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
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
+
+chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
+oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
+sprintf, substr, tr///, uc, ucfirst, y///
+
+=item Regular expressions and pattern matching
+
+m//, pos, quotemeta, s///, split, study
+
+=item Numeric functions
+
+abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
+srand
+
+=item Functions for real @ARRAYs
+
+pop, push, shift, splice, unshift
+
+=item Functions for list data
+
+grep, join, map, qw/STRING/, reverse, sort, unpack
+
+=item Functions for real %HASHes
+
+delete, each, exists, keys, values
+
+=item Input and output functions
+
+binmode, close, closedir, dbmclose, dbmopen, die, eof,
+fileno, flock, format, getc, print, printf, read, readdir,
+rewinddir, seek, seekdir, select, syscall, sysread,
+syswrite, tell, telldir, truncate, warn, write
+
+=item Functions for fixed length data or records
+
+pack, read, syscall, sysread, syswrite, unpack, vec
+
+=item Functions for filehandles, files, or directories
+
+I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
+lstat, mkdir, open, opendir, readlink, rename, rmdir,
+stat, symlink, umask, unlink, utime
+
+=item Keywords related to the control flow of your perl program
+
+caller, continue, die, do, dump, eval, exit, goto, last,
+next, redo, return, sub, wantarray
+
+=item Keywords related to scoping
+
+caller, import, local, my, package, use
+
+=item Miscellaneous functions
+
+defined, dump, eval, formline, local, my, reset, scalar,
+undef, wantarray
+
+=item Functions for processes and process groups
+
+alarm, exec, fork, getpgrp, getppid, getpriority, kill,
+pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
+times, wait, waitpid
+
+=item Keywords related to perl modules
+
+do, import, no, package, require, use
+
+=item Keywords related to classes and object-orientedness
+
+bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
+
+=item Low-level socket functions
+
+accept, bind, connect, getpeername, getsockname,
+getsockopt, listen, recv, send, setsockopt, shutdown,
+socket, socketpair
+
+=item System V interprocess communication functions
+
+msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
+shmctl, shmget, shmread, shmwrite
+
+=item Fetching user and group info
+
+endgrent, endhostent, endnetent, endpwent, getgrent,
+getgrgid, getgrnam, getlogin, getpwent, getpwnam,
+getpwuid, setgrent, setpwent
+
+=item Fetching network info
+
+endprotoent, endservent, gethostbyaddr, gethostbyname,
+gethostent, getnetbyaddr, getnetbyname, getnetent,
+getprotobyname, getprotobynumber, getprotoent,
+getservbyname, getservbyport, getservent, sethostent,
+setnetent, setprotoent, setservent
+
+=item Time-related functions
+
+gmtime, localtime, time, times
+
+=item Functions new in perl5
+
+abs, bless, chomp, chr, exists, formline, glob, import, lc,
+lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
+ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, 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
+
+dbmclose, dbmopen
+
+=back
+
+=head2 Alphabetical Listing of Perl Functions
+
=over 8
=item -X FILEHANDLE
-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
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
=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
or else see L</select()> below. It is not advised 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
=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--otherwise your DOS C library
-may mangle your file. 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 DOS
+and similarly archaic systems, it may be imperative--otherwise your
+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,CLASSNAME
=item bless REF
-This function tells the referenced object (passed as REF) that it is now
+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, since a bless() is often the last thing in a constructor.
+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
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 C<L<eval>>. In such a case additional elements $evaltext and
+$is_require are set: $is_require is true if the frame is created by
+C<L<require>> or C<L<use>> statement, $evaltext contains the text of
+C<L<eval EXPR>> statement. In particular, for C<L<eval BLOCK>>
+statement $filename is C<"(eval)">, but $evaltext is undefined. (Note
+also that C<L<use>> statement creates a C<L<require>> frame inside
+an C<L<eval EXPR>>) frame.
Furthermore, when called from within the DB package, caller returns more
detailed information: it sets the list variable @DB::args to be the
This is a slightly safer version of chop (see below). It removes any
line ending that corresponds to the current value of C<$/> (also known as
-$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
-of characters removed. It's often used to remove the newline from the
-end of an input record when you're worried that the final record may be
-missing its newline. When in paragraph mode (C<$/ = "">), it removes all
-trailing newlines from the string. If VARIABLE is omitted, it chomps
-$_. Example:
+$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
=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.
+If NUMBER is omitted, uses $_.
+
=item chroot FILENAME
+=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 is unaffected.) For security
+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 $_.
Closes the file or pipe associated with the file handle, returning TRUE
only if stdio successfully flushes buffers and closes the system file
descriptor. You don't have to close FILEHANDLE if you are immediately
-going to do another open() on it, since open() will close it for you. (See
+going to do another open() on it, because open() will close it for you. (See
open().) However, an explicit close on an input file resets the line
counter ($.), while the implicit close done by open() does not. Also,
closing a pipe will wait for the process executing on the pipe to
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
print "ok\n";
}
-Of course, typing in your own password to whoever asks you
+Of course, typing in your own password to whomever asks you
for it is unwise.
=item dbmclose ASSOC_ARRAY
[This function has been superseded by the tie() function.]
-This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB 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 any). 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.
-In order versions of Perl,
-if your system had neither DBM nor ndbm, calling dbmopen() produced a
-fatal error; it now falls back to sdbm(3).
+This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB 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 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
associative array variables, not set them. If you want to test whether
}
dbmclose(%HIST);
-See also L<DB_File> for many other interesting possibilities.
+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
+=item defined
+
+Returns a boolean value saying whether EXPR has a real value
+or not. If EXPR is not present, $_ will be checked. 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
See also undef().
+Note: many folks tend to overuse defined(), and then are surprised to
+discover that the number 0 and the null string are, in fact, defined
+concepts. For example, if you say
+
+ "ab" =~ /a(.*)b/;
+
+the pattern match succeeds, and $1 is defined, despite the fact that it
+matched "nothing". But it didn't really match nothing--rather, it
+matched something that happened to be 0 characters long. This is all
+very above-board and honest. When a function returns an undefined value,
+it's an admission that it couldn't give you an honest answer. So
+you should use defined() only when you're questioning the integrity
+of what you're trying to do. At other times, a simple comparison to
+0 or "" is what you want.
+
+Another surprise is that using defined() on an entire array or
+hash reports whether memory for that aggregate has ever been
+allocated. So an array you set to the empty list appears undefined
+initially, and one that once was full and that you then set to
+the empty list still appears defined. You should instead use a
+simple test for size:
+
+ if (@an_array) { print "has array elements\n" }
+ if (%a_hash) { print "has hash members\n" }
+
+Using undef() on these, however, does clear their memory and then report
+them as not defined anymore, but you shoudln't do that unless you don't
+plan to use them again, because it saves time when you load them up
+again to have memory already ready to be filled.
+
+This counter-intuitive behaviour of defined() on aggregates may be
+changed, fixed, or broken in a future release of Perl.
+
=item delete EXPR
-Deletes the specified value from its hash array. Returns the deleted
-value, or the undefined value if nothing was deleted. Deleting from
-C<$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
+array. 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 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.)
The following deletes all the values of an associative array:
- 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; this makes die()
-the way to raise an exception.
+the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
+C<($? E<gt>E<gt> 8)> (back-tick `command` status). If C<($? E<gt>E<gt> 8)>
+is 0, exits with 255. Inside an eval(), the error message is stuffed into
+C<$@>, and the eval() is terminated with the undefined value; this makes
+die() the way to raise an exception.
Equivalent examples:
See also exit() and warn().
+You can arrange for a callback to be called just before the die() does
+its deed, by setting the C<$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> for details on
+setting C<%SIG> entries, and eval() for some examples.
+
=item do BLOCK
Not really a function. Returns the value of the last command in the
current filename for error messages, and searches all the B<-I>
libraries if the file isn't in the current directory (see also the @INC
array in L<perlvar/Predefined Names>). It's the same, however, in that it does
-reparse the file every time you call it, so you probably don't want to
+re-parse 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
=item each ASSOC_ARRAY
-Returns a 2-element array consisting of the key and value for the next
-value of an associative array, so that you can iterate over it.
+When called in a list context, returns a 2-element array consisting
+of the key and value for the next element of an associative array,
+so that you can iterate over it. When called in a scalar context,
+returns the key for only the next element in the associative array.
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
+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. 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
+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:
An C<eof> without an argument uses the last file read as argument.
Empty parentheses () may be used to indicate
-the pseudofile 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
+the pseudo file formed of the files listed on the command line, i.e.,
+C<eof()> is reasonable to use inside a 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 (<>) loop. Examples:
+test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
# reset line numbering on each input file
while (<>) {
}
Practical hint: you almost never need to use C<eof> in Perl, because the
-input operators return undef when they run out of data. Testing C<eof>
+input operators return undef when they run out of data.
=item eval EXPR
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.
+variable settings or subroutine and 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.
+return statement may be used, just as with subroutines. The last
+expression is evaluated in scalar or array context, depending on the
+context of the eval.
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 $_. The final semicolon, if
-any, may be omitted from the expression.
+string. If EXPR is omitted, evaluates C<$_>. The final semicolon, if
+any, may be omitted from the expression. 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 warn() and L<perlvar>.
-Note that, since eval() traps otherwise-fatal errors, it is useful for
+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.
# a run-time error
eval '$answer ='; # sets $@
+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 foofs here" };
+ print $@ if $@; # prints "bar barfs here"
+ }
+
With an eval(), you should be especially careful to remember what's
being looked at when:
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
+likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
is a place where normally you I<WOULD> like to use double quotes, except
-in that particular situation, you can just use symbolic references
+that in that particular situation, you can just use symbolic references
instead, as in case 6.
=item exec LIST
-The exec() function executes a system command I<AND NEVER RETURNS>. Use
-the system() function if you want it to return.
+The exec() function executes a system command I<AND NEVER RETURNS>,
+unless the command does not exist and is executed directly instead of
+via C</bin/sh -c> (see below). Use system() instead of exec() if you
+want it to return.
If there is more than one argument in LIST, or if LIST is an array with
more than one value, calls execvp(3) with the arguments in LIST. If
metacharacters. If there are any, the entire argument is passed to
C</bin/sh -c> for parsing. If there are none, the argument is split
into words and passed directly to execvp(), which is more efficient.
-Note: exec() (and system(0) do not flush your output buffer, so you may
+Note: exec() and system() do not flush your output buffer, so you may
need to set C<$|> to avoid lost output. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
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 $ans =~ /^[Xx]/;
-See also die(). If EXPR is omitted, exits with 0 status.
+See also die(). If EXPR is omitted, exits with 0 status. The only
+univerally portable values for EXPR are 0 for success and 1 for error;
+all other values are subject to unpredictable interpretation depending
+on the environment in which the Perl program is running.
+
+You shouldn't use exit() to abort a subroutine if there's any chance that
+someone might want to trap whatever error happened. Use die() instead,
+which can be trapped by an eval().
=item exp EXPR
+=item exp
+
Returns I<e> (the natural logarithm base) to the power of EXPR.
If EXPR is omitted, gives C<exp($_)>.
=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 either flock(2) or
-fcntl(2). (fcntl(2) will be automatically used if flock(2) is missing.)
+Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
+success, FALSE on failure. Will produce 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 will lock
+only entire files, not records.
+
+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 you pull them in with an explicit
+request to the Fcntl module. The names can be requested as a group with
+the :flock tag (or they can be requested individually, of course).
+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).
+
+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.
- $LOCK_SH = 1;
- $LOCK_EX = 2;
- $LOCK_NB = 4;
- $LOCK_UN = 8;
+ use Fcntl ':flock'; # import LOCK_* constants
sub lock {
- flock(MBOX,$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 $msg,"\n\n";
unlock();
-Note that many versions of flock() cannot lock things over the network.
-You need to do locking with fcntl() for that.
+See also L<DB_File> for other flock() examples.
=item fork
Does a fork(2) system call. Returns the child pid to the parent process
and 0 to the child process, or C<undef> if the fork is unsuccessful.
Note: unflushed buffers remain unflushed in both processes, which means
-you may need to set C<$|> ($AUTOFLUSH in English) or call the
-autoflush() FileHandle method to avoid duplicate output.
+you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
+method of IO::Handle to avoid duplicate output.
If you fork() without ever waiting on your children, you will accumulate
zombies:
}
waitpid($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 with use by the write() function. For
+example:
+
+ format Something =
+ Test: @<<<<<<<< @||||| @>>>>>
+ $str, $%, '$' . int($num)
+ .
+
+ $str = "widget";
+ $num = $cost/$quantity;
+ $~ = 'Something';
+ write;
+
+See L<perlform> for many details and examples.
+
=item formline PICTURE, LIST
You may therefore need to use multiple formlines to implement a single
record format, just like the format compiler.
-Be careful if you put double quotes around the picture, since an "C<@>"
+Be careful if you put double quotes around the picture, because an "C<@>"
character may be taken to mean the beginning of an array name.
formline() always returns TRUE. See L<perlform> for other examples.
Returns the next character from the input file attached to FILEHANDLE,
or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
This is not particularly efficient. It cannot be used to get unbuffered
-single-character
+single-characters, however. For that, try something more like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
- system "stty", '-icanon',
- system "stty", 'eol', "\001";
+ system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
- system "stty", 'icanon';
- system "stty", 'eol', '^@'; # ascii null
+ system "stty", 'icanon', 'eol', '^@'; # ASCII null
}
print "\n";
-Determination of whether to whether $BSD_STYLE should be set
-is left as an exercise to the reader.
+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 authorentication: it is not as
+Do not consider getlogin() for authentication: it is not as
secure as getpwuid().
=item getpeername SOCKET
=item getpgrp PID
-Returns the current process group for the specified PID, 0 for the
+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 glob EXPR
+=item glob
+
Returns the value of EXPR with filename expansions such as a shell
-would do. This is the internal function implementing the <*.*>
+would do. This is the internal function implementing the E<lt>*.*E<gt>
operator, except it's easier to use.
+If EXPR is omitted, $_ is used.
=item gmtime EXPR
Converts a time as returned by the time function to a 9-element array
-with the time localized for the standard Greenwich timezone.
+with the time localized for the standard Greenwich time zone.
Typically used as follows:
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())>.
+In a scalar context, prints out the ctime(3) value:
+
+ $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
+
+Also see the F<timegm.pl> library, and the strftime(3) function available
+via the POSIX module.
+
=item goto LABEL
=item goto EXPR
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).
@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.
=item hex EXPR
+=item hex
+
Interprets EXPR as a hex string and returns the corresponding decimal
value. (To convert strings that might start with 0 or 0x see
oct().) If EXPR is omitted, uses $_.
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 $[
+the string. The return value is based at 0 (or whatever you've set the C<$[>
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
}
To sort an array by value, you'll need to use a C<sort{}>
-function. Here's a descending numeric sort by value:
+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}, $key;
}
+As an lvalue C<keys> allows you to increase the number of hash buckets
+allocated for the given associative array. This can gain you a measure
+of efficiency if you know the hash is going to get big. (This is
+similar to pre-extending an array by assigning a larger number to
+$#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
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 the L<perlipc/"Signals"> man page for details.
+use a signal name in quotes. See L<perlipc/"Signals"> for details.
=item last LABEL
=item lc EXPR
+=item lc
+
Returns an lowercased version of EXPR. This is the internal function
implementing the \L escape in double-quoted strings.
-Should respect any POSIX setlocale() settings.
+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.
-Should respect any POSIX setlocale() settings.
+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 $_.
=item local EXPR
-In general, you should be using "my" instead of "local", because it's
-faster and safer. Format variables often use "local" though, as
-do other variables whose current value must be visible to called
-subroutines. This is known as dynamic scoping. Lexical scoping is
-done with "my", which works more like C's auto declarations.
-
A local modifies the listed variables to be local to the enclosing block,
-subroutine, eval or "do". If more than one value is listed, the list
-must be placed in parens. All the listed elements must be legal
-lvalues. This operator works by saving the current values of those
-variables in LIST on a hidden stack and restoring them upon exiting the
-block, subroutine or eval. This means that called subroutines can also
-reference the local variable, but not the global one. The LIST may be
-assigned to if desired, which allows you to initialize your local
-variables. (If no initializer is given for a particular variable, it
-is created with an undefined value.) Commonly this is used to name the
-parameters to a subroutine. Examples:
-
- sub RANGEVAL {
- local($min, $max, $thunk) = @_;
- local $result = '';
- local $i;
-
- # Presumably $thunk makes reference to $i
-
- for ($i = $min; $i < $max; $i++) {
- $result .= eval $thunk;
- }
-
- $result;
- }
-
-
- if ($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 ($base12) {
- # (NOTE: not claiming this is efficient!)
- local(%digits) = (%digits,'t',10,'e',11);
- parse_num();
- }
-
-Note that local() is a run-time command, and so gets executed every
-time through a loop. In Perl 4 it used more stack storage each
-time until the loop was exited. Perl 5 reclaims the space each time
-through, but it's still more efficient to declare your variables
-outside the loop.
-
-A local is simply a modifier on an lvalue expression.
-When you assign to a localized EXPR, the local doesn't change whether
-EXPR is viewed as a scalar or an array. So
+subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
+list must be placed in parentheses. See L<perlsub/"Temporary Values via
+local()"> for details.
- local($foo) = <STDIN>;
- local @FOO = <STDIN>;
-
-both supply a list context to the righthand side, while
-
- local $foo = <STDIN>;
-
-supplies a scalar context.
+But you really probably want to be using my() instead, because local() isn't
+what most people think of as "local"). See L<perlsub/"Private Variables
+via my()"> for details.
=item localtime EXPR
Converts a time as returned by the time function to a 9-element array
-with the time analyzed for the local timezone. Typically used as
+with the time analyzed for the local time zone. Typically used as
follows:
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
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 and $year is year-1900, that is, $year is 123 in year
+2023. If EXPR is omitted, uses the current time ("localtime(time)").
-In a scalar context, prints out the ctime(3) value:
+In a scalar context, returns 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"
-See also L<perlmod/timelocal> and the strftime(3) function available
-via the POSIX modulie.
+Also see the Time::Local module, and the strftime(3) function available
+via the POSIX module.
=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>.
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 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, C<eval>, or C<do/require/use>'d file. 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. You also cannot use my() on a package variable.
-In particular, you're not allowed to say
-
- my $_; # Illegal!
- my $pack::$var; # 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'">.
-
-Variables declared with "my" are not part of any package and
-are therefore never fully qualified with the package name.
-However, you may declare a "my" variable at the outer most
-scope of a file to totally hide any such identifiers from the
-outside world. This is similar to a C's static variables
-at the file level. To do this with a subroutine requires the
-use of a closure (anonymous function):
-
- my $secret_version = '1.001-beta';
- my $secret_sub = { print $secret_version };
- &$secret_sub();
-
-This does not work with object methods, however;
-all object methods have to be in the symbol table of some
-package to be found.
-
-Just because the "my" variable is lexically scoped doesn't mean that
-within a function it works like a C static. Here's a mechanism for giving
-a function private variables with both lexical scoping and a static
-lifetime.
-
- #!/usr/bin/perl -l
- $var = "global";
- { my $count = 0;
- my $var = "static";
- sub foo {
- $count++;
- print "$var (call # $count)";
- }
- }
- print $var; foo();
- print $var; foo();
- print $var; foo();
+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
=item oct EXPR
+=item oct
+
Interprets EXPR as an octal string and returns the corresponding
decimal value. (If EXPR happens to start off with 0x, interprets it as
a hex string instead.) The following will handle decimal, octal, and
=item open FILEHANDLE
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 See L<perlipc/"Using open() for IPC">
-for more examples of this. as command which pipes input to us. (You may
-not have a command that pipes both in and out, but see See L<open2>,
-L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)
-Opening '-' opens STDIN and opening '>-' opens STDOUT. Open returns
+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.
+(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
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:
+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.
+
+Examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
+ open(DBASE, '+<dbase.mine'); # open for update
+
open(ARTICLE, "caesar <$article |"); # decrypt article
open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
}
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
+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
See L<perlipc/"Safe Pipe Opens"> for more examples of this.
Explicitly closing any piped filehandle causes the parent process to
-wait for the child to finish, and returns the status value in $?.
+wait for the child to finish, and returns the status value in C<$?>.
Note: on any operation which may do a fork, unflushed buffers remain
-unflushed in both processes, which means you may need to set $| to
+unflushed in both processes, which means you may need to set C<$|> to
avoid duplicate output.
+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 $_.
u A uuencoded string.
+ w A BER compressed integer. Bytes give an unsigned integer base
+ 128, most significant digit first, with as few digits as
+ possible, and with the bit 8 of each byte except the last set
+ to "1."
+
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,
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<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
+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
+=item pos
+
Returns the offset of where the last C<m//g> search left off for the variable
-in question. May be modified to change that offset.
+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
+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 LIST
+=item printf FILEHANDLE FORMAT, LIST
-=item printf LIST
+=item printf FORMAT, LIST
-Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
-of the list will be interpreted as the printf format.
+Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. 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>.
+
+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.
=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 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
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 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 recv SOCKET,SCALAR,LEN,FLAGS
=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
=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,
+If EXPR is a bare word, 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
+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 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///
=item scalar EXPR
Forces EXPR to be interpreted in a scalar context and returns the value
-of EXPR.
+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
plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
this from POSIX module. Returns 1 upon success, 0 otherwise.
+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 pointer:
+
+ 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. First the simple trick listed above to clear the
+filepointer. 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
Sets the current position for the readdir() routine on DIRHANDLE. POS
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(2) system 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 = '';
($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
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
=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::sin()
+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()> below.
+
+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/"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.
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
+to be ordered. (The E<lt>=E<gt> 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.
+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;
}
@sortedclass = sort byage @class;
+ # this sorts the %age associative arrays 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
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):
+following equivalences hold (assuming C<$[ == 0>):
push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
pop(@a) splice(@a,-1)
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
(Note that $shell above will still have a newline on it. See L</chop>,
L</chomp>, and L</join>.)
-=item sprintf FORMAT,LIST
+=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
+language. See L<sprintf(3)> or L<printf(3)> on your system for details.
+(The * character for an indirectly specified length is not
supported, but you can get the same effect by interpolating a variable
-into the pattern.) Some C libraries' implementations of sprintf() can dump core
-when fed ludiocrous arguments.
+into the pattern.) 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>.
+Some C libraries' implementations of sprintf() can
+dump core when fed ludicrous arguments.
=item sqrt EXPR
+=item sqrt
+
Return the square root of EXPR. If EXPR is omitted, returns square
root of $_.
=item srand EXPR
-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.
+Sets the random number seed for the C<rand> operator. If EXPR is 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 ^
+($$ + ($$ << 15))>), but that isn't necessary any more.
+
+You need something much more random than the default seed for
+cryptographic purposes, though. 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:
+
+ dev device number of filesystem
+ ino inode number
+ mode file mode (type and permissions)
+ nlink number of (hard) links to the file
+ uid numeric user ID of file's owner
+ gid numeric group ID of file's owner
+ rdev the device identifier (special files only)
+ size total size of file, in bytes
+ atime last access time since the epoch
+ mtime last modify time since the epoch
+ ctime inode change time (NOT creation time!) since the epoch
+ blksize preferred block size for file system I/O
+ 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
+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
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
=item substr EXPR,OFFSET
require 'syscall.ph'; # may need to run h2ph
syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
-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.
+=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
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.
+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 system LIST
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 back-ticks or
+qx//, as described in L<perlop/"`STRING`">.
+
+Because system() and back-ticks 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 coredumps.
+
+ $rc = 0xffff & system @args;
+ printf "system(%s) returned %#04x: ", "@args", $rc;
+ if ($rc == 0) {
+ print "ran with normal exit\n";
+ }
+ elsif ($rc == 0xff00) {
+ print "command failed: $!\n";
+ }
+ elsif ($rc > 0x80) {
+ $rc >>= 8;
+ print "ran with non-zero exit status $rc\n";
+ }
+ else {
+ print "ran with ";
+ if ($rc & 0x80) {
+ $rc &= ~0x80;
+ print "coredump from ";
+ }
+ print "signal $rc\n"
+ }
+ $ok = ($rc != 0);
=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.
+number of bytes actually written, or undef if there was an error.
+If the length is greater than the available data, 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
+from that many bytes counting backwards from the end of the string.
=item tell FILEHANDLE
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
+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
# print out history file offsets
use NDBM_File;
- tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0);
+ tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
for you--you need to do that explicitly yourself. See L<DB_File>
or the F<Config> module for interesting tie() implementations.
+=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 uc EXPR
+=item uc
+
Returns an uppercased version of EXPR. This is the internal function
implementing the \U escape in double-quoted strings.
-Should respect any POSIX setlocale() settings.
+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.
-Should respect any POSIX setlocale() settings.
+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.
+omitted, returns merely the current umask.
=item undef EXPR
=item undef
-Undefines the value of EXPR, which must be an lvalue. Use only on a
+Undefines the value of EXPR, which must be an lvalue. Use on only a
scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
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
=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 bare word.
+
+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 $Module::VERSION. (Note that there is not a
+comma after VERSION!)
Because this is a wide-open interface, pragmas (compiler directives)
are also implemented this way. Currently implemented pragmas are:
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
-These pseudomodules import semantics into the current block scope, unlike
+These pseudo-modules import semantics into the current block scope, unlike
ordinary modules, which import symbols into the current package (which are
effective through the end of the file).
There's a corresponding "no" command that unimports meanings imported
-by use.
+by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
no integer;
no strict 'refs';
+If no unimport method can be found the call fails with a fatal error.
+
See L<perlmod> for a list of standard modules and pragmas.
=item utime LIST
associative array. (In a scalar context, returns the number of
values.) The values are returned in an apparently random order, but it
is the same order as either the keys() or each() function would produce
-on the same array. See also keys() and each().
+on the same array. See also keys(), each(), and sort().
=item vec EXPR,OFFSET,BITS
-Treats a string as a vector of unsigned integers, and returns the value
-of the bitfield specified. May also be assigned to. BITS must be a
-power of two from 1 to 32.
+Treats the string in EXPR as a vector of unsigned integers, and
+returns the value of the bit field specified by OFFSET. BITS specifies
+the number of bits that are reserved for each entry in the bit
+vector. This must be a power of two from 1 to 32. vec() may also be
+assigned to, in which case parentheses are needed to give the expression
+the correct precedence as in
+
+ vec($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
=item warn LIST
-Produces a message on STDERR just like die(), but doesn't exit or
-on an exception.
+Produces a message on STDERR just like die(), but doesn't exit or throw
+an exception.
+
+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
using the format associated with that file. By default the format for
a file is the one having the same name is the filehandle, but the
format for the current output channel (see the select() function) may be set
-explicitly by assigning the name of the format to the $~ variable.
+explicitly by assigning the name of the format to the C<$~> 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 translation operator. See L<perlop>.
=back
projects / p5sagit/p5-mst-13.2.git / blobdiff