occur in the child process after system() has
+ # forked, so the errno value is not visible in the parent.
+ printf "command failed: %s\n", ($! || "Unknown system() error");
+ }
+ elsif (($rc & 0xff) == 0) {
+ $rc >>= 8;
+ print "ran with non-zero exit status $rc\n";
+ }
+ else {
+ print "ran with ";
+ if ($rc & 0x80) {
+ $rc &= ~0x80;
+ print "core dump from ";
+ }
+ print "signal $rc\n"
+ }
+ $ok = ($rc != 0);
+
+When the arguments get executed via the system shell, results will
+be subject to its quirks and capabilities. See L<perlop/"`STRING`">
+for details.
=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
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.
+stdio, so mixing this with reads (other than sysread()), print(),
+write(), seek(), or tell() may cause confusion because stdio usually
+buffers data. Returns the number of bytes actually written, or 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.
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
-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 (meaning TIESCALAR, TIEARRAY, or TIEHASH). Typically these
-are arguments such as might be passed to the dbm_open() function of C.
+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. CLASSNAME is the name of a class implementing objects
+of correct type. Any additional arguments are passed to the "new"
+method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
+Typically these are arguments such as might be passed to the dbm_open()
+function of C. The object returned by the "new" method is also
+returned by the tie() function, which would be useful if you want to
+access other methods in CLASSNAME.
Note that functions such as keys() and values() may return huge array
values when used on large objects, like DBM files. You may prefer to
use the 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
+ TIEHASH classname, LIST
DESTROY this
FETCH this, key
STORE this, key, value
FIRSTKEY this
NEXTKEY this, lastkey
-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
+ TIEARRAY classname, LIST
DESTROY this
FETCH this, key
STORE this, key, value
[others TBD]
-A package implementing a scalar should have the following methods:
+A class implementing a scalar should have the following methods:
- TIESCALAR objectname, LIST
+ TIESCALAR classname, LIST
DESTROY this
- FETCH this,
+ FETCH this,
STORE this, value
+Unlike dbmopen(), the 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 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 tie() call which 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 gmtime() and localtime().
=item times
=item tr///
-The translation operator. See L<perlop>.
+The transliteration operator. Same as 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.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
+
+If EXPR is omitted, uses $_.
=item ucfirst EXPR
+=item ucfirst
+
Returns the value of EXPR with the first character uppercased. This is
the internal function implementing the \u escape in double-quoted strings.
+Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
+
+If EXPR is omitted, uses $_.
=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. If umask(2) is
+not implemented on your system, 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 "@"), a hash (using "%"), a subroutine
+(using "&"), 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
=item unlink LIST
+=item unlink
+
Deletes a list of files. Returns the number of files successfully
deleted.
met, be warned that unlinking a directory can inflict damage on your
filesystem. Use rmdir instead.
+If LIST is omitted, uses $_.
+
=item unpack TEMPLATE,EXPR
Unpack does the reverse of pack: it takes a string representing a
structure and expands it out into a list value, returning the array
-value. (In a scalar context, it merely returns the first value
+value. (In a scalar context, it returns merely the first value
produced.) The TEMPLATE has the same format as in the pack function.
Here's a subroutine that does substring:
sub ordinal { unpack("c",$_[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:
=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 which 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
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.
+is defined in the Exporter module. See L<Exporter>. If no 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 $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
+These pseudo-modules 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).
There's a corresponding "no" command that unimports meanings imported
-by use.
+by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
no integer;
no strict 'refs';
+If no 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 "touch"
+command if the files already exist:
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
-=item values ASSOC_ARRAY
+=item values HASH
-Returns a normal array consisting of all the values of the named
-associative array. (In a scalar context, returns the number of
-values.) The values are returned in an apparently random order, but it
-is the same order as either the keys() or each() function would produce
-on the same array. See also keys() and each().
+Returns a normal array consisting of all the values of the named hash.
+(In a scalar context, returns the number of values.) The values are
+returned in an apparently random order, but it is the same order as either
+the keys() or each() function would produce on the same hash. As a side
+effect, it resets HASH's iterator. See also keys(), each(), and sort().
=item vec EXPR,OFFSET,BITS
-Treats a string as a vector of unsigned integers, and returns the value
-of the bitfield specified. May also be assigned to. BITS must be a
-power of two from 1 to 32.
+Treats the string in EXPR as a vector of unsigned integers, and
+returns the value of the bit field specified by OFFSET. BITS specifies
+the number of bits that are reserved for each entry in the bit
+vector. This must be a power of two from 1 to 32. vec() may also be
+assigned to, in which case parentheses are needed to give the expression
+the correct precedence as in
+
+ vec($image, $max_x * $x + $y, 8) = 3;
Vectors created with vec() can also be manipulated with the logical
-operators |, & and ^, which will assume a bit vector operation is
+operators |, &, and ^, which will assume a bit vector operation is
desired when both operands are strings.
To transform a bit vector into a string or array of 0's and 1's, use these:
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 $?.
+returned in C<$?>.
=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 $?. If you say
+status is returned in C<$?>. If you say
- use POSIX "wait_h";
+ use POSIX ":sys_wait_h";
...
waitpid(-1,&WNOHANG);
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
+is 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
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.
+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";
=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 die(), but doesn't exit or throw
+an exception.
+
+If LIST is empty and $@ already contains a value (typically from a
+previous eval) that value is used after appending "\t...caught"
+to $@. This is useful for staying almost, but not entirely similar to
+die().
+
+If $@ is empty then the string "Warning: Something's wrong" is used.
+
+No message is printed if there is a C<$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 die()). Most
+handlers must therefore make arrangements to actually display the
+warnings that they are not prepared to deal with, by calling 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 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.
=item write FILEHANDLE
Writes a formatted record (possibly multi-line) to the specified file,
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
+a file is the one having the same name as 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 $~ variable.
+explicitly by assigning the name of the format to the C<$~> 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 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
=item y///
-The translation operator. See L<perlop/tr///>.
+The transliteration operator. Same as tr///. See L<perlop>.
=back
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