[perl #81290] assertion failure with lock &lvsub

[perl5.git] / pod / perlfunc.pod

=head1 NAME
X<function>

perlfunc - Perl builtin functions

=head1 DESCRIPTION

The functions in this section can serve as terms in an expression.
They fall into two major categories: list operators and named unary
operators.  These differ in their precedence relationship with a
following comma.  (See the precedence table in L<perlop>.)  List
operators take more than one argument, while unary operators can never
take more than one argument.  Thus, a comma terminates the argument of
a unary operator, but merely separates the arguments of a list
operator.  A unary operator generally provides scalar context to its
argument, while a list operator may provide either scalar or list
contexts for its arguments.  If it does both, scalar arguments 
come first and list argument follow, and there can only ever
be one such list argument.  For instance, splice() has three scalar
arguments followed by a list, whereas gethostbyname() has four scalar
arguments.

In the syntax descriptions that follow, list operators that expect a
list (and provide list context for elements of the list) are shown
with LIST as an argument.  Such a list may consist of any combination
of scalar arguments or list values; the list values will be included
in the list as if each individual element were interpolated at that
point in the list, forming a longer single-dimensional list value.
Commas should separate literal elements of the LIST.

Any function in the list below may be used either with or without
parentheses around its arguments.  (The syntax descriptions omit the
parentheses.)  If you use 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.  Whitespace
between the function and left parenthesis doesn't count, so sometimes
you need to be careful:

    print 1+2+4;      # Prints 7.
    print(1+2) + 4;   # Prints 3.
    print (1+2)+4;    # Also prints 3!
    print +(1+2)+4;   # Prints 7.
    print ((1+2)+4);  # Prints 7.

If you run Perl with the B<-w> switch it can warn you about this.  For
example, the third line above produces:

    print (...) interpreted as function at - line 1.
    Useless use of integer addition in void context at - line 1.

A few functions take no arguments at all, and therefore work as neither
unary nor list operators.  These include such functions as C<time>
and C<endpwent>.  For example, C<time+86_400> always means
C<time() + 86_400>.

For functions that can be used in either a scalar or list context,
nonabortive failure is generally indicated in scalar context by
returning the undefined value, and in list context by returning the
empty list.

Remember the following important rule: There is B<no rule> that relates
the behavior of an expression in list context to its behavior in scalar
context, or vice versa.  It might do two totally different things.
Each operator and function decides which sort of value would be most
appropriate to return in scalar context.  Some operators return the
length of the list that would have been returned in list context.  Some
operators return the first value in the list.  Some operators return the
last value in the list.  Some operators return a count of successful
operations.  In general, they do what you want, unless you want
consistency.
X<context>

A named array in scalar context is quite different from what would at
first glance appear to be a list in scalar context.  You can't get a list
like C<(1,2,3)> into being in scalar context, because the compiler knows
the context at compile time.  It would generate the scalar comma operator
there, not the list construction version of the comma.  That means it
was never a list to start with.

In general, functions in Perl that serve as wrappers for system calls ("syscalls")
of the same name (like chown(2), fork(2), closedir(2), etc.) return
true when they succeed and C<undef> otherwise, as is usually mentioned
in the descriptions below.  This is different from the C interfaces,
which return C<-1> on failure.  Exceptions to this rule include C<wait>,
C<waitpid>, and C<syscall>.  System calls also set the special C<$!>
variable on failure.  Other functions do not, except accidentally.

Extension modules can also hook into the Perl parser to define new
kinds of keyword-headed expression.  These may look like functions, but
may also look completely different.  The syntax following the keyword
is defined entirely by the extension.  If you are an implementor, see
L<perlapi/PL_keyword_plugin> for the mechanism.  If you are using such
a module, see the module's documentation for details of the syntax that
it defines.

=head2 Perl Functions by Category
X<function>

Here are Perl's functions (including things that look like
functions, like some keywords and named operators)
arranged by category.  Some functions appear in more
than one place.

=over 4

=item Functions for SCALARs or strings
X<scalar> X<string> X<character>

C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>

=item Regular expressions and pattern matching
X<regular expression> X<regex> X<regexp>

C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>

=item Numeric functions
X<numeric> X<number> X<trigonometric> X<trigonometry>

C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
C<sin>, C<sqrt>, C<srand>

=item Functions for real @ARRAYs
X<array>

C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>

=item Functions for list data
X<list>

C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>

=item Functions for real %HASHes
X<hash>

C<delete>, C<each>, C<exists>, C<keys>, C<values>

=item Input and output functions
X<I/O> X<input> X<output> X<dbm>

C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
C<warn>, C<write>

=item Functions for fixed-length data or records

C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>

=item Functions for filehandles, files, or directories
X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>

C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
C<umask>, C<unlink>, C<utime>

=item Keywords related to the control flow of your Perl program
X<control flow>

C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>

=item Keywords related to the switch feature

C<break>, C<continue>, C<default>, C<given>, C<when>

Except for C<continue>, these are available only if you enable the
C<"switch"> feature or use the C<CORE::> prefix.
See L<feature> and L<perlsyn/"Switch statements">.  
Alternately, include a C<use v5.10> or later to the current scope.  In Perl
5.14 and earlier, C<continue> required the C<"switch"> feature, like the
other keywords.

=item Keywords related to scoping

C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>

C<state> is available only if the C<"state"> feature
is enabled or if it is prefixed with C<CORE::>. See
L<feature>.  Alternately, include a C<use v5.10> or later to the current scope.

=item Miscellaneous functions

C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>

=item Functions for processes and process groups
X<process> X<pid> X<process id>

C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
C<times>, C<wait>, C<waitpid>

=item Keywords related to Perl modules
X<module>

C<do>, C<import>, C<no>, C<package>, C<require>, C<use>

=item Keywords related to classes and object-orientation
X<object> X<class> X<package>

C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
C<untie>, C<use>

=item Low-level socket functions
X<socket> X<sock>

C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
C<socket>, C<socketpair>

=item System V interprocess communication functions
X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>

C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>

=item Fetching user and group info
X<user> X<group> X<password> X<uid> X<gid>  X<passwd> X</etc/passwd>

C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
C<getpwuid>, C<setgrent>, C<setpwent>

=item Fetching network info
X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>

C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
C<setnetent>, C<setprotoent>, C<setservent>

=item Time-related functions
X<time> X<date>

C<gmtime>, C<localtime>, C<time>, C<times>

=item Functions new in perl5
X<perl5>

C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>, 
C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
C<ucfirst>, C<untie>, C<use>, C<when>

* C<sub> was a keyword in Perl 4, but in Perl 5 it is an
operator, which can be used in expressions.

=item Functions obsoleted in perl5

C<dbmclose>, C<dbmopen>

=back

=head2 Portability
X<portability> X<Unix> X<portable>

Perl was born in Unix and can therefore access all common Unix
system calls.  In non-Unix environments, the functionality of some
Unix system calls may not be available or details of the available
functionality may differ slightly.  The Perl functions affected
by this are:

C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
C<shmwrite>, C<socket>, C<socketpair>,
C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
C<times>, C<truncate>, C<umask>, C<unlink>,
C<utime>, C<wait>, C<waitpid>

For more information about the portability of these functions, see
L<perlport> and other available platform-specific documentation.

=head2 Alphabetical Listing of Perl Functions

=over 

=item -X FILEHANDLE
X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>

=item -X EXPR

=item -X DIRHANDLE

=item -X

A file test, where X is one of the letters listed below.  This unary
operator takes one argument, either a filename, a filehandle, or a dirhandle, 
and tests the associated file to see if something is true about it.  If the
argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
Unless otherwise documented, it returns C<1> for true and C<''> for false, or
the undefined value if the file doesn't exist.  Despite the funny
names, precedence is the same as any other named unary operator.  The
operator may be any of:

    -r  File is readable by effective uid/gid.
    -w  File is writable by effective uid/gid.
    -x  File is executable by effective uid/gid.
    -o  File is owned by effective uid.

    -R  File is readable by real uid/gid.
    -W  File is writable by real uid/gid.
    -X  File is executable by real uid/gid.
    -O  File is owned by real uid.

    -e  File exists.
    -z  File has zero size (is empty).
    -s  File has nonzero size (returns size in bytes).

    -f  File is a plain file.
    -d  File is a directory.
    -l  File is a symbolic link.
    -p  File is a named pipe (FIFO), or Filehandle is a pipe.
    -S  File is a socket.
    -b  File is a block special file.
    -c  File is a character special file.
    -t  Filehandle is opened to a tty.

    -u  File has setuid bit set.
    -g  File has setgid bit set.
    -k  File has sticky bit set.

    -T  File is an ASCII text file (heuristic guess).
    -B  File is a "binary" file (opposite of -T).

    -M  Script start time minus file modification time, in days.
    -A  Same for access time.
    -C  Same for inode change time (Unix, may differ for other platforms)

Example:

    while (<>) {
        chomp;
        next unless -f $_;  # ignore specials
        #...
    }

Note that C<-s/a/b/> does not do a negated substitution.  Saying
C<-exp($foo)> still works as expected, however: only single letters
following a minus are interpreted as file tests.

These operators are exempt from the "looks like a function rule" described
above. That is, an opening parenthesis after the operator does not affect
how much of the following code constitutes the argument. Put the opening
parentheses before the operator to separate it from code that follows (this
applies only to operators with higher precedence than unary operators, of
course):

    -s($file) + 1024   # probably wrong; same as -s($file + 1024)
    (-s $file) + 1024  # correct

The interpretation of the file permission operators C<-r>, C<-R>,
C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
of the file and the uids and gids of the user.  There may be other
reasons you can't actually read, write, or execute the file: for
example network filesystem access controls, ACLs (access control lists),
read-only filesystems, and unrecognized executable formats.  Note
that the use of these six specific operators to verify if some operation
is possible is usually a mistake, because it may be open to race
conditions.

Also note that, for the superuser on the local filesystems, the C<-r>,
C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
if any execute bit is set in the mode.  Scripts run by the superuser
may thus need to do a stat() to determine the actual mode of the file,
or temporarily set their effective uid to something else.

If you are using ACLs, there is a pragma called C<filetest> that may
produce more accurate results than the bare stat() mode bits.
When under C<use filetest 'access'> the above-mentioned filetests
test whether the permission can(not) be granted using the
access(2) family of system calls.  Also note that the C<-x> and C<-X> may
under this pragma return true even if there are no execute permission
bits set (nor any extra execute permission ACLs).  This strangeness is
due to the underlying system calls' definitions. Note also that, due to
the implementation of C<use filetest 'access'>, the C<_> special
filehandle won't cache the results of the file tests when this pragma is
in effect.  Read the documentation for the C<filetest> pragma for more
information.

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 strange characters (>30%)
are found, it's a C<-B> file; otherwise it's a C<-T> file.  Also, any file
containing a zero byte in the first block is considered a binary file.  If C<-T>
or C<-B> is used on a filehandle, the current IO buffer is examined
rather than the first block.  Both C<-T> and C<-B> return true on an empty
file, or a file at EOF when testing a filehandle.  Because you have to
read a file to do the C<-T> test, on most occasions you want to use a C<-f>
against the file first, as in C<next unless -f $file && -T $file>.

If any of the file tests (or either the C<stat> or C<lstat> operator) is 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> leave values in the stat structure for the
symbolic link, not the real file.)  (Also, if the stat buffer was filled by
an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
Example:

    print "Can do.\n" if -r $a || -w _ || -x _;

    stat($filename);
    print "Readable\n" if -r _;
    print "Writable\n" if -w _;
    print "Executable\n" if -x _;
    print "Setuid\n" if -u _;
    print "Setgid\n" if -g _;
    print "Sticky\n" if -k _;
    print "Text\n" if -T _;
    print "Binary\n" if -B _;

As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
test operators, in a way that C<-f -w -x $file> is equivalent to
C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
the return value of C<-f $file> as an argument to another filetest
operator, no special magic will happen.)

Portability issues: L<perlport/-X>.

=item abs VALUE
X<abs> X<absolute>

=item abs

Returns the absolute value of its argument.
If VALUE is omitted, uses C<$_>.

=item accept NEWSOCKET,GENERICSOCKET
X<accept>

Accepts an incoming socket connect, just as accept(2) 
does.  Returns the packed address if it succeeded, false otherwise.
See the example in L<perlipc/"Sockets: Client/Server Communication">.

On systems that support a close-on-exec flag on files, the flag will
be set for the newly opened file descriptor, as determined by the
value of $^F.  See L<perlvar/$^F>.

=item alarm SECONDS
X<alarm>
X<SIGALRM>
X<timer>

=item alarm

Arranges to have a SIGALRM delivered to this process after the
specified number of wallclock seconds has elapsed.  If SECONDS is not
specified, the value stored in C<$_> is used. (On some machines,
unfortunately, the elapsed time may be up to one second less or more
than you specified because of how seconds are counted, and process
scheduling may delay the delivery of the signal even further.)

Only one timer may be counting at once.  Each call disables the
previous timer, and an argument of C<0> may be supplied to cancel the
previous timer without starting a new one.  The returned value is the
amount of time remaining on the previous timer.

For delays of finer granularity than one second, the Time::HiRes module
(from CPAN, and starting from Perl 5.8 part of the standard
distribution) provides ualarm().  You may also use Perl's four-argument
version of select() leaving the first three arguments undefined, or you
might be able to use the C<syscall> interface to access setitimer(2) if
your system supports it. See L<perlfaq8> for details.

It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
C<sleep> may be internally implemented on your system with C<alarm>.

If you want to use C<alarm> to time out a system call you need to use an
C<eval>/C<die> pair.  You can't rely on the alarm causing the system call to
fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
restart system calls on some systems.  Using C<eval>/C<die> always works,
modulo the caveats given in L<perlipc/"Signals">.

    eval {
        local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
        alarm $timeout;
        $nread = sysread SOCKET, $buffer, $size;
        alarm 0;
    };
    if ($@) {
        die unless $@ eq "alarm\n";   # propagate unexpected errors
        # timed out
    }
    else {
        # didn't
    }

For more information see L<perlipc>.

Portability issues: L<perlport/alarm>.

=item atan2 Y,X
X<atan2> X<arctangent> X<tan> X<tangent>

Returns the arctangent of Y/X in the range -PI to PI.

For the tangent operation, you may use the C<Math::Trig::tan>
function, or use the familiar relation:

    sub tan { sin($_[0]) / cos($_[0])  }

The return value for C<atan2(0,0)> is implementation-defined; consult
your atan2(3) manpage for more information.

Portability issues: L<perlport/atan2>.

=item bind SOCKET,NAME
X<bind>

Binds a network address to a socket, just as bind(2)
does.  Returns true if it succeeded, false otherwise.  NAME should be a
packed address of the appropriate type for the socket.  See the examples in
L<perlipc/"Sockets: Client/Server Communication">.

=item binmode FILEHANDLE, LAYER
X<binmode> X<binary> X<text> X<DOS> X<Windows>

=item binmode FILEHANDLE

Arranges for FILEHANDLE to be read or written in "binary" or "text"
mode on systems where the run-time libraries distinguish between
binary and text files.  If FILEHANDLE is an expression, the value is
taken as the name of the filehandle.  Returns true on success,
otherwise it returns C<undef> and sets C<$!> (errno).

On some systems (in general, DOS- and Windows-based systems) binmode()
is necessary when you're not working with a text file.  For the sake
of portability it is a good idea always to use it when appropriate,
and never to use it when it isn't appropriate.  Also, people can
set their I/O to be by default UTF8-encoded Unicode, not bytes.

In other words: regardless of platform, use binmode() on binary data,
like images, for example.

If LAYER is present it is a single string, but may contain multiple
directives. The directives alter the behaviour of the filehandle.
When LAYER is present, using binmode on a text file makes sense.

If LAYER is omitted or specified as C<:raw> the filehandle is made
suitable for passing binary data. This includes turning off possible CRLF
translation and marking it as bytes (as opposed to Unicode characters).
Note that, despite what may be implied in I<"Programming Perl"> (the
Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
Other layers that would affect the binary nature of the stream are
I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
PERLIO environment variable.

The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
form C<:...>, are called I/O I<layers>.  The C<open> pragma can be used to
establish default I/O layers.  See L<open>.

I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
in "Programming Perl, 3rd Edition".  However, since the publishing of this
book, by many known as "Camel III", the consensus of the naming of this
functionality has moved from "discipline" to "layer".  All documentation
of this version of Perl therefore refers to "layers" rather than to
"disciplines".  Now back to the regularly scheduled documentation...>

To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
C<:utf8> just marks the data as UTF-8 without further checking,
while C<:encoding(UTF-8)> checks the data for actually being valid
UTF-8. More details can be found in L<PerlIO::encoding>.

In general, binmode() should be called after open() but before any I/O
is done on the filehandle.  Calling binmode() normally flushes any
pending buffered output data (and perhaps pending input data) on the
handle.  An exception to this is the C<:encoding> layer that
changes the default character encoding of the handle; see L</open>.
The C<:encoding> layer sometimes needs to be called in
mid-stream, and it doesn't flush the stream.  The C<:encoding>
also implicitly pushes on top of itself the C<:utf8> layer because
internally Perl operates on UTF8-encoded Unicode characters.

The operating system, device drivers, C libraries, and Perl run-time
system all conspire to let the programmer treat a single
character (C<\n>) as the line terminator, irrespective of external
representation.  On many operating systems, the native text file
representation matches the internal representation, but on some
platforms the external representation of C<\n> is made up of more than
one character.

All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
a single character to end each line in the external representation of text
(even though that single character is CARRIAGE RETURN on old, pre-Darwin
flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
systems like OS/2, DOS, and the various flavors of MS-Windows, your program
sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
two characters C<\cM\cJ>.  That means that if you don't use binmode() on
these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
output.  This is what you want for text files, but it can be disastrous for
binary files.

Another consequence of using binmode() (on some systems) is that
special end-of-file markers will be seen as part of the data stream.
For systems from the Microsoft family this means that, if your binary
data contain C<\cZ>, the I/O subsystem will regard it as the end of
the file, unless you use binmode().

binmode() is important not only for readline() and print() operations,
but also when using read(), seek(), sysread(), syswrite() and tell()
(see L<perlport> for more details).  See the C<$/> and C<$\> variables
in L<perlvar> for how to manually set your input and output
line-termination sequences.

Portability issues: L<perlport/binmode>.

=item bless REF,CLASSNAME
X<bless>

=item bless REF

This function tells the thingy referenced by REF that it is now an object
in the CLASSNAME package.  If CLASSNAME is omitted, the current package
is used.  Because a C<bless> is often the last thing in a constructor,
it returns the reference for convenience.  Always use the two-argument
version if a derived class might inherit the function doing the blessing.
See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
of objects.

Consider always blessing objects in CLASSNAMEs that are mixed case.
Namespaces with all lowercase names are considered reserved for
Perl pragmata.  Builtin types have all uppercase names. To prevent
confusion, you may wish to avoid such package names as well.  Make sure
that CLASSNAME is a true value.

See L<perlmod/"Perl Modules">.

=item break

Break out of a C<given()> block.

This keyword is enabled by the C<"switch"> feature: see
L<feature> for more information.  You can also access it by
prefixing it with C<CORE::>.  Alternately, include a C<use
v5.10> or later to the current scope.

=item caller EXPR
X<caller> X<call stack> X<stack> X<stack trace>

=item caller

Returns the context of the current subroutine call.  In scalar context,
returns the caller's package name if there I<is> a caller (that is, if
we're in a subroutine or C<eval> or C<require>) and the undefined value
otherwise.  In list context, returns

    # 0         1          2
    ($package, $filename, $line) = caller;

With EXPR, it returns some extra information that the debugger uses to
print a stack trace.  The value of EXPR indicates how many call frames
to go back before the current one.

    #  0         1          2      3            4
    ($package, $filename, $line, $subroutine, $hasargs,

    #  5          6          7            8       9         10
    $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
     = caller($i);

Here $subroutine may be C<(eval)> if the frame is not a subroutine
call, but an C<eval>.  In such a case additional elements $evaltext and
C<$is_require> are set: C<$is_require> is true if the frame is created by a
C<require> or C<use> statement, $evaltext contains the text of the
C<eval EXPR> statement.  In particular, for an C<eval BLOCK> statement,
$subroutine is C<(eval)>, but $evaltext is undefined.  (Note also that
each C<use> statement creates a C<require> frame inside an C<eval EXPR>
frame.)  $subroutine may also be C<(unknown)> if this particular
subroutine happens to have been deleted from the symbol table.
C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
compiled with.  The C<$hints> and C<$bitmask> values are subject to change
between versions of Perl, and are not meant for external use.

C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
of this hash, as they are the actual values stored in the optree.

Furthermore, when called from within the DB package, caller returns more
detailed information: it sets the list variable C<@DB::args> to be the
arguments with which the subroutine was invoked.

Be aware that the optimizer might have optimized call frames away before
C<caller> had a chance to get the information.  That means that C<caller(N)>
might not return information about the call frame you expect it to, for
C<< N > 1 >>.  In particular, C<@DB::args> might have information from the
previous time C<caller> was called.

Be aware that setting C<@DB::args> is I<best effort>, intended for
debugging or generating backtraces, and should not be relied upon. In
particular, as C<@_> contains aliases to the caller's arguments, Perl does
not take a copy of C<@_>, so C<@DB::args> will contain modifications the
subroutine makes to C<@_> or its contents, not the original values at call
time. C<@DB::args>, like C<@_>, does not hold explicit references to its
elements, so under certain cases its elements may have become freed and
reallocated for other variables or temporary values. Finally, a side effect
of the current implementation is that the effects of C<shift @_> can
I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
elements), so C<@DB::args> is actually a hybrid of the current state and
initial state of C<@_>. Buyer beware.

=item chdir EXPR
X<chdir>
X<cd>
X<directory, change>

=item chdir FILEHANDLE

=item chdir DIRHANDLE

=item chdir

Changes the working directory to EXPR, if possible. If EXPR is omitted,
changes to the directory specified by C<$ENV{HOME}>, if set; if not,
changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
neither is set, C<chdir> does nothing. It returns true on success,
false otherwise. See the example under C<die>.

On systems that support fchdir(2), you may pass a filehandle or
directory handle as the argument.  On systems that don't support fchdir(2),
passing handles raises an exception.

=item chmod LIST
X<chmod> X<permission> X<mode>

Changes the permissions of a list of files.  The first element of the
list must be the numeric mode, which should probably be an octal
number, and which definitely should I<not> be a string of octal digits:
C<0644> is okay, but C<"0644"> is not.  Returns the number of files
successfully changed.  See also L</oct> if all you have is a string.

    $cnt = chmod 0755, "foo", "bar";
    chmod 0755, @executables;
    $mode = "0644"; chmod $mode, "foo";      # !!! sets mode to
                                             # --w----r-T
    $mode = "0644"; chmod oct($mode), "foo"; # this is better
    $mode = 0644;   chmod $mode, "foo";      # this is best

On systems that support fchmod(2), you may pass filehandles among the
files.  On systems that don't support fchmod(2), passing filehandles raises
an exception.  Filehandles must be passed as globs or glob references to be
recognized; barewords are considered filenames.

    open(my $fh, "<", "foo");
    my $perm = (stat $fh)[2] & 07777;
    chmod($perm | 0600, $fh);

You can also import the symbolic C<S_I*> constants from the C<Fcntl>
module:

    use Fcntl qw( :mode );
    chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
    # Identical to the chmod 0755 of the example above.

Portability issues: L<perlport/chmod>.

=item chomp VARIABLE
X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>

=item chomp( LIST )

=item chomp

This safer version of L</chop> removes any trailing string
that corresponds to the current value of C<$/> (also known as
$INPUT_RECORD_SEPARATOR in the C<English> module).  It returns the total
number of characters removed from all its arguments.  It's often used to
remove the newline from the end of an input record when you're worried
that the final record may be missing its newline.  When in paragraph
mode (C<$/ = "">), it removes all trailing newlines from the string.
When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
a reference to an integer or the like; see L<perlvar>) chomp() won't
remove anything.
If VARIABLE is omitted, it chomps C<$_>.  Example:

    while (<>) {
        chomp;  # avoid \n on last field
        @array = split(/:/);
        # ...
    }

If VARIABLE is a hash, it chomps the hash's values, but not its keys.

You can actually chomp anything that's an lvalue, including an assignment:

    chomp($cwd = `pwd`);
    chomp($answer = <STDIN>);

If you chomp a list, each element is chomped, and the total number of
characters removed is returned.

Note that parentheses are necessary when you're chomping anything
that is not a simple variable.  This is because C<chomp $cwd = `pwd`;>
is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
C<chomp( $cwd = `pwd` )> which you might expect.  Similarly,
C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
as C<chomp($a, $b)>.

=item chop VARIABLE
X<chop>

=item chop( LIST )

=item chop

Chops off the last character of a string and returns the character
chopped.  It is much more efficient than C<s/.$//s> because it neither
scans nor copies the string.  If VARIABLE is omitted, chops C<$_>.
If VARIABLE is a hash, it chops the hash's values, but not its keys.

You can actually chop anything that's an lvalue, including an assignment.

If you chop a list, each element is chopped.  Only the value of the
last C<chop> is returned.

Note that C<chop> returns the last character.  To return all but the last
character, use C<substr($string, 0, -1)>.

See also L</chomp>.

=item chown LIST
X<chown> X<owner> X<user> X<group>

Changes the owner (and group) of a list of files.  The first two
elements of the list must be the I<numeric> uid and gid, in that
order.  A value of -1 in either position is interpreted by most
systems to leave that value unchanged.  Returns the number of files
successfully changed.

    $cnt = chown $uid, $gid, 'foo', 'bar';
    chown $uid, $gid, @filenames;

On systems that support fchown(2), you may pass filehandles among the
files.  On systems that don't support fchown(2), passing filehandles raises
an exception.  Filehandles must be passed as globs or glob references to be
recognized; barewords are considered filenames.

Here's an example that looks up nonnumeric uids in the passwd file:

    print "User: ";
    chomp($user = <STDIN>);
    print "Files: ";
    chomp($pattern = <STDIN>);

    ($login,$pass,$uid,$gid) = getpwnam($user)
        or die "$user not in passwd file";

    @ary = glob($pattern);  # expand filenames
    chown $uid, $gid, @ary;

On most systems, you are not allowed to change the ownership of the
file unless you're the superuser, although you should be able to change
the group to any of your secondary groups.  On insecure systems, these
restrictions may be relaxed, but this is not a portable assumption.
On POSIX systems, you can detect this condition this way:

    use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
    $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);

Portability issues: L<perlport/chmod>.

=item chr NUMBER
X<chr> X<character> X<ASCII> X<Unicode>

=item chr

Returns the character represented by that NUMBER in the character set.
For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
chr(0x263a) is a Unicode smiley face.  

Negative values give the Unicode replacement character (chr(0xfffd)),
except under the L<bytes> pragma, where the low eight bits of the value
(truncated to an integer) are used.

If NUMBER is omitted, uses C<$_>.

For the reverse, use L</ord>.

Note that characters from 128 to 255 (inclusive) are by default
internally not encoded as UTF-8 for backward compatibility reasons.

See L<perlunicode> for more about Unicode.

=item chroot FILENAME
X<chroot> X<root>

=item chroot

This function works like the system call by the same name: it makes the
named directory the new root directory for all further pathnames that
begin with a C</> by your process and all its children.  (It doesn't
change your current working directory, which is unaffected.)  For security
reasons, this call is restricted to the superuser.  If FILENAME is
omitted, does a C<chroot> to C<$_>.

Portability issues: L<perlport/chroot>.

=item close FILEHANDLE
X<close>

=item close

Closes the file or pipe associated with the filehandle, flushes the IO
buffers, and closes the system file descriptor.  Returns true if those
operations succeed and if no error was reported by any PerlIO
layer.  Closes the currently selected filehandle if the argument is
omitted.

You don't have to close FILEHANDLE if you are immediately going to do
another C<open> on it, because C<open> closes it for you.  (See
L<open|/open FILEHANDLE>.)  However, an explicit C<close> on an input file resets the line
counter (C<$.>), while the implicit close done by C<open> does not.

If the filehandle came from a piped open, C<close> returns false if one of
the other syscalls involved fails or if its program exits with non-zero
status.  If the only problem was that the program exited non-zero, C<$!>
will be set to C<0>.  Closing a pipe also waits for the process executing
on the pipe to exit--in case you wish to look at the output of the pipe
afterwards--and implicitly puts the exit status value of that command into
C<$?> and C<${^CHILD_ERROR_NATIVE}>.

If there are multiple threads running, C<close> on a filehandle from a
piped open returns true without waiting for the child process to terminate,
if the filehandle is still open in another thread.

Closing the read end of a pipe before the process writing to it at the
other end is done writing results in the writer receiving a SIGPIPE.  If
the other end can't handle that, be sure to read all the data before
closing the pipe.

Example:

    open(OUTPUT, '|sort >foo')  # pipe to sort
        or die "Can't start sort: $!";
    #...                        # print stuff to output
    close OUTPUT                # wait for sort to finish
        or warn $! ? "Error closing sort pipe: $!"
                   : "Exit status $? from sort";
    open(INPUT, 'foo')          # get sort's results
        or die "Can't open 'foo' for input: $!";

FILEHANDLE may be an expression whose value can be used as an indirect
filehandle, usually the real filehandle name or an autovivified handle.

=item closedir DIRHANDLE
X<closedir>

Closes a directory opened by C<opendir> and returns the success of that
system call.

=item connect SOCKET,NAME
X<connect>

Attempts to connect to a remote socket, just like connect(2).
Returns true if it succeeded, false otherwise.  NAME should be a
packed address of the appropriate type for the socket.  See the examples in
L<perlipc/"Sockets: Client/Server Communication">.

=item continue BLOCK
X<continue>

=item continue

When followed by a BLOCK, C<continue> is actually a
flow control statement rather than a function.  If
there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
C<foreach>), it is always executed just before the conditional is about to
be evaluated again, just like the third part of a C<for> loop in C.  Thus
it can be used to increment a loop variable, even when the loop has been
continued via the C<next> statement (which is similar to the C C<continue>
statement).

C<last>, C<next>, or C<redo> may appear within a C<continue>
block; C<last> and C<redo> behave as if they had been executed within
the main block.  So will C<next>, but since it will execute a C<continue>
block, it may be more entertaining.

    while (EXPR) {
        ### redo always comes here
        do_something;
    } continue {
        ### next always comes here
        do_something_else;
        # then back the top to re-check EXPR
    }
    ### last always comes here

Omitting the C<continue> section is equivalent to using an
empty one, logically enough, so C<next> goes directly back
to check the condition at the top of the loop.

When there is no BLOCK, C<continue> is a function that
falls through the current C<when> or C<default> block instead of iterating
a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
In Perl 5.14 and earlier, this form of C<continue> was
only available when the C<"switch"> feature was enabled.
See L<feature> and L<perlsyn/"Switch statements"> for more
information.

=item cos EXPR
X<cos> X<cosine> X<acos> X<arccosine>

=item cos

Returns the cosine of EXPR (expressed in radians).  If EXPR is omitted,
takes the cosine of C<$_>.

For the inverse cosine operation, you may use the C<Math::Trig::acos()>
function, or use this relation:

    sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }

=item crypt PLAINTEXT,SALT
X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
X<decrypt> X<cryptography> X<passwd> X<encrypt>

Creates a digest string exactly like the crypt(3) function in the C
library (assuming that you actually have a version there that has not
been extirpated as a potential munition).

crypt() is a one-way hash function.  The PLAINTEXT and SALT are turned
into a short string, called a digest, which is returned.  The same
PLAINTEXT and SALT will always return the same string, but there is no
(known) way to get the original PLAINTEXT from the hash.  Small
changes in the PLAINTEXT or SALT will result in large changes in the
digest.

There is no decrypt function.  This function isn't all that useful for
cryptography (for that, look for F<Crypt> modules on your nearby CPAN
mirror) and the name "crypt" is a bit of a misnomer.  Instead it is
primarily used to check if two pieces of text are the same without
having to transmit or store the text itself.  An example is checking
if a correct password is given.  The digest of the password is stored,
not the password itself.  The user types in a password that is
crypt()'d with the same salt as the stored digest.  If the two digests
match, the password is correct.

When verifying an existing digest string you should use the digest as
the salt (like C<crypt($plain, $digest) eq $digest>).  The SALT used
to create the digest is visible as part of the digest.  This ensures
crypt() will hash the new string with the same salt as the digest.
This allows your code to work with the standard L<crypt|/crypt> and
with more exotic implementations.  In other words, assume
nothing about the returned string itself nor about how many bytes 
of SALT may matter.

Traditionally the result is a string of 13 bytes: two first bytes of
the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
the first eight bytes of PLAINTEXT mattered. But alternative
hashing schemes (like MD5), higher level security schemes (like C2),
and implementations on non-Unix platforms may produce different
strings.

When choosing a new salt create a random two character string whose
characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).  This set of
characters is just a recommendation; the characters allowed in
the salt depend solely on your system's crypt library, and Perl can't
restrict what salts C<crypt()> accepts.

Here's an example that makes sure that whoever runs this program knows
their password:

    $pwd = (getpwuid($<))[1];

    system "stty -echo";
    print "Password: ";
    chomp($word = <STDIN>);
    print "\n";
    system "stty echo";

    if (crypt($word, $pwd) ne $pwd) {
        die "Sorry...\n";
    } else {
        print "ok\n";
    }

Of course, typing in your own password to whoever asks you
for it is unwise.

The L<crypt|/crypt> function is unsuitable for hashing large quantities
of data, not least of all because you can't get the information
back.  Look at the L<Digest> module for more robust algorithms.

If using crypt() on a Unicode string (which I<potentially> has
characters with codepoints above 255), Perl tries to make sense
of the situation by trying to downgrade (a copy of)
the string back to an eight-bit byte string before calling crypt()
(on that copy).  If that works, good.  If not, crypt() dies with
C<Wide character in crypt>.

Portability issues: L<perlport/crypt>.

=item dbmclose HASH
X<dbmclose>

[This function has been largely superseded by the C<untie> function.]

Breaks the binding between a DBM file and a hash.

Portability issues: L<perlport/dbmclose>.

=item dbmopen HASH,DBNAME,MASK
X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>

[This function has been largely superseded by the
L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]

This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
hash.  HASH is the name of the hash.  (Unlike normal C<open>, the first
argument is I<not> a filehandle, even though it looks like one).  DBNAME
is the name of the database (without the F<.dir> or F<.pag> extension if
any).  If the database does not exist, it is created with protection
specified by MASK (as modified by the C<umask>).  If your system supports
only the older DBM functions, you may make only one C<dbmopen> call in your
program.  In older versions of Perl, if your system had neither DBM nor
ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
sdbm(3).

If you don't have write access to the DBM file, you can only read hash
variables, not set them.  If you want to test whether you can write,
either use file tests or try setting a dummy hash entry inside an C<eval> 
to trap the error.

Note that functions such as C<keys> and C<values> may return huge lists
when used on large DBM files.  You may prefer to use the C<each>
function to iterate over large DBM files.  Example:

    # print out history file offsets
    dbmopen(%HIST,'/usr/lib/news/history',0666);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    dbmclose(%HIST);

See also L<AnyDBM_File> for a more general description of the pros and
cons of the various dbm approaches, as well as L<DB_File> for a particularly
rich implementation.

You can control which DBM library you use by loading that library
before you call dbmopen():

    use DB_File;
    dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
        or die "Can't open netscape history file: $!";

Portability issues: L<perlport/dbmopen>.

=item default BLOCK

Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
that's always true.  Only available after Perl 5.10, and only if the
C<switch> feature has been requested or if the keyword is prefixed with
C<CORE::>.  See L</when>.

=item defined EXPR
X<defined> X<undef> X<undefined>

=item defined

Returns a Boolean value telling whether EXPR has a value other than
the undefined value C<undef>.  If EXPR is not present, C<$_> is
checked.

Many operations return C<undef> to indicate failure, end of file,
system error, uninitialized variable, and other exceptional
conditions.  This function allows you to distinguish C<undef> from
other values.  (A simple Boolean test will not distinguish among
C<undef>, zero, the empty string, and C<"0">, which are all equally
false.)  Note that since C<undef> is a valid scalar, its presence
doesn't I<necessarily> indicate an exceptional condition: C<pop>
returns C<undef> when its argument is an empty array, I<or> when the
element to return happens to be C<undef>.

You may also use C<defined(&func)> to check whether subroutine C<&func>
has ever been defined.  The return value is unaffected by any forward
declarations of C<&func>.  A subroutine that is not defined
may still be callable: its package may have an C<AUTOLOAD> method that
makes it spring into existence the first time that it is called; see
L<perlsub>.

Use of C<defined> on aggregates (hashes and arrays) is deprecated.  It
used to report whether memory for that aggregate had ever been
allocated.  This behavior may disappear in future versions of Perl.
You should instead use a simple test for size:

    if (@an_array) { print "has array elements\n" }
    if (%a_hash)   { print "has hash members\n"   }

When used on a hash element, it tells you whether the value is defined,
not whether the key exists in the hash.  Use L</exists> for the latter
purpose.

Examples:

    print if defined $switch{D};
    print "$val\n" while defined($val = pop(@ary));
    die "Can't readlink $sym: $!"
        unless defined($value = readlink $sym);
    sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
    $debugging = 0 unless defined $debugging;

Note:  Many folks tend to overuse C<defined> and are then surprised to
discover that the number C<0> and C<""> (the zero-length string) are, in fact,
defined values.  For example, if you say

    "ab" =~ /a(.*)b/;

The pattern match succeeds and C<$1> is defined, although it
matched "nothing".  It didn't really fail to match anything.  Rather, it
matched something that happened to be zero characters long.  This is all
very above-board and honest.  When a function returns an undefined value,
it's an admission that it couldn't give you an honest answer.  So you
should use C<defined> only when questioning the integrity of what
you're trying to do.  At other times, a simple comparison to C<0> or C<""> is
what you want.

See also L</undef>, L</exists>, L</ref>.

=item delete EXPR
X<delete>

Given an expression that specifies an element or slice of a hash, C<delete>
deletes the specified elements from that hash so that exists() on that element
no longer returns true.  Setting a hash element to the undefined value does
not remove its key, but deleting it does; see L</exists>.

In list context, returns the value or values deleted, or the last such
element in scalar context.  The return list's length always matches that of
the argument list: deleting non-existent elements returns the undefined value
in their corresponding positions.

delete() may also be used on arrays and array slices, but its behavior is less
straightforward.  Although exists() will return false for deleted entries,
deleting array elements never changes indices of existing values; use shift()
or splice() for that.  However, if all deleted elements fall at the end of an
array, the array's size shrinks to the position of the highest element that
still tests true for exists(), or to 0 if none do.

B<WARNING:> Calling delete on array values is deprecated and likely to
be removed in a future version of Perl.

Deleting from C<%ENV> modifies the environment.  Deleting from a hash tied to
a DBM file deletes the entry from the DBM file.  Deleting from a C<tied> hash
or array may not necessarily return anything; it depends on the implementation
of the C<tied> package's DELETE method, which may do whatever it pleases.

The C<delete local EXPR> construct localizes the deletion to the current
block at run time.  Until the block exits, elements locally deleted
temporarily no longer exist.  See L<perlsub/"Localized deletion of elements
of composite types">.

    %hash = (foo => 11, bar => 22, baz => 33);
    $scalar = delete $hash{foo};             # $scalar is 11
    $scalar = delete @hash{qw(foo bar)};     # $scalar is 22
    @array  = delete @hash{qw(foo bar baz)}; # @array  is (undef,undef,33)

The following (inefficiently) deletes all the values of %HASH and @ARRAY:

    foreach $key (keys %HASH) {
        delete $HASH{$key};
    }

    foreach $index (0 .. $#ARRAY) {
        delete $ARRAY[$index];
    }

And so do these:

    delete @HASH{keys %HASH};

    delete @ARRAY[0 .. $#ARRAY];

But both are slower than assigning the empty list
or undefining %HASH or @ARRAY, which is the customary 
way to empty out an aggregate:

    %HASH = ();     # completely empty %HASH
    undef %HASH;    # forget %HASH ever existed

    @ARRAY = ();    # completely empty @ARRAY
    undef @ARRAY;   # forget @ARRAY ever existed

The EXPR can be arbitrarily complicated provided its
final operation is an element or slice of an aggregate:

    delete $ref->[$x][$y]{$key};
    delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};

    delete $ref->[$x][$y][$index];
    delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];

=item die LIST
X<die> X<throw> X<exception> X<raise> X<$@> X<abort>

C<die> raises an exception. Inside an C<eval> the error message is stuffed
into C<$@> and the C<eval> is terminated with the undefined value.
If the exception is outside of all enclosing C<eval>s, then the uncaught
exception prints LIST to C<STDERR> and exits with a non-zero value. If you
need to exit the process with a specific exit code, see L</exit>.

Equivalent examples:

    die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
    chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"

If the last element of LIST does not end in a newline, the current
script line number and input line number (if any) are also printed,
and a newline is supplied.  Note that the "input line number" (also
known as "chunk") is subject to whatever notion of "line" happens to
be currently in effect, and is also available as the special variable
C<$.>.  See L<perlvar/"$/"> and L<perlvar/"$.">.

Hint: sometimes appending C<", stopped"> to your message will cause it
to make better sense when the string C<"at foo line 123"> is appended.
Suppose you are running script "canasta".

    die "/etc/games is no good";
    die "/etc/games is no good, stopped";

produce, respectively

    /etc/games is no good at canasta line 123.
    /etc/games is no good, stopped at canasta line 123.

If the output is empty and C<$@> already contains a value (typically from a
previous eval) that value is reused after appending C<"\t...propagated">.
This is useful for propagating exceptions:

    eval { ... };
    die unless $@ =~ /Expected exception/;

If the output is empty and C<$@> contains an object reference that has a
C<PROPAGATE> method, that method will be called with additional file
and line number parameters.  The return value replaces the value in
C<$@>;  i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
were called.

If C<$@> is empty then the string C<"Died"> is used.

If an uncaught exception results in interpreter exit, the exit code is
determined from the values of C<$!> and C<$?> with this pseudocode:

    exit $! if $!;              # errno
    exit $? >> 8 if $? >> 8;    # child exit status
    exit 255;                   # last resort

The intent is to squeeze as much possible information about the likely cause
into the limited space of the system exit code. However, as C<$!> is the value
of C's C<errno>, which can be set by any system call, this means that the value
of the exit code used by C<die> can be non-predictable, so should not be relied
upon, other than to be non-zero.

You can also call C<die> with a reference argument, and if this is trapped
within an C<eval>, C<$@> contains that reference.  This permits more
elaborate exception handling using objects that maintain arbitrary state
about the exception.  Such a scheme is sometimes preferable to matching
particular string values of C<$@> with regular expressions.  Because C<$@> 
is a global variable and C<eval> may be used within object implementations,
be careful that analyzing the error object doesn't replace the reference in
the global variable.  It's easiest to make a local copy of the reference
before any manipulations.  Here's an example:

    use Scalar::Util "blessed";

    eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
    if (my $ev_err = $@) {
        if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
            # handle Some::Module::Exception
        }
        else {
            # handle all other possible exceptions
        }
    }

Because Perl stringifies uncaught exception messages before display,
you'll probably want to overload stringification operations on
exception objects.  See L<overload> for details about that.

You can arrange for a callback to be run just before the C<die>
does its deed, by setting the C<$SIG{__DIE__}> hook.  The associated
handler is called with the error text and can change the error
message, if it sees fit, by calling C<die> again.  See
L<perlvar/%SIG> for details on setting C<%SIG> entries, and
L<"eval BLOCK"> for some examples.  Although this feature was 
to be run only right before your program was to exit, this is not
currently so: the C<$SIG{__DIE__}> hook is currently called
even inside eval()ed blocks/strings!  If one wants the hook to do
nothing in such situations, put

    die @_ if $^S;

as the first line of the handler (see L<perlvar/$^S>).  Because
this promotes strange action at a distance, this counterintuitive
behavior may be fixed in a future release.

See also exit(), warn(), and the Carp module.

=item do BLOCK
X<do> X<block>

Not really a function.  Returns the value of the last command in the
sequence of commands indicated by BLOCK.  When modified by the C<while> or
C<until> loop modifier, executes the BLOCK once before testing the loop
condition. (On other statements the loop modifiers test the conditional
first.)

C<do BLOCK> does I<not> count as a loop, so the loop control statements
C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
See L<perlsyn> for alternative strategies.

=item do SUBROUTINE(LIST)
X<do>

This form of subroutine call is deprecated.  SUBROUTINE can be a bareword,
a scalar variable or a subroutine beginning with C<&>.

=item do EXPR
X<do>

Uses the value of EXPR as a filename and executes the contents of the
file as a Perl script.

    do 'stat.pl';

is just like

    eval `cat stat.pl`;

except that it's more efficient and concise, keeps track of the current
filename for error messages, searches the C<@INC> directories, and updates
C<%INC> if the file is found.  See L<perlvar/@INC> and L<perlvar/%INC> for
these variables.  It also differs in that code evaluated with C<do FILENAME>
cannot see lexicals in the enclosing scope; C<eval STRING> does.  It's the
same, however, in that it does reparse the file every time you call it,
so you probably don't want to do this inside a loop.

If C<do> can read the file but cannot compile it, it returns C<undef> and sets
an error message in C<$@>.  If C<do> cannot read the file, it returns undef
and sets C<$!> to the error.  Always check C<$@> first, as compilation
could fail in a way that also sets C<$!>.  If the file is successfully
compiled, C<do> returns the value of the last expression evaluated.

Inclusion of library modules is better done with the
C<use> and C<require> operators, which also do automatic error checking
and raise an exception if there's a problem.

You might like to use C<do> to read in a program configuration
file.  Manual error checking can be done this way:

    # read in config files: system first, then user
    for $file ("/share/prog/defaults.rc",
               "$ENV{HOME}/.someprogrc")
    {
        unless ($return = do $file) {
            warn "couldn't parse $file: $@" if $@;
            warn "couldn't do $file: $!"    unless defined $return;
            warn "couldn't run $file"       unless $return;
        }
    }

=item dump LABEL
X<dump> X<core> X<undump>

=item dump

This function causes an immediate core dump.  See also the B<-u>
command-line switch in L<perlrun>, which does the same thing.
Primarily this is so that you can use the B<undump> program (not
supplied) to turn your core dump into an executable binary after
having initialized all your variables at the beginning of the
program.  When the new binary is executed it will begin by executing
a C<goto LABEL> (with all the restrictions that C<goto> suffers).
Think of it as a goto with an intervening core dump and reincarnation.
If C<LABEL> is omitted, restarts the program from the top.

B<WARNING>: Any files opened at the time of the dump will I<not>
be open any more when the program is reincarnated, with possible
resulting confusion by Perl.

This function is now largely obsolete, mostly because it's very hard to
convert a core file into an executable. That's why you should now invoke
it as C<CORE::dump()>, if you don't want to be warned against a possible
typo.

Portability issues: L<perlport/dump>.

=item each HASH
X<each> X<hash, iterator>

=item each ARRAY
X<array, iterator>

=item each EXPR

When called in list context, returns a 2-element list consisting of the key
and value for the next element of a hash, or the index and value for the
next element of an array, so that you can iterate over it.  When called in
scalar context, returns only the key (not the value) in a hash, or the index
in an array.

Hash entries are returned in an apparently random order.  The actual random
order is subject to change in future versions of Perl, but it is
guaranteed to be in the same order as either the C<keys> or C<values>
function would produce on the same (unmodified) hash.  Since Perl
5.8.2 the ordering can be different even between different runs of Perl
for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).

After C<each> has returned all entries from the hash or array, the next
call to C<each> returns the empty list in list context and C<undef> in
scalar context.  The next call following that one restarts iteration.  Each
hash or array has its own internal iterator, accessed by C<each>, C<keys>,
and C<values>.  The iterator is implicitly reset when C<each> has reached
the end as just described; it can be explicitly reset by calling C<keys> or
C<values> on the hash or array.  If you add or delete a hash's elements
while iterating over it, entries may be skipped or duplicated--so don't do
that.  Exception: It is always safe to delete the item most recently
returned by C<each()>, so the following code works properly:

        while (($key, $value) = each %hash) {
          print $key, "\n";
          delete $hash{$key};   # This is safe
        }

This prints out your environment like the printenv(1) program,
but in a different order:

    while (($key,$value) = each %ENV) {
        print "$key=$value\n";
    }

Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
reference to an unblessed hash or array.  The argument will be dereferenced
automatically.  This aspect of C<each> is considered highly experimental.
The exact behaviour may change in a future version of Perl.

    while (($key,$value) = each $hashref) { ... }

See also C<keys>, C<values>, and C<sort>.

=item eof FILEHANDLE
X<eof>
X<end of file>
X<end-of-file>

=item eof ()

=item eof

Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
FILEHANDLE is not open.  FILEHANDLE may be an expression whose value
gives the real filehandle.  (Note that this function actually
reads a character and then C<ungetc>s it, so isn't useful in an
interactive context.)  Do not read from a terminal file (or call
C<eof(FILEHANDLE)> on it) after end-of-file is reached.  File types such
as terminals may lose the end-of-file condition if you do.

An C<eof> without an argument uses the last file read.  Using C<eof()>
with empty parentheses is different.  It refers to the pseudo file
formed from the files listed on the command line and accessed via the
C<< <> >> operator.  Since C<< <> >> isn't explicitly opened,
as a normal filehandle is, an C<eof()> before C<< <> >> has been
used will cause C<@ARGV> to be examined to determine if input is
available.   Similarly, an C<eof()> after C<< <> >> has returned
end-of-file will assume you are processing another C<@ARGV> list,
and if you haven't set C<@ARGV>, will read input from C<STDIN>;
see L<perlop/"I/O Operators">.

In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
detect the end of each file, whereas C<eof()> will detect the end 
of the very last file only.  Examples:

    # reset line numbering on each input file
    while (<>) {
        next if /^\s*#/;  # skip comments
        print "$.\t$_";
    } continue {
        close ARGV if eof;  # Not eof()!
    }

    # insert dashes just before last line of last file
    while (<>) {
        if (eof()) {  # check for end of last file
            print "--------------\n";
        }
        print;
        last if eof();          # needed if we're reading from a terminal
    }

Practical hint: you almost never need to use C<eof> in Perl, because the
input operators typically return C<undef> when they run out of data or 
encounter an error.

=item eval EXPR
X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
X<error, handling> X<exception, handling>

=item eval BLOCK

=item eval

In the first form, the return value of EXPR is parsed and executed as if it
were a little Perl program.  The value of the expression (which is itself
determined within scalar context) is first parsed, and if there were no
errors, executed as a block within the lexical context of the current Perl
program. This means, that in particular, any outer lexical variables are
visible to it, and any package variable settings or subroutine and format
definitions remain afterwards.

Note that the value is parsed every time the C<eval> executes.
If EXPR is omitted, evaluates C<$_>.  This form is typically used to
delay parsing and subsequent execution of the text of EXPR until run time.

In the second form, the code within the BLOCK is parsed only once--at the
same time the code surrounding the C<eval> itself was parsed--and executed
within the context of the current Perl program.  This form is typically
used to trap exceptions more efficiently than the first (see below), while
also providing the benefit of checking the code within BLOCK at compile
time.

The final semicolon, if any, may be omitted from the value of EXPR or within
the BLOCK.

In both forms, the value returned is the value of the last expression
evaluated inside the mini-program; a return statement may be also used, just
as with subroutines.  The expression providing the return value is evaluated
in void, scalar, or list context, depending on the context of the C<eval> 
itself.  See L</wantarray> for more on how the evaluation context can be 
determined.

If there is a syntax error or runtime error, or a C<die> statement is
executed, C<eval> returns C<undef> in scalar context
or an empty list--or, for syntax errors, a list containing a single
undefined value--in list context, and C<$@> is set to the error
message.  The discrepancy in the return values in list context is
considered a bug by some, and will probably be fixed in a future
release.  If there was no error, C<$@> is guaranteed to be the empty
string.  Beware that using C<eval> neither silences Perl from printing
warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.

Note that, because C<eval> traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as C<socket> or C<symlink>)
is implemented.  It is also Perl's exception-trapping mechanism, where
the die operator is used to raise exceptions.

If you want to trap errors when loading an XS module, some problems with
the binary interface (such as Perl version skew) may be fatal even with
C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.

If the code to be executed doesn't vary, you may use the eval-BLOCK
form to trap run-time errors without incurring the penalty of
recompiling each time.  The error, if any, is still returned in C<$@>.
Examples:

    # make divide-by-zero nonfatal
    eval { $answer = $a / $b; }; warn $@ if $@;

    # same thing, but less efficient
    eval '$answer = $a / $b'; warn $@ if $@;

    # a compile-time error
    eval { $answer = }; # WRONG

    # a run-time error
    eval '$answer =';   # sets $@

Using the C<eval{}> form as an exception trap in libraries does have some
issues.  Due to the current arguably broken state of C<__DIE__> hooks, 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 this example shows:

    # a private exception trap for divide-by-zero
    eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
    warn $@ if $@;

This is especially significant, given that C<__DIE__> hooks can call
C<die> again, which has the effect of changing their error messages:

    # __DIE__ hooks may modify error messages
    {
       local $SIG{'__DIE__'} =
              sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
       eval { die "foo lives here" };
       print $@ if $@;                # prints "bar lives here"
    }

Because this promotes action at a distance, this counterintuitive behavior
may be fixed in a future release.

With an C<eval>, you should be especially careful to remember what's
being looked at when:

    eval $x;        # CASE 1
    eval "$x";      # CASE 2

    eval '$x';      # CASE 3
    eval { $x };    # CASE 4

    eval "\$$x++";  # CASE 5
    $$x++;          # CASE 6

Cases 1 and 2 above behave identically: they run the code contained in
the variable $x.  (Although case 2 has misleading double quotes making
the reader wonder what else might be happening (nothing is).)  Cases 3
and 4 likewise behave in the same way: they run the code C<'$x'>, which
does nothing but return the value of $x.  (Case 4 is preferred for
purely visual reasons, but it also has the advantage of compiling at
compile-time instead of at run-time.)  Case 5 is a place where
normally you I<would> like to use double quotes, except that in this
particular situation, you can just use symbolic references instead, as
in case 6.

Before Perl 5.14, the assignment to C<$@> occurred before restoration 
of localised variables, which means that for your code to run on older
versions, a temporary is required if you want to mask some but not all
errors:

    # alter $@ on nefarious repugnancy only
    {
       my $e;
       {
          local $@; # protect existing $@
          eval { test_repugnancy() };
          # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
          $@ =~ /nefarious/ and $e = $@;
       }
       die $e if defined $e
    }

C<eval BLOCK> does I<not> count as a loop, so the loop control statements
C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.

An C<eval ''> executed within the C<DB> package doesn't see the usual
surrounding lexical scope, but rather the scope of the first non-DB piece
of code that called it. You don't normally need to worry about this unless
you are writing a Perl debugger.

=item exec LIST
X<exec> X<execute>

=item exec PROGRAM LIST

The C<exec> function executes a system command I<and never returns>;
use C<system> instead of C<exec> if you want it to return.  It fails and
returns false only if the command does not exist I<and> it is executed
directly instead of via your system's command shell (see below).

Since it's a common mistake to use C<exec> instead of C<system>, Perl
warns you if there is a following statement that isn't C<die>, C<warn>,
or C<exit> (if C<-w> is set--but you always do that, right?).   If you
I<really> want to follow an C<exec> with some other statement, you
can use one of these styles to avoid the warning:

    exec ('foo')   or print STDERR "couldn't exec foo: $!";
    { exec ('foo') }; print STDERR "couldn't exec foo: $!";

If there is more than one argument in LIST, or if LIST is an array
with more than one value, calls execvp(3) with the arguments in LIST.
If there is only one scalar argument or an array with one element in it,
the argument is checked for shell metacharacters, and if there are any,
the entire argument is passed to the system's command shell for parsing
(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
If there are no shell metacharacters in the argument, it is split into
words and passed directly to C<execvp>, which is more efficient.
Examples:

    exec '/bin/echo', 'Your arguments are: ', @ARGV;
    exec "sort $outfile | uniq";

If you don't really want to execute the first argument, but want to lie
to the program you are executing about its own name, you can specify
the program you actually want to run as an "indirect object" (without a
comma) in front of the LIST.  (This always forces interpretation of the
LIST as a multivalued list, even if there is only a single scalar in
the list.)  Example:

    $shell = '/bin/csh';
    exec $shell '-sh';    # pretend it's a login shell

or, more directly,

    exec {'/bin/csh'} '-sh';  # pretend it's a login shell

When the arguments get executed via the system shell, results are
subject to its quirks and capabilities.  See L<perlop/"`STRING`">
for details.

Using an indirect object with C<exec> or C<system> is also more
secure.  This usage (which also works fine with system()) forces
interpretation of the arguments as a multivalued list, even if the
list had just one argument.  That way you're safe from the shell
expanding wildcards or splitting up words with whitespace in them.

    @args = ( "echo surprise" );

    exec @args;               # subject to shell escapes
                                # if @args == 1
    exec { $args[0] } @args;  # safe even with one-arg list

The first version, the one without the indirect object, ran the I<echo>
program, passing it C<"surprise"> an argument.  The second version didn't;
it tried to run a program named I<"echo surprise">, didn't find it, and set
C<$?> to a non-zero value indicating failure.

Beginning with v5.6.0, Perl attempts to flush all files opened for
output before the exec, but this may not be supported on some platforms
(see L<perlport>).  To be safe, you may need to set C<$|> ($AUTOFLUSH
in English) or call the C<autoflush()> method of C<IO::Handle> on any
open handles to avoid lost output.

Note that C<exec> will not call your C<END> blocks, nor will it invoke
C<DESTROY> methods on your objects.

Portability issues: L<perlport/exec>.

=item exists EXPR
X<exists> X<autovivification>

Given an expression that specifies an element of a hash, returns true if the
specified element in the hash has ever been initialized, even if the
corresponding value is undefined.

    print "Exists\n"    if exists $hash{$key};
    print "Defined\n"   if defined $hash{$key};
    print "True\n"      if $hash{$key};

exists may also be called on array elements, but its behavior is much less
obvious and is strongly tied to the use of L</delete> on arrays.  B<Be aware>
that calling exists on array values is deprecated and likely to be removed in
a future version of Perl.

    print "Exists\n"    if exists $array[$index];
    print "Defined\n"   if defined $array[$index];
    print "True\n"      if $array[$index];

A hash or array element can be true only if it's defined and defined only if
it exists, but the reverse doesn't necessarily hold true.

Given an expression that specifies the name of a subroutine,
returns true if the specified subroutine has ever been declared, even
if it is undefined.  Mentioning a subroutine name for exists or defined
does not count as declaring it.  Note that a subroutine that does not
exist may still be callable: its package may have an C<AUTOLOAD>
method that makes it spring into existence the first time that it is
called; see L<perlsub>.

    print "Exists\n"  if exists &subroutine;
    print "Defined\n" if defined &subroutine;

Note that the EXPR can be arbitrarily complicated as long as the final
operation is a hash or array key lookup or subroutine name:

    if (exists $ref->{A}->{B}->{$key})  { }
    if (exists $hash{A}{B}{$key})       { }

    if (exists $ref->{A}->{B}->[$ix])   { }
    if (exists $hash{A}{B}[$ix])        { }

    if (exists &{$ref->{A}{B}{$key}})   { }

Although the mostly deeply nested array or hash will not spring into
existence just because its existence was tested, any intervening ones will.
Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
into existence due to the existence test for the $key element above.
This happens anywhere the arrow operator is used, including even here:

    undef $ref;
    if (exists $ref->{"Some key"})    { }
    print $ref;  # prints HASH(0x80d3d5c)

This surprising autovivification in what does not at first--or even
second--glance appear to be an lvalue context may be fixed in a future
release.

Use of a subroutine call, rather than a subroutine name, as an argument
to exists() is an error.

    exists &sub;    # OK
    exists &sub();  # Error

=item exit EXPR
X<exit> X<terminate> X<abort>

=item exit

Evaluates EXPR and exits immediately with that value.    Example:

    $ans = <STDIN>;
    exit 0 if $ans =~ /^[Xx]/;

See also C<die>.  If EXPR is omitted, exits with C<0> status.  The only
universally recognized values for EXPR are C<0> for success and C<1>
for error; other values are subject to interpretation depending on the
environment in which the Perl program is running.  For example, exiting
69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
the mailer to return the item undelivered, but that's not true everywhere.

Don't use C<exit> to abort a subroutine if there's any chance that
someone might want to trap whatever error happened.  Use C<die> instead,
which can be trapped by an C<eval>.

The exit() function does not always exit immediately.  It calls any
defined C<END> routines first, but these C<END> routines may not
themselves abort the exit.  Likewise any object destructors that need to
be called are called before the real exit.  C<END> routines and destructors
can change the exit status by modifying C<$?>. If this is a problem, you
can call C<POSIX:_exit($status)> to avoid END and destructor processing.
See L<perlmod> for details.

Portability issues: L<perlport/exit>.

=item exp EXPR
X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>

=item exp

Returns I<e> (the natural logarithm base) to the power of EXPR.
If EXPR is omitted, gives C<exp($_)>.

=item fcntl FILEHANDLE,FUNCTION,SCALAR
X<fcntl>

Implements the fcntl(2) function.  You'll probably have to say

    use Fcntl;

first to get the correct constant definitions.  Argument processing and
value returned work just like C<ioctl> below.
For example:

    use Fcntl;
    fcntl($filehandle, F_GETFL, $packed_return_buffer)
        or die "can't fcntl F_GETFL: $!";

You don't have to check for C<defined> on the return from C<fcntl>.
Like C<ioctl>, it maps a C<0> return from the system call into
C<"0 but true"> in Perl.  This string is true in boolean context and C<0>
in numeric context.  It is also exempt from the normal B<-w> warnings
on improper numeric conversions.

Note that C<fcntl> raises an exception if used on a machine that
doesn't implement fcntl(2).  See the Fcntl module or your fcntl(2)
manpage to learn what functions are available on your system.

Here's an example of setting a filehandle named C<REMOTE> to be
non-blocking at the system level.  You'll have to negotiate C<$|>
on your own, though.

    use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);

    $flags = fcntl(REMOTE, F_GETFL, 0)
                or die "Can't get flags for the socket: $!\n";

    $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
                or die "Can't set flags for the socket: $!\n";

Portability issues: L<perlport/fcntl>.

=item fileno FILEHANDLE
X<fileno>

Returns the file descriptor for a filehandle, or undefined if the
filehandle is not open.  If there is no real file descriptor at the OS
level, as can happen with filehandles connected to memory objects via
C<open> with a reference for the third argument, -1 is returned.

This is mainly useful for constructing
bitmaps for C<select> and low-level POSIX tty-handling operations.
If FILEHANDLE is an expression, the value is taken as an indirect
filehandle, generally its name.

You can use this to find out whether two handles refer to the
same underlying descriptor:

    if (fileno(THIS) == fileno(THAT)) {
        print "THIS and THAT are dups\n";
    }

=item flock FILEHANDLE,OPERATION
X<flock> X<lock> X<locking>

Calls flock(2), or an emulation of it, on FILEHANDLE.  Returns true
for success, false on failure.  Produces a fatal error if used on a
machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
C<flock> is Perl's portable file-locking interface, although it locks
entire files only, not records.

Two potentially non-obvious but traditional C<flock> semantics are
that it waits indefinitely until the lock is granted, and that its locks
are B<merely advisory>.  Such discretionary locks are more flexible, but
offer fewer guarantees.  This means that programs that do not also use
C<flock> may modify files locked with C<flock>.  See L<perlport>, 
your port's specific documentation, and your system-specific local manpages
for details.  It's best to assume traditional behavior if you're writing
portable programs.  (But if you're not, you should as always feel perfectly
free to write for your own system's idiosyncrasies (sometimes called
"features").  Slavish adherence to portability concerns shouldn't get
in the way of your getting your job done.)

OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
LOCK_NB.  These constants are traditionally valued 1, 2, 8 and 4, but
you can use the symbolic names if you import them from the L<Fcntl> module,
either individually, or as a group using the C<:flock> tag.  LOCK_SH
requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
releases a previously requested lock.  If LOCK_NB is bitwise-or'ed with
LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
waiting for the lock; check the return status to see if you got it.

To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
before locking or unlocking it.

Note that the emulation built with lockf(3) doesn't provide shared
locks, and it requires that FILEHANDLE be open with write intent.  These
are the semantics that lockf(3) implements.  Most if not all systems
implement lockf(3) in terms of fcntl(2) locking, though, so the
differing semantics shouldn't bite too many people.

Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
be open with read intent to use LOCK_SH and requires that it be open
with write intent to use LOCK_EX.

Note also that some versions of C<flock> cannot lock things over the
network; you would need to use the more system-specific C<fcntl> for
that.  If you like you can force Perl to ignore your system's flock(2)
function, and so provide its own fcntl(2)-based emulation, by passing
the switch C<-Ud_flock> to the F<Configure> program when you configure
and build a new Perl.

Here's a mailbox appender for BSD systems.

    use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants

    sub lock {
        my ($fh) = @_;
        flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";

        # and, in case someone appended while we were waiting...
        seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
    }

    sub unlock {
        my ($fh) = @_;
        flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
    }

    open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
        or die "Can't open mailbox: $!";

    lock($mbox);
    print $mbox $msg,"\n\n";
    unlock($mbox);

On systems that support a real flock(2), locks are inherited across fork()
calls, whereas those that must resort to the more capricious fcntl(2)
function lose their locks, making it seriously harder to write servers.

See also L<DB_File> for other flock() examples.

Portability issues: L<perlport/flock>.

=item fork
X<fork> X<child> X<parent>

Does a fork(2) system call to create a new process running the
same program at the same point.  It returns the child pid to the
parent process, C<0> to the child process, or C<undef> if the fork is
unsuccessful.  File descriptors (and sometimes locks on those descriptors)
are shared, while everything else is copied.  On most systems supporting
fork(), great care has gone into making it extremely efficient (for
example, using copy-on-write technology on data pages), making it the
dominant paradigm for multitasking over the last few decades.

Beginning with v5.6.0, Perl attempts to flush all files opened for
output before forking the child process, but this may not be supported
on some platforms (see L<perlport>).  To be safe, you may need to set
C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
C<IO::Handle> on any open handles to avoid duplicate output.

If you C<fork> without ever waiting on your children, you will
accumulate zombies.  On some systems, you can avoid this by setting
C<$SIG{CHLD}> to C<"IGNORE">.  See also L<perlipc> for more examples of
forking and reaping moribund children.

Note that if your forked child inherits system file descriptors like
STDIN and STDOUT that are actually connected by a pipe or socket, even
if you exit, then the remote server (such as, say, a CGI script or a
backgrounded job launched from a remote shell) won't think you're done.
You should reopen those to F</dev/null> if it's any issue.

On some platforms such as Windows, where the fork() system call is not available,
Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to,
at the level of the Perl program, be as compatible as possible with the "Unix" fork().
However it has limitations that have to be considered in code intended to be portable.
See L<perlfork> for more details.

Portability issues: L<perlport/fork>.

=item format
X<format>

Declare a picture format for use by the C<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
X<formline>

This is an internal function used by C<format>s, though you may call it,
too.  It formats (see L<perlform>) a list of values according to the
contents of PICTURE, placing the output into the format output
accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
Eventually, when a C<write> is done, the contents of
C<$^A> are written to some filehandle.  You could also read C<$^A>
and then set C<$^A> back to C<"">.  Note that a format typically
does one C<formline> per line of form, but the C<formline> function itself
doesn't care how many newlines are embedded in the PICTURE.  This means
that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines to implement a single
record format, just like the C<format> compiler.

Be careful if you put double quotes around the picture, because an C<@>
character may be taken to mean the beginning of an array name.
C<formline> always returns true.  See L<perlform> for other examples.

If you are trying to use this instead of C<write> to capture the output,
you may find it easier to open a filehandle to a scalar
(C<< open $fh, ">", \$output >>) and write to that instead.

=item getc FILEHANDLE
X<getc> X<getchar> X<character> X<file, read>

=item getc

Returns the next character from the input file attached to FILEHANDLE,
or the undefined value at end of file or if there was an error (in
the latter case C<$!> is set).  If FILEHANDLE is omitted, reads from
STDIN.  This is not particularly efficient.  However, it cannot be
used by itself to fetch single characters without waiting for the user
to hit enter.  For that, try something more like:

    if ($BSD_STYLE) {
        system "stty cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system "stty", '-icanon', 'eol', "\001";
    }

    $key = getc(STDIN);

    if ($BSD_STYLE) {
        system "stty -cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
        system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
    }
    print "\n";

Determination of whether $BSD_STYLE should be set
is left as an exercise to the reader.

The C<POSIX::getattr> function can do this more portably on
systems purporting POSIX compliance.  See also the C<Term::ReadKey>
module from your nearest CPAN site; details on CPAN can be found under
L<perlmodlib/CPAN>.

=item getlogin
X<getlogin> X<login>

This implements the C library function of the same name, which on most
systems returns the current login from F</etc/utmp>, if any.  If it
returns the empty string, use C<getpwuid>.

    $login = getlogin || getpwuid($<) || "Kilroy";

Do not consider C<getlogin> for authentication: it is not as
secure as C<getpwuid>.

Portability issues: L<perlport/getlogin>.

=item getpeername SOCKET
X<getpeername> X<peer>

Returns the packed sockaddr address of the other end of the SOCKET
connection.

    use Socket;
    $hersockaddr    = getpeername(SOCK);
    ($port, $iaddr) = sockaddr_in($hersockaddr);
    $herhostname    = gethostbyaddr($iaddr, AF_INET);
    $herstraddr     = inet_ntoa($iaddr);

=item getpgrp PID
X<getpgrp> X<group>

Returns the current process group for the specified PID.  Use
a PID of C<0> to get the current process group for the
current process.  Will raise an exception if used on a machine that
doesn't implement getpgrp(2).  If PID is omitted, returns the process
group of the current process.  Note that the POSIX version of C<getpgrp>
does not accept a PID argument, so only C<PID==0> is truly portable.

Portability issues: L<perlport/getpgrp>.

=item getppid
X<getppid> X<parent> X<pid>

Returns the process id of the parent process.

Note for Linux users: on Linux, the C functions C<getpid()> and
C<getppid()> return different values from different threads. In order to
be portable, this behavior is not reflected by the Perl-level function
C<getppid()>, that returns a consistent value across threads. If you want
to call the underlying C<getppid()>, you may use the CPAN module
C<Linux::Pid>.

Portability issues: L<perlport/getppid>.

=item getpriority WHICH,WHO
X<getpriority> X<priority> X<nice>

Returns the current priority for a process, a process group, or a user.
(See L<getpriority(2)>.)  Will raise a fatal exception if used on a
machine that doesn't implement getpriority(2).

Portability issues: L<perlport/getpriority>.

=item getpwnam NAME
X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
X<endnetent> X<endprotoent> X<endservent> 

=item getgrnam NAME

=item gethostbyname NAME

=item getnetbyname NAME

=item getprotobyname NAME

=item getpwuid UID

=item getgrgid GID

=item getservbyname NAME,PROTO

=item gethostbyaddr ADDR,ADDRTYPE

=item getnetbyaddr ADDR,ADDRTYPE

=item getprotobynumber NUMBER

=item getservbyport PORT,PROTO

=item getpwent

=item getgrent

=item gethostent

=item getnetent

=item getprotoent

=item getservent

=item setpwent

=item setgrent

=item sethostent STAYOPEN

=item setnetent STAYOPEN

=item setprotoent STAYOPEN

=item setservent STAYOPEN

=item endpwent

=item endgrent

=item endhostent

=item endnetent

=item endprotoent

=item endservent

These routines are the same as their counterparts in the
system C library.  In list context, the return values from the
various get routines are as follows:

    ($name,$passwd,$uid,$gid,
       $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
    ($name,$passwd,$gid,$members) = getgr*
    ($name,$aliases,$addrtype,$length,@addrs) = gethost*
    ($name,$aliases,$addrtype,$net) = getnet*
    ($name,$aliases,$proto) = getproto*
    ($name,$aliases,$port,$proto) = getserv*

(If the entry doesn't exist you get an empty list.)

The exact meaning of the $gcos field varies but it usually contains
the real name of the user (as opposed to the login name) and other
information pertaining to the user.  Beware, however, that in many
system users are able to change this information and therefore it
cannot be trusted and therefore the $gcos is tainted (see
L<perlsec>).  The $passwd and $shell, user's encrypted password and
login shell, are also tainted, for the same reason.

In scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever it is.
(If the entry doesn't exist you get the undefined value.)  For example:

    $uid   = getpwnam($name);
    $name  = getpwuid($num);
    $name  = getpwent();
    $gid   = getgrnam($name);
    $name  = getgrgid($num);
    $name  = getgrent();
    #etc.

In I<getpw*()> the fields $quota, $comment, and $expire are special
in that they are unsupported on many systems.  If the
$quota is unsupported, it is an empty scalar.  If it is supported, it
usually encodes the disk quota.  If the $comment field is unsupported,
it is an empty scalar.  If it is supported it usually encodes some
administrative comment about the user.  In some systems the $quota
field may be $change or $age, fields that have to do with password
aging.  In some systems the $comment field may be $class.  The $expire
field, if present, encodes the expiration period of the account or the
password.  For the availability and the exact meaning of these fields
in your system, please consult getpwnam(3) and your system's 
F<pwd.h> file.  You can also find out from within Perl what your
$quota and $comment fields mean and whether you have the $expire field
by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.  Shadow password
files are supported only if your vendor has implemented them in the
intuitive fashion that calling the regular C library routines gets the
shadow versions if you're running under privilege or if there exists
the shadow(3) functions as found in System V (this includes Solaris
and Linux).  Those systems that implement a proprietary shadow password
facility are unlikely to be supported.

The $members value returned by I<getgr*()> is a space-separated list of
the login names of the members of the group.

For the I<gethost*()> functions, if the C<h_errno> variable is supported in
C, it will be returned to you via C<$?> if the function call fails.  The
C<@addrs> value returned by a successful call is a list of raw
addresses returned by the corresponding library call.  In the
Internet domain, each address is four bytes long; you can unpack it
by saying something like:

    ($a,$b,$c,$d) = unpack('W4',$addr[0]);

The Socket library makes this slightly easier:

    use Socket;
    $iaddr = inet_aton("127.1"); # or whatever address
    $name  = gethostbyaddr($iaddr, AF_INET);

    # or going the other way
    $straddr = inet_ntoa($iaddr);

In the opposite way, to resolve a hostname to the IP address
you can write this:

    use Socket;
    $packed_ip = gethostbyname("www.perl.org");
    if (defined $packed_ip) {
        $ip_address = inet_ntoa($packed_ip);
    }

Make sure <gethostbyname()> is called in SCALAR context and that
its return value is checked for definedness.

If you get tired of remembering which element of the return list
contains which return value, by-name interfaces are provided
in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
and C<User::grent>.  These override the normal built-ins, supplying
versions that return objects with the appropriate names
for each field.  For example:

   use File::stat;
   use User::pwent;
   $is_his = (stat($filename)->uid == pwent($whoever)->uid);

Even though it looks as though they're the same method calls (uid),
they aren't, because a C<File::stat> object is different from
a C<User::pwent> object.

Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.

=item getsockname SOCKET
X<getsockname>

Returns the packed sockaddr address of this end of the SOCKET connection,
in case you don't know the address because you have several different
IPs that the connection might have come in on.

    use Socket;
    $mysockaddr = getsockname(SOCK);
    ($port, $myaddr) = sockaddr_in($mysockaddr);
    printf "Connect to %s [%s]\n",
       scalar gethostbyaddr($myaddr, AF_INET),
       inet_ntoa($myaddr);

=item getsockopt SOCKET,LEVEL,OPTNAME
X<getsockopt>

Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
Options may exist at multiple protocol levels depending on the socket
type, but at least the uppermost socket level SOL_SOCKET (defined in the
C<Socket> module) will exist. To query options at another level the
protocol number of the appropriate protocol controlling the option
should be supplied. For example, to indicate that an option is to be
interpreted by the TCP protocol, LEVEL should be set to the protocol
number of TCP, which you can get using C<getprotobyname>.

The function returns a packed string representing the requested socket
option, or C<undef> on error, with the reason for the error placed in
C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
consult getsockopt(2) for details.  A common case is that the option is an
integer, in which case the result is a packed integer, which you can decode
using C<unpack> with the C<i> (or C<I>) format.

Here's an example to test whether Nagle's algorithm is enabled on a socket:

    use Socket qw(:all);

    defined(my $tcp = getprotobyname("tcp"))
        or die "Could not determine the protocol number for tcp";
    # my $tcp = IPPROTO_TCP; # Alternative
    my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
        or die "getsockopt TCP_NODELAY: $!";
    my $nodelay = unpack("I", $packed);
    print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";

Portability issues: L<perlport/getsockopt>.

=item given EXPR BLOCK
X<given>

=item given BLOCK

C<given> is analogous to the C<switch> keyword in other languages. C<given>
and C<when> are used in Perl to implement C<switch>/C<case> like statements.
Only available after Perl 5.10.  For example:

    use v5.10;
    given ($fruit) {
        when (/apples?/) {
            print "I like apples."
        }
        when (/oranges?/) {
            print "I don't like oranges."
        }
        default {
            print "I don't like anything"
        }
    }

See L<perlsyn/"Switch statements"> for detailed information.

=item glob EXPR
X<glob> X<wildcard> X<filename, expansion> X<expand>

=item glob

In list context, returns a (possibly empty) list of filename expansions on
the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
scalar context, glob iterates through such filename expansions, returning
undef when the list is exhausted. This is the internal function
implementing the C<< <*.c> >> operator, but you can use it directly. If
EXPR is omitted, C<$_> is used.  The C<< <*.c> >> operator is discussed in
more detail in L<perlop/"I/O Operators">.

Note that C<glob> splits its arguments on whitespace and treats
each segment as separate pattern.  As such, C<glob("*.c *.h")> 
matches all files with a F<.c> or F<.h> extension.  The expression
C<glob(".* *")> matches all files in the current working directory.

If non-empty braces are the only wildcard characters used in the
C<glob>, no filenames are matched, but potentially many strings
are returned.  For example, this produces nine strings, one for
each pairing of fruits and colors:

    @many =  glob "{apple,tomato,cherry}={green,yellow,red}";

Beginning with v5.6.0, this operator is implemented using the standard
C<File::Glob> extension.  See L<File::Glob> for details, including
C<bsd_glob> which does not treat whitespace as a pattern separator.

Portability issues: L<perlport/glob>.

=item gmtime EXPR
X<gmtime> X<UTC> X<Greenwich>

=item gmtime

Works just like L</localtime> but the returned values are
localized for the standard Greenwich time zone.

Note: When called in list context, $isdst, the last value
returned by gmtime, is always C<0>.  There is no
Daylight Saving Time in GMT.

Portability issues: L<perlport/gmtime>.

=item goto LABEL
X<goto> X<jump> X<jmp>

=item goto EXPR

=item goto &NAME

The C<goto-LABEL> form finds the statement labeled with LABEL and
resumes execution there. It can't be used to get out of a block or
subroutine given to C<sort>.  It can be used to go almost anywhere
else within the dynamic scope, including out of subroutines, but it's
usually better to use some other construct such as C<last> or C<die>.
The author of Perl has never felt the need to use this form of C<goto>
(in Perl, that is; C is another matter).  (The difference is that C
does not offer named loops combined with loop control.  Perl does, and
this replaces most structured uses of C<goto> in other languages.)

The C<goto-EXPR> form expects a label name, whose scope will be resolved
dynamically.  This allows for computed C<goto>s per FORTRAN, but isn't
necessarily recommended if you're optimizing for maintainability:

    goto ("FOO", "BAR", "GLARCH")[$i];

As shown in this example, C<goto-EXPR> is exempt from the "looks like a
function" rule. A pair of parentheses following it does not (necessarily)
delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.

Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
deprecated and will issue a warning.  Even then, it may not be used to
go into any construct that requires initialization, such as a
subroutine or a C<foreach> loop.  It also can't be used to go into a
construct that is optimized away.

The C<goto-&NAME> form is quite different from the other forms of
C<goto>.  In fact, it isn't a goto in the normal sense at all, and
doesn't have the stigma associated with other gotos.  Instead, it
exits the current subroutine (losing any changes set by local()) and
immediately calls in its place the named subroutine using the current
value of @_.  This is used by C<AUTOLOAD> subroutines that wish to
load another subroutine and then pretend that the other subroutine had
been called in the first place (except that any modifications to C<@_>
in the current subroutine are propagated to the other subroutine.)
After the C<goto>, not even C<caller> will be able to tell that this
routine was called first.

NAME needn't be the name of a subroutine; it can be a scalar variable
containing a code reference or a block that evaluates to a code
reference.

=item grep BLOCK LIST
X<grep>

=item grep EXPR,LIST

This is similar in spirit to, but not the same as, grep(1) and its
relatives.  In particular, it is not limited to using regular expressions.

Evaluates the BLOCK or EXPR for each element of LIST (locally setting
C<$_> to each element) and returns the list value consisting of those
elements for which the expression evaluated to true.  In scalar
context, returns the number of times the expression was true.

    @foo = grep(!/^#/, @bar);    # weed out comments

or equivalently,

    @foo = grep {!/^#/} @bar;    # weed out comments

Note that C<$_> is an alias to the list value, so it can be used to
modify the elements of the LIST.  While this is useful and supported,
it can cause bizarre results if the elements of LIST are not variables.
Similarly, grep returns aliases into the original list, much as a for
loop's index variable aliases the list elements.  That is, modifying an
element of a list returned by grep (for example, in a C<foreach>, C<map>
or another C<grep>) actually modifies the element in the original list.
This is usually something to be avoided when writing clear code.

If C<$_> is lexical in the scope where the C<grep> appears (because it has
been declared with C<my $_>) then, in addition to being locally aliased to
the list elements, C<$_> keeps being lexical inside the block; i.e., it
can't be seen from the outside, avoiding any potential side-effects.

See also L</map> for a list composed of the results of the BLOCK or EXPR.

=item hex EXPR
X<hex> X<hexadecimal>

=item hex

Interprets EXPR as a hex string and returns the corresponding value.
(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
L</oct>.)  If EXPR is omitted, uses C<$_>.

    print hex '0xAf'; # prints '175'
    print hex 'aF';   # same

Hex strings may only represent integers.  Strings that would cause
integer overflow trigger a warning.  Leading whitespace is not stripped,
unlike oct(). To present something as hex, look into L</printf>,
L</sprintf>, and L</unpack>.

=item import LIST
X<import>

There is no builtin C<import> function.  It is just an ordinary
method (subroutine) defined (or inherited) by modules that wish to export
names to another module.  The C<use> function calls the C<import> method
for the package used.  See also L</use>, L<perlmod>, and L<Exporter>.

=item index STR,SUBSTR,POSITION
X<index> X<indexOf> X<InStr>

=item index STR,SUBSTR

The index function searches for one string within another, but without
the wildcard-like behavior of a full regular-expression pattern match.
It returns the position of the first occurrence of SUBSTR in STR at
or after POSITION.  If POSITION is omitted, starts searching from the
beginning of the string.  POSITION before the beginning of the string
or after its end is treated as if it were the beginning or the end,
respectively.  POSITION and the return value are based at C<0> (or whatever
you've set the C<$[> variable to--but don't do that).  If the substring
is not found, C<index> returns one less than the base, ordinarily C<-1>.

=item int EXPR
X<int> X<integer> X<truncate> X<trunc> X<floor>

=item int

Returns the integer portion of EXPR.  If EXPR is omitted, uses C<$_>.
You should not use this function for rounding: one because it truncates
towards C<0>, and two because machine representations of floating-point
numbers can sometimes produce counterintuitive results.  For example,
C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
because it's really more like -268.99999999999994315658 instead.  Usually,
the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
functions will serve you better than will int().

=item ioctl FILEHANDLE,FUNCTION,SCALAR
X<ioctl>

Implements the ioctl(2) function.  You'll probably first have to say

    require "sys/ioctl.ph";  # probably in $Config{archlib}/sys/ioctl.ph

to get the correct function definitions.  If F<sys/ioctl.ph> doesn't
exist or doesn't have the correct definitions you'll have to roll your
own, based on your C header files such as F<< <sys/ioctl.h> >>.
(There is a Perl script called B<h2ph> that comes with the Perl kit that
may help you in this, but it's nontrivial.)  SCALAR will be read and/or
written depending on the FUNCTION; a C pointer to the string value of SCALAR
will be passed as the third argument of the actual C<ioctl> call.  (If SCALAR
has no string value but does have a numeric value, that value will be
passed rather than a pointer to the string value.  To guarantee this to be
true, add a C<0> to the scalar before using it.)  The C<pack> and C<unpack>
functions may be needed to manipulate the values of structures used by
C<ioctl>.

The return value of C<ioctl> (and C<fcntl>) is as follows:

    if OS returns:      then Perl returns:
        -1               undefined value
         0              string "0 but true"
    anything else           that number

Thus Perl returns true on success and false on failure, yet you can
still easily determine the actual value returned by the operating
system:

    $retval = ioctl(...) || -1;
    printf "System returned %d\n", $retval;

The special string C<"0 but true"> is exempt from B<-w> complaints
about improper numeric conversions.

Portability issues: L<perlport/ioctl>.

=item join EXPR,LIST
X<join>

Joins the separate strings of LIST into a single string with fields
separated by the value of EXPR, and returns that new string.  Example:

    $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);

Beware that unlike C<split>, C<join> doesn't take a pattern as its
first argument.  Compare L</split>.

=item keys HASH
X<keys> X<key>

=item keys ARRAY

=item keys EXPR

Returns a list consisting of all the keys of the named hash, or the indices
of an array. (In scalar context, returns the number of keys or indices.)

The keys of a hash are returned in an apparently random order.  The actual
random order is subject to change in future versions of Perl, but it
is guaranteed to be the same order as either the C<values> or C<each>
function produces (given that the hash has not been modified).  Since
Perl 5.8.1 the ordering can be different even between different runs of
Perl for security reasons (see L<perlsec/"Algorithmic Complexity
Attacks">).

As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
(see L</each>).  In particular, calling keys() in void context resets
the iterator with no other overhead.

Here is yet another way to print your environment:

    @keys = keys %ENV;
    @values = values %ENV;
    while (@keys) {
        print pop(@keys), '=', pop(@values), "\n";
    }

or how about sorted by key:

    foreach $key (sort(keys %ENV)) {
        print $key, '=', $ENV{$key}, "\n";
    }

The returned values are copies of the original keys in the hash, so
modifying them will not affect the original hash.  Compare L</values>.

To sort a hash by value, you'll need to use a C<sort> function.
Here's a descending numeric sort of a hash by its values:

    foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
        printf "%4d %s\n", $hash{$key}, $key;
    }

Used as an lvalue, C<keys> allows you to increase the number of hash buckets
allocated for the given hash.  This can gain you a measure of efficiency if
you know the hash is going to get big.  (This is similar to pre-extending
an array by assigning a larger number to $#array.)  If you say

    keys %hash = 200;

then C<%hash> will have at least 200 buckets allocated for it--256 of them,
in fact, since it rounds up to the next power of two.  These
buckets will be retained even if you do C<%hash = ()>, use C<undef
%hash> if you want to free the storage while C<%hash> is still in scope.
You can't shrink the number of buckets allocated for the hash using
C<keys> in this way (but you needn't worry about doing this by accident,
as trying has no effect). C<keys @array> in an lvalue context is a syntax
error.

Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
a reference to an unblessed hash or array.  The argument will be
dereferenced automatically.  This aspect of C<keys> is considered highly
experimental.  The exact behaviour may change in a future version of Perl.

    for (keys $hashref) { ... }
    for (keys $obj->get_arrayref) { ... }

See also C<each>, C<values>, and C<sort>.

=item kill SIGNAL, LIST
X<kill> X<signal>

Sends a signal to a list of processes.  Returns the number of
processes successfully signaled (which is not necessarily the
same as the number actually killed).

    $cnt = kill 1, $child1, $child2;
    kill 9, @goners;

If SIGNAL is zero, no signal is sent to the process, but C<kill>
checks whether it's I<possible> to send a signal to it (that
means, to be brief, that the process is owned by the same user, or we are
the super-user).  This is useful to check that a child process is still
alive (even if only as a zombie) and hasn't changed its UID.  See
L<perlport> for notes on the portability of this construct.

Unlike in the shell, if SIGNAL is negative, it kills process groups instead
of processes. That means you usually want to use positive not negative signals.
You may also use a signal name in quotes.

The behavior of kill when a I<PROCESS> number is zero or negative depends on
the operating system.  For example, on POSIX-conforming systems, zero will
signal the current process group and -1 will signal all processes.

See L<perlipc/"Signals"> for more details.

On some platforms such as Windows where the fork() system call is not available.
Perl can be built to emulate fork() at the interpreter level.
This emulation has limitations related to kill that have to be considered,
for code running on Windows and in code intended to be portable.

See L<perlfork> for more details.

Portability issues: L<perlport/kill>.

=item last LABEL
X<last> X<break>

=item last

The C<last> command is like the C<break> statement in C (as used in
loops); it immediately exits the loop in question.  If the LABEL is
omitted, the command refers to the innermost enclosing loop.  The
C<continue> block, if any, is not executed:

    LINE: while (<STDIN>) {
        last LINE if /^$/;  # exit when done with header
        #...
    }

C<last> cannot be used to exit a block that returns a value such as
C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
a grep() or map() operation.

Note that a block by itself is semantically identical to a loop
that executes once.  Thus C<last> can be used to effect an early
exit out of such a block.

See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.

=item lc EXPR
X<lc> X<lowercase>

=item lc

Returns a lowercased version of EXPR.  This is the internal function
implementing the C<\L> escape in double-quoted strings.

If EXPR is omitted, uses C<$_>.

What gets returned depends on several factors:

=over

=item If C<use bytes> is in effect:

=over

=item On EBCDIC platforms

The results are what the C language system call C<tolower()> returns.

=item On ASCII platforms

The results follow ASCII semantics.  Only characters C<A-Z> change, to C<a-z>
respectively.

=back

=item Otherwise, If EXPR has the UTF8 flag set

If the current package has a subroutine named C<ToLower>, it will be used to
change the case
(See L<perlunicode/"User-Defined Case Mappings (for serious hackers only)">.)
Otherwise Unicode semantics are used for the case change.

=item Otherwise, if C<use locale> is in effect

Respects current LC_CTYPE locale.  See L<perllocale>.

=item Otherwise, if C<use feature 'unicode_strings'> is in effect:

Unicode semantics are used for the case change.  Any subroutine named
C<ToLower> will be ignored.

=item Otherwise:

=over

=item On EBCDIC platforms

The results are what the C language system call C<tolower()> returns.

=item On ASCII platforms

ASCII semantics are used for the case change.  The lowercase of any character
outside the ASCII range is the character itself.

=back

=back

=item lcfirst EXPR
X<lcfirst> X<lowercase>

=item lcfirst

Returns the value of EXPR with the first character lowercased.  This
is the internal function implementing the C<\l> escape in
double-quoted strings.

If EXPR is omitted, uses C<$_>.

This function behaves the same way under various pragmata, such as in a locale,
as L</lc> does.

=item length EXPR
X<length> X<size>

=item length

Returns the length in I<characters> of the value of EXPR.  If EXPR is
omitted, returns the length of C<$_>.  If EXPR is undefined, returns
C<undef>.

This function cannot be used on an entire array or hash to find out how
many elements these have.  For that, use C<scalar @array> and C<scalar keys
%hash>, respectively.

Like all Perl character operations, length() normally deals in logical
characters, not physical bytes.  For how many bytes a string encoded as
UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
to C<use Encode> first).  See L<Encode> and L<perlunicode>.

=item link OLDFILE,NEWFILE
X<link>

Creates a new filename linked to the old filename.  Returns true for
success, false otherwise.

Portability issues: L<perlport/link>.

=item listen SOCKET,QUEUESIZE
X<listen>

Does the same thing that the listen(2) system call does.  Returns true if
it succeeded, false otherwise.  See the example in
L<perlipc/"Sockets: Client/Server Communication">.

=item local EXPR
X<local>

You really probably want to be using C<my> instead, because C<local> isn't
what most people think of as "local".  See
L<perlsub/"Private Variables via my()"> for details.

A local modifies the listed variables to be local to the enclosing
block, file, or eval.  If more than one value is listed, the list must
be placed in parentheses.  See L<perlsub/"Temporary Values via local()">
for details, including issues with tied arrays and hashes.

The C<delete local EXPR> construct can also be used to localize the deletion
of array/hash elements to the current block.
See L<perlsub/"Localized deletion of elements of composite types">.

=item localtime EXPR
X<localtime> X<ctime>

=item localtime

Converts a time as returned by the time function to a 9-element list
with the time analyzed for the local time zone.  Typically used as
follows:

    #  0    1    2     3     4    5     6     7     8
    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                                localtime(time);

All list elements are numeric and come straight out of the C `struct
tm'.  C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
of the specified time.

C<$mday> is the day of the month and C<$mon> the month in
the range C<0..11>, with 0 indicating January and 11 indicating December.
This makes it easy to get a month name from a list:

    my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
    print "$abbr[$mon] $mday";
    # $mon=9, $mday=18 gives "Oct 18"

C<$year> is the number of years since 1900, B<not> just the last two digits
of the year.  That is, C<$year> is C<123> in year 2023.  The proper way
to get a 4-digit year is simply:

    $year += 1900;

Otherwise you create non-Y2K-compliant programs--and you wouldn't want
to do that, would you?

To get the last two digits of the year (e.g., "01" in 2001) do:

    $year = sprintf("%02d", $year % 100);

C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
Wednesday.  C<$yday> is the day of the year, in the range C<0..364>
(or C<0..365> in leap years.)

C<$isdst> is true if the specified time occurs during Daylight Saving
Time, false otherwise.

If EXPR is omitted, C<localtime()> uses the current time (as returned
by time(3)).

In scalar context, C<localtime()> returns the ctime(3) value:

    $now_string = localtime;  # e.g., "Thu Oct 13 04:54:34 1994"

This scalar value is B<not> locale-dependent but is a Perl builtin. For GMT
instead of local time use the L</gmtime> builtin. See also the
C<Time::Local> module (for converting seconds, minutes, hours, and such back to
the integer value returned by time()), and the L<POSIX> module's strftime(3)
and mktime(3) functions.

To get somewhat similar but locale-dependent date strings, set up your
locale environment variables appropriately (please see L<perllocale>) and
try for example:

    use POSIX qw(strftime);
    $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
    # or for GMT formatted appropriately for your locale:
    $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;

Note that the C<%a> and C<%b>, the short forms of the day of the week
and the month of the year, may not necessarily be three characters wide.

The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
by-name access mechanism to the gmtime() and localtime() functions,
respectively.

For a comprehensive date and time representation look at the
L<DateTime> module on CPAN.

Portability issues: L<perlport/localtime>.

=item lock THING
X<lock>

This function places an advisory lock on a shared variable or referenced
object contained in I<THING> until the lock goes out of scope.

lock() is a "weak keyword" : this means that if you've defined a function
by this name (before any calls to it), that function will be called
instead.  If you are not under C<use threads::shared> this does nothing.
See L<threads::shared>.

=item log EXPR
X<log> X<logarithm> X<e> X<ln> X<base>

=item log

Returns the natural logarithm (base I<e>) of EXPR.  If EXPR is omitted,
returns the log of C<$_>.  To get the
log of another base, use basic algebra:
The base-N log of a number is equal to the natural log of that number
divided by the natural log of N.  For example:

    sub log10 {
        my $n = shift;
        return log($n)/log(10);
    }

See also L</exp> for the inverse operation.

=item lstat EXPR
X<lstat>

=item lstat

Does the same thing as the C<stat> function (including setting the
special C<_> filehandle) but stats a symbolic link instead of the file
the symbolic link points to.  If symbolic links are unimplemented on
your system, a normal C<stat> is done.  For much more detailed
information, please see the documentation for C<stat>.

If EXPR is omitted, stats C<$_>.

Portability issues: L<perlport/lstat>.

=item m//

The match operator.  See L<perlop/"Regexp Quote-Like Operators">.

=item map BLOCK LIST
X<map>

=item map EXPR,LIST

Evaluates the BLOCK or EXPR for each element of LIST (locally setting
C<$_> to each element) and returns the list value composed of the
results of each such evaluation.  In scalar context, returns the
total number of elements so generated.  Evaluates BLOCK or EXPR in
list context, so each element of LIST may produce zero, one, or
more elements in the returned value.

    @chars = map(chr, @numbers);

translates a list of numbers to the corresponding characters.

    my @squares = map { $_ * $_ } @numbers;

translates a list of numbers to their squared values.

    my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;

shows that number of returned elements can differ from the number of
input elements. To omit an element, return an empty list ().
This could also be achieved by writing

    my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;

which makes the intention more clear.

Map always returns a list, which can be
assigned to a hash such that the elements
become key/value pairs. See L<perldata> for more details.

    %hash = map { get_a_key_for($_) => $_ } @array;

is just a funny way to write

    %hash = ();
    foreach (@array) {
        $hash{get_a_key_for($_)} = $_;
    }

Note that C<$_> is an alias to the list value, so it can be used to
modify the elements of the LIST.  While this is useful and supported,
it can cause bizarre results if the elements of LIST are not variables.
Using a regular C<foreach> loop for this purpose would be clearer in
most cases.  See also L</grep> for an array composed of those items of
the original list for which the BLOCK or EXPR evaluates to true.

If C<$_> is lexical in the scope where the C<map> appears (because it has
been declared with C<my $_>), then, in addition to being locally aliased to
the list elements, C<$_> keeps being lexical inside the block; that is, it
can't be seen from the outside, avoiding any potential side-effects.

C<{> starts both hash references and blocks, so C<map { ...> could be either
the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
ahead for the closing C<}> it has to take a guess at which it's dealing with
based on what it finds just after the C<{>. Usually it gets it right, but if it
doesn't it won't realize something is wrong until it gets to the C<}> and
encounters the missing (or unexpected) comma. The syntax error will be
reported close to the C<}>, but you'll need to change something near the C<{>
such as using a unary C<+> to give Perl some help:

    %hash = map {  "\L$_" => 1  } @array  # perl guesses EXPR.  wrong
    %hash = map { +"\L$_" => 1  } @array  # perl guesses BLOCK. right
    %hash = map { ("\L$_" => 1) } @array  # this also works
    %hash = map {  lc($_) => 1  } @array  # as does this.
    %hash = map +( lc($_) => 1 ), @array  # this is EXPR and works!

    %hash = map  ( lc($_), 1 ),   @array  # evaluates to (1, @array)

or to force an anon hash constructor use C<+{>:

   @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end

to get a list of anonymous hashes each with only one entry apiece.

=item mkdir FILENAME,MASK
X<mkdir> X<md> X<directory, create>

=item mkdir FILENAME

=item mkdir

Creates the directory specified by FILENAME, with permissions
specified by MASK (as modified by C<umask>).  If it succeeds it
returns true; otherwise it returns false and sets C<$!> (errno).
MASK defaults to 0777 if omitted, and FILENAME defaults
to C<$_> if omitted.

In general, it is better to create directories with a permissive MASK
and let the user modify that with their C<umask> than it is to supply
a restrictive MASK and give the user no way to be more permissive.
The exceptions to this rule are when the file or directory should be
kept private (mail files, for instance).  The perlfunc(1) entry on
C<umask> discusses the choice of MASK in more detail.

Note that according to the POSIX 1003.1-1996 the FILENAME may have any
number of trailing slashes.  Some operating and filesystems do not get
this right, so Perl automatically removes all trailing slashes to keep
everyone happy.

To recursively create a directory structure, look at
the C<mkpath> function of the L<File::Path> module.

=item msgctl ID,CMD,ARG
X<msgctl>

Calls the System V IPC function msgctl(2).  You'll probably have to say

    use IPC::SysV;

first to get the correct constant definitions.  If CMD is C<IPC_STAT>,
then ARG must be a variable that will hold the returned C<msqid_ds>
structure.  Returns like C<ioctl>: the undefined value for error,
C<"0 but true"> for zero, or the actual return value otherwise.  See also
L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
C<IPC::Semaphore>.

Portability issues: L<perlport/msgctl>.

=item msgget KEY,FLAGS
X<msgget>

Calls the System V IPC function msgget(2).  Returns the message queue
id, or C<undef> on error.  See also
L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
C<IPC::Msg>.

Portability issues: L<perlport/msgget>.

=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
X<msgrcv>

Calls the System V IPC function msgrcv to receive a message from
message queue ID into variable VAR with a maximum message size of
SIZE.  Note that when a message is received, the message type as a
native long integer will be the first thing in VAR, followed by the
actual message.  This packing may be opened with C<unpack("l! a*")>.
Taints the variable.  Returns true if successful, false 
on error.  See also L<perlipc/"SysV IPC"> and the documentation for
C<IPC::SysV> and C<IPC::SysV::Msg>.

Portability issues: L<perlport/msgrcv>.

=item msgsnd ID,MSG,FLAGS
X<msgsnd>

Calls the System V IPC function msgsnd to send the message MSG to the
message queue ID.  MSG must begin with the native long integer message
type, be followed by the length of the actual message, and then finally
the message itself.  This kind of packing can be achieved with
C<pack("l! a*", $type, $message)>.  Returns true if successful,
false on error.  See also the C<IPC::SysV>
and C<IPC::SysV::Msg> documentation.

Portability issues: L<perlport/msgsnd>.

=item my EXPR
X<my>

=item my TYPE EXPR

=item my EXPR : ATTRS

=item my TYPE EXPR : ATTRS

A C<my> declares the listed variables to be local (lexically) to the
enclosing block, file, or C<eval>.  If more than one value is listed,
the list must be placed in parentheses.

The exact semantics and interface of TYPE and ATTRS are still
evolving.  TYPE is currently bound to the use of the C<fields> pragma,
and attributes are handled using the C<attributes> pragma, or starting
from Perl 5.8.0 also via the C<Attribute::Handlers> module.  See
L<perlsub/"Private Variables via my()"> for details, and L<fields>,
L<attributes>, and L<Attribute::Handlers>.

=item next LABEL
X<next> X<continue>

=item next

The C<next> command is like the C<continue> statement in C; it starts
the next iteration of the loop:

    LINE: while (<STDIN>) {
        next LINE if /^#/;  # discard comments
        #...
    }

Note that if there were a C<continue> block on the above, it would get
executed even on discarded lines.  If LABEL is omitted, the command
refers to the innermost enclosing loop.

C<next> cannot be used to exit a block which returns a value such as
C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
a grep() or map() operation.

Note that a block by itself is semantically identical to a loop
that executes once.  Thus C<next> will exit such a block early.

See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.

=item no MODULE VERSION LIST
X<no declarations>
X<unimporting>

=item no MODULE VERSION

=item no MODULE LIST

=item no MODULE

=item no VERSION

See the C<use> function, of which C<no> is the opposite.

=item oct EXPR
X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>

=item oct

Interprets EXPR as an octal string and returns the corresponding
value.  (If EXPR happens to start off with C<0x>, interprets it as a
hex string.  If EXPR starts off with C<0b>, it is interpreted as a
binary string.  Leading whitespace is ignored in all three cases.)
The following will handle decimal, binary, octal, and hex in standard
Perl notation:

    $val = oct($val) if $val =~ /^0/;

If EXPR is omitted, uses C<$_>.   To go the other way (produce a number
in octal), use sprintf() or printf():

    $dec_perms = (stat("filename"))[2] & 07777;
    $oct_perm_str = sprintf "%o", $perms;

The oct() function is commonly used when a string such as C<644> needs
to be converted into a file mode, for example.  Although Perl 
automatically converts strings into numbers as needed, this automatic
conversion assumes base 10.

Leading white space is ignored without warning, as too are any trailing 
non-digits, such as a decimal point (C<oct> only handles non-negative
integers, not negative integers or floating point).

=item open FILEHANDLE,EXPR
X<open> X<pipe> X<file, open> X<fopen>

=item open FILEHANDLE,MODE,EXPR

=item open FILEHANDLE,MODE,EXPR,LIST

=item open FILEHANDLE,MODE,REFERENCE

=item open FILEHANDLE

Opens the file whose filename is given by EXPR, and associates it with
FILEHANDLE.

Simple examples to open a file for reading:

    open(my $fh, "<", "input.txt") 
        or die "cannot open < input.txt: $!";

and for writing:

    open(my $fh, ">", "output.txt") 
        or die "cannot open > output.txt: $!";

(The following is a comprehensive reference to open(): for a gentler
introduction you may consider L<perlopentut>.)

If FILEHANDLE is an undefined scalar variable (or array or hash element), a
new filehandle is autovivified, meaning that the variable is assigned a
reference to a newly allocated anonymous filehandle.  Otherwise if
FILEHANDLE is an expression, its value is the real filehandle.  (This is
considered a symbolic reference, so C<use strict "refs"> should I<not> be
in effect.)

If EXPR is omitted, the global (package) scalar variable of the same
name as the FILEHANDLE contains the filename.  (Note that lexical 
variables--those declared with C<my> or C<state>--will not work for this
purpose; so if you're using C<my> or C<state>, specify EXPR in your
call to open.)

If three (or more) arguments are specified, the open mode (including
optional encoding) in the second argument are distinct from the filename in
the third.  If MODE is C<< < >> or nothing, the file is opened for input.
If MODE is C<< > >>, the file is opened for output, with existing files
first being truncated ("clobbered") and nonexisting files newly created.
If MODE is C<<< >> >>>, the file is opened for appending, again being
created if necessary.

You can put a C<+> in front of the C<< > >> or C<< < >> to
indicate that you want both read and write access to the file; thus
C<< +< >> is almost always preferred for read/write updates--the 
C<< +> >> mode would clobber the file first.  You cant usually use
either read-write mode for updating textfiles, since they have
variable-length records.  See the B<-i> switch in L<perlrun> for a
better approach.  The file is created with permissions of C<0666>
modified by the process's C<umask> value.

These various prefixes correspond to the fopen(3) modes of C<r>,
C<r+>, C<w>, C<w+>, C<a>, and C<a+>.

In the one- and two-argument forms of the call, the mode and filename
should be concatenated (in that order), preferably separated by white
space.  You can--but shouldn't--omit the mode in these forms when that mode
is C<< < >>.  It is always safe to use the two-argument form of C<open> if
the filename argument is a known literal.

For three or more arguments if MODE is C<|->, the filename is
interpreted as a command to which output is to be piped, and if MODE
is C<-|>, the filename is interpreted as a command that pipes
output to us.  In the two-argument (and one-argument) form, one should
replace dash (C<->) with the command.
See L<perlipc/"Using open() for IPC"> for more examples of this.
(You are not allowed to C<open> to a command that pipes both in I<and>
out, but see L<IPC::Open2>, L<IPC::Open3>, and
L<perlipc/"Bidirectional Communication with Another Process"> for
alternatives.)

In the form of pipe opens taking three or more arguments, if LIST is specified
(extra arguments after the command name) then LIST becomes arguments
to the command invoked if the platform supports it.  The meaning of
C<open> with more than three arguments for non-pipe modes is not yet
defined, but experimental "layers" may give extra LIST arguments
meaning.

In the two-argument (and one-argument) form, opening C<< <- >> 
or C<-> opens STDIN and opening C<< >- >> opens STDOUT.

You may (and usually should) use the three-argument form of open to specify
I/O layers (sometimes referred to as "disciplines") to apply to the handle
that affect how the input and output are processed (see L<open> and
L<PerlIO> for more details). For example:

  open(my $fh, "<:encoding(UTF-8)", "filename")
    || die "can't open UTF-8 encoded filename: $!";

opens the UTF8-encoded file containing Unicode characters;
see L<perluniintro>. Note that if layers are specified in the
three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
usually set by the B<open> pragma or the switch B<-CioD>) are ignored.

Open returns nonzero on success, the undefined value otherwise.  If
the C<open> involved a pipe, the return value happens to be the pid of
the subprocess.

If you're running Perl on a system that distinguishes between text
files and binary files, then you should check out L</binmode> for tips
for dealing with this.  The key distinction between systems that need
C<binmode> and those that don't is their text file formats.  Systems
like Unix, Mac OS, and Plan 9, that end lines with a single
character and encode that character in C as C<"\n"> do not
need C<binmode>.  The rest need it.

When opening a file, it's seldom a good idea to continue 
if the request failed, so C<open> is frequently used with
C<die>.  Even if C<die> won't do what you want (say, in a CGI script,
where you want to format a suitable error message (but there are
modules that can help with that problem)) always check
the return value from opening a file.  

As a special case the three-argument form with a read/write mode and the third
argument being C<undef>:

    open(my $tmp, "+>", undef) or die ...

opens a filehandle to an anonymous temporary file.  Also using C<< +< >>
works for symmetry, but you really should consider writing something
to the temporary file first.  You will need to seek() to do the
reading.

Since v5.8.0, Perl has built using PerlIO by default.  Unless you've
changed this (such as building Perl with C<Configure -Uuseperlio>), you can
open filehandles directly to Perl scalars via:

    open($fh, ">", \$variable) || ..

To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:

    close STDOUT;
    open(STDOUT, ">", \$variable)
        or die "Can't open STDOUT: $!";

General examples:

    $ARTICLE = 100;
    open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
    while (<ARTICLE>) {...

    open(LOG, ">>/usr/spool/news/twitlog");  # (log is reserved)
    # if the open fails, output is discarded

    open(my $dbase, "+<", "dbase.mine")      # open for update
        or die "Can't open 'dbase.mine' for update: $!";

    open(my $dbase, "+<dbase.mine")          # ditto
        or die "Can't open 'dbase.mine' for update: $!";

    open(ARTICLE, "-|", "caesar <$article")  # decrypt article
        or die "Can't start caesar: $!";

    open(ARTICLE, "caesar <$article |")      # ditto
        or die "Can't start caesar: $!";

    open(EXTRACT, "|sort >Tmp$$")            # $$ is our process id
        or die "Can't start sort: $!";

    # in-memory files
    open(MEMORY, ">", \$var)
        or die "Can't open memory file: $!";
    print MEMORY "foo!\n";                   # output will appear in $var

    # process argument list of files along with any includes

    foreach $file (@ARGV) {
        process($file, "fh00");
    }

    sub process {
        my($filename, $input) = @_;
        $input++;    # this is a string increment
        unless (open($input, "<", $filename)) {
            print STDERR "Can't open $filename: $!\n";
            return;
        }

        local $_;
        while (<$input>) {    # note use of indirection
            if (/^#include "(.*)"/) {
                process($1, $input);
                next;
            }
            #...          # whatever
        }
    }

See L<perliol> for detailed info on PerlIO.

You may also, in the Bourne shell tradition, specify an EXPR beginning
with C<< >& >>, in which case the rest of the string is interpreted
as the name of a filehandle (or file descriptor, if numeric) to be
duped (as C<dup(2)>) and opened.  You may use C<&> after C<< > >>,
C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
The mode you specify should match the mode of the original filehandle.
(Duping a filehandle does not take into account any existing contents
of IO buffers.) If you use the three-argument form, then you can pass either a
number, the name of a filehandle, or the normal "reference to a glob".

Here is a script that saves, redirects, and restores C<STDOUT> and
C<STDERR> using various methods:

    #!/usr/bin/perl
    open(my $oldout, ">&STDOUT")     or die "Can't dup STDOUT: $!";
    open(OLDERR,     ">&", \*STDERR) or die "Can't dup STDERR: $!";

    open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
    open(STDERR, ">&STDOUT")     or die "Can't dup STDOUT: $!";

    select STDERR; $| = 1;  # make unbuffered
    select STDOUT; $| = 1;  # make unbuffered

    print STDOUT "stdout 1\n";  # this works for
    print STDERR "stderr 1\n";  # subprocesses too

    open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
    open(STDERR, ">&OLDERR")    or die "Can't dup OLDERR: $!";

    print STDOUT "stdout 2\n";
    print STDERR "stderr 2\n";

If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
that file descriptor (and not call C<dup(2)>); this is more
parsimonious of file descriptors.  For example:

    # open for input, reusing the fileno of $fd
    open(FILEHANDLE, "<&=$fd")

or

    open(FILEHANDLE, "<&=", $fd)

or

    # open for append, using the fileno of OLDFH
    open(FH, ">>&=", OLDFH)

or

    open(FH, ">>&=OLDFH")

Being parsimonious on filehandles is also useful (besides being
parsimonious) for example when something is dependent on file
descriptors, like for example locking using flock().  If you do just
C<< open(A, ">>&B") >>, the filehandle A will not have the same file
descriptor as B, and therefore flock(A) will not flock(B) nor vice
versa.  But with C<< open(A, ">>&=B") >>, the filehandles will share
the same underlying system file descriptor.

Note that under Perls older than 5.8.0, Perl uses the standard C library's'
fdopen() to implement the C<=> functionality.  On many Unix systems,
fdopen() fails when file descriptors exceed a certain value, typically 255.
For Perls 5.8.0 and later, PerlIO is (most often) the default.

You can see whether your Perl was built with PerlIO by running C<perl -V>
and looking for the C<useperlio=> line.  If C<useperlio> is C<define>, you
have PerlIO; otherwise you don't.

If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
with the one- or two-argument forms of C<open>), 
an implicit C<fork> is done, so C<open> returns twice: in the parent
process it returns the pid
of the child process, and in the child process it returns (a defined) C<0>.
Use C<defined($pid)> or C<//> to determine whether the open was successful.

For example, use either

    $child_pid = open(FROM_KID, "-|")   // die "can't fork: $!";

or
    $child_pid = open(TO_KID,   "|-")   // die "can't fork: $!";

followed by 

    if ($child_pid) {
        # am the parent:
        # either write TO_KID or else read FROM_KID
        ...
        wait $child_pid;
    } else {
        # am the child; use STDIN/STDOUT normally
        ...
        exit;
    } 

The filehandle behaves normally for the parent, but I/O to that
filehandle is piped from/to the STDOUT/STDIN of the child process.
In the child process, the filehandle isn't opened--I/O happens from/to
the new STDOUT/STDIN.  Typically this is used like the normal
piped open when you want to exercise more control over just how the
pipe command gets executed, such as when running setuid and
you don't want to have to scan shell commands for metacharacters.

The following blocks are more or less equivalent:

    open(FOO, "|tr '[a-z]' '[A-Z]'");
    open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
    open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
    open(FOO, "|-", "tr", '[a-z]', '[A-Z]');

    open(FOO, "cat -n '$file'|");
    open(FOO, "-|", "cat -n '$file'");
    open(FOO, "-|") || exec "cat", "-n", $file;
    open(FOO, "-|", "cat", "-n", $file);

The last two examples in each block show the pipe as "list form", which is
not yet supported on all platforms.  A good rule of thumb is that if
your platform has a real C<fork()> (in other words, if your platform is
Unix, including Linux and MacOS X), you can use the list form.  You would 
want to use the list form of the pipe so you can pass literal arguments
to the command without risk of the shell interpreting any shell metacharacters
in them.  However, this also bars you from opening pipes to commands
that intentionally contain shell metacharacters, such as:

    open(FOO, "|cat -n | expand -4 | lpr")
        // die "Can't open pipeline to lpr: $!";

See L<perlipc/"Safe Pipe Opens"> for more examples of this.

Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before any operation that may do a fork, but this may not be
supported on some platforms (see L<perlport>).  To be safe, you may need
to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
of C<IO::Handle> on any open handles.

On systems that support a close-on-exec flag on files, the flag will
be set for the newly opened file descriptor as determined by the value
of C<$^F>.  See L<perlvar/$^F>.

Closing any piped filehandle causes the parent process to wait for the
child to finish, then returns the status value in C<$?> and
C<${^CHILD_ERROR_NATIVE}>.

The filename passed to the one- and two-argument forms of open() will
have leading and trailing whitespace deleted and normal
redirection characters honored.  This property, known as "magic open",
can often be used to good effect.  A user could specify a filename of
F<"rsh cat file |">, or you could change certain filenames as needed:

    $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
    open(FH, $filename) or die "Can't open $filename: $!";

Use the three-argument form to open a file with arbitrary weird characters in it,

    open(FOO, "<", $file)
        || die "can't open < $file: $!";

otherwise it's necessary to protect any leading and trailing whitespace:

    $file =~ s#^(\s)#./$1#;
    open(FOO, "< $file\0")
        || die "open failed: $!";

(this may not work on some bizarre filesystems).  One should
conscientiously choose between the I<magic> and I<three-argument> form
of open():

    open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";

will allow the user to specify an argument of the form C<"rsh cat file |">,
but will not work on a filename that happens to have a trailing space, while

    open(IN, "<", $ARGV[0])
        || die "can't open < $ARGV[0]: $!";

will have exactly the opposite restrictions.

If you want a "real" C C<open> (see L<open(2)> on your system), then you
should use the C<sysopen> function, which involves no such magic (but may
use subtly different filemodes than Perl open(), which is mapped to C
fopen()).  This is another way to protect your filenames from
interpretation.  For example:

    use IO::Handle;
    sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
        or die "sysopen $path: $!";
    $oldfh = select(HANDLE); $| = 1; select($oldfh);
    print HANDLE "stuff $$\n";
    seek(HANDLE, 0, 0);
    print "File contains: ", <HANDLE>;

Using the constructor from the C<IO::Handle> package (or one of its
subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
filehandles that have the scope of the variables used to hold them, then
automatically (but silently) close once their reference counts become
zero, typically at scope exit:

    use IO::File;
    #...
    sub read_myfile_munged {
        my $ALL = shift;
        # or just leave it undef to autoviv
        my $handle = IO::File->new;
        open($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.
        return $first;                      # Or here.
    }

B<WARNING:> The previous example has a bug because the automatic
close that happens when the refcount on C<handle> does not
properly detect and report failures.  I<Always> close the handle
yourself and inspect the return value.

    close($handle) 
        || warn "close failed: $!";

See L</seek> for some details about mixing reading and writing.

Portability issues: L<perlport/open>.

=item opendir DIRHANDLE,EXPR
X<opendir>

Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
C<seekdir>, C<rewinddir>, and C<closedir>.  Returns true if successful.
DIRHANDLE may be an expression whose value can be used as an indirect
dirhandle, usually the real dirhandle name.  If DIRHANDLE is an undefined
scalar variable (or array or hash element), the variable is assigned a
reference to a new anonymous dirhandle; that is, it's autovivified.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.

See the example at C<readdir>.

=item ord EXPR
X<ord> X<encoding>

=item ord

Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
or Unicode) value of the first character of EXPR.  
If EXPR is an empty string, returns 0.  If EXPR is omitted, uses C<$_>.
(Note I<character>, not byte.)

For the reverse, see L</chr>.
See L<perlunicode> for more about Unicode.

=item our EXPR
X<our> X<global>

=item our TYPE EXPR

=item our EXPR : ATTRS

=item our TYPE EXPR : ATTRS

C<our> associates a simple name with a package variable in the current
package for use within the current scope.  When C<use strict 'vars'> is in
effect, C<our> lets you use declared global variables without qualifying
them with package names, within the lexical scope of the C<our> declaration.
In this way C<our> differs from C<use vars>, which is package-scoped.

Unlike C<my> or C<state>, which allocates storage for a variable and
associates a simple name with that storage for use within the current
scope, C<our> associates a simple name with a package (read: global)
variable in the current package, for use within the current lexical scope.
In other words, C<our> has the same scoping rules as C<my> or C<state>, but
does not necessarily create a variable.

If more than one value is listed, the list must be placed
in parentheses.

    our $foo;
    our($bar, $baz);

An C<our> declaration declares a global variable that will be visible
across its entire lexical scope, even across package boundaries.  The
package in which the variable is entered is determined at the point
of the declaration, not at the point of use.  This means the following
behavior holds:

    package Foo;
    our $bar;      # declares $Foo::bar for rest of lexical scope
    $bar = 20;

    package Bar;
    print $bar;    # prints 20, as it refers to $Foo::bar

Multiple C<our> declarations with the same name in the same lexical
scope are allowed if they are in different packages.  If they happen
to be in the same package, Perl will emit warnings if you have asked
for them, just like multiple C<my> declarations.  Unlike a second
C<my> declaration, which will bind the name to a fresh variable, a
second C<our> declaration in the same package, in the same scope, is
merely redundant.

    use warnings;
    package Foo;
    our $bar;      # declares $Foo::bar for rest of lexical scope
    $bar = 20;

    package Bar;
    our $bar = 30; # declares $Bar::bar for rest of lexical scope
    print $bar;    # prints 30

    our $bar;      # emits warning but has no other effect
    print $bar;    # still prints 30

An C<our> declaration may also have a list of attributes associated
with it.

The exact semantics and interface of TYPE and ATTRS are still
evolving.  TYPE is currently bound to the use of C<fields> pragma,
and attributes are handled using the C<attributes> pragma, or starting
from Perl 5.8.0 also via the C<Attribute::Handlers> module.  See
L<perlsub/"Private Variables via my()"> for details, and L<fields>,
L<attributes>, and L<Attribute::Handlers>.

=item pack TEMPLATE,LIST
X<pack>

Takes a LIST of values and converts it into a string using the rules
given by the TEMPLATE.  The resulting string is the concatenation of
the converted values.  Typically, each converted value looks
like its machine-level representation.  For example, on 32-bit machines
an integer may be represented by a sequence of 4 bytes, which  will in
Perl be presented as a string that's 4 characters long. 

See L<perlpacktut> for an introduction to this function.

The TEMPLATE is a sequence of characters that give the order and type
of values, as follows:

    a  A string with arbitrary binary data, will be null padded.
    A  A text (ASCII) string, will be space padded.
    Z  A null-terminated (ASCIZ) string, will be null padded.

    b  A bit string (ascending bit order inside each byte, like vec()).
    B  A bit string (descending bit order inside each byte).
    h  A hex string (low nybble first).
    H  A hex string (high nybble first).

    c  A signed char (8-bit) value.
    C  An unsigned char (octet) value.
    W  An unsigned char value (can be greater than 255).

    s  A signed short (16-bit) value.
    S  An unsigned short value.

    l  A signed long (32-bit) value.
    L  An unsigned long value.

    q  A signed quad (64-bit) value.
    Q  An unsigned quad value.
      (Quads are available only if your system supports 64-bit
       integer values _and_ if Perl has been compiled to support those.
           Raises an exception otherwise.)

    i  A signed integer value.
    I  A unsigned integer value.
      (This 'integer' is _at_least_ 32 bits wide.  Its exact
           size depends on what a local C compiler calls 'int'.)

    n  An unsigned short (16-bit) in "network" (big-endian) order.
    N  An unsigned long (32-bit) in "network" (big-endian) order.
    v  An unsigned short (16-bit) in "VAX" (little-endian) order.
    V  An unsigned long (32-bit) in "VAX" (little-endian) order.

    j   A Perl internal signed integer value (IV).
    J   A Perl internal unsigned integer value (UV).

    f  A single-precision float in native format.
    d  A double-precision float in native format.

    F  A Perl internal floating-point value (NV) in native format
    D  A float of long-double precision in native format.
      (Long doubles are available only if your system supports long
       double values _and_ if Perl has been compiled to support those.
           Raises an exception otherwise.)

    p  A pointer to a null-terminated string.
    P  A pointer to a structure (fixed-length string).

    u  A uuencoded string.
    U  A Unicode character number.  Encodes to a character in character mode
        and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.

    w  A BER compressed integer (not an ASN.1 BER, see perlpacktut for
       details).  Its bytes represent an unsigned integer in base 128,
       most significant digit first, with as few digits as possible.  Bit
       eight (the high bit) is set on each byte except the last.

    x  A null byte (a.k.a ASCII NUL, "\000", chr(0))
    X  Back up a byte.
    @  Null-fill or truncate to absolute position, counted from the
       start of the innermost ()-group.
    .  Null-fill or truncate to absolute position specified by the value.
    (  Start of a ()-group.

One or more modifiers below may optionally follow certain letters in the
TEMPLATE (the second column lists letters for which the modifier is valid):

    !   sSlLiI     Forces native (short, long, int) sizes instead
                   of fixed (16-/32-bit) sizes.

        xX         Make x and X act as alignment commands.

        nNvV       Treat integers as signed instead of unsigned.

        @.         Specify position as byte offset in the internal
                   representation of the packed string. Efficient but
                   dangerous.

    >   sSiIlLqQ   Force big-endian byte-order on the type.
        jJfFdDpP   (The "big end" touches the construct.)

    <   sSiIlLqQ   Force little-endian byte-order on the type.
        jJfFdDpP   (The "little end" touches the construct.)

The C<< > >> and C<< < >> modifiers can also be used on C<()> groups 
to force a particular byte-order on all components in that group, 
including all its subgroups.

The following rules apply:

=over 

=item *

Each letter may optionally be followed by a number indicating the repeat
count.  A numeric repeat count may optionally be enclosed in brackets, as
in C<pack("C[80]", @arr)>.  The repeat count gobbles that many values from
the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
something else, dscribed below.  Supplying a C<*> for the repeat count
instead of a number means to use however many items are left, except for:

=over 

=item * 

C<@>, C<x>, and C<X>, where it is equivalent to C<0>.

=item * 

<.>, where it means relative to the start of the string.

=item * 

C<u>, where it is equivalent to 1 (or 45, which here is equivalent).

=back 

One can replace a numeric repeat count with a template letter enclosed in
brackets to use the packed byte length of the bracketed template for the
repeat count.

For example, the template C<x[L]> skips as many bytes as in a packed long,
and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
variable-expanded) unpacks.  If the template in brackets contains alignment
commands (such as C<x![d]>), its packed length is calculated as if the
start of the template had the maximal possible alignment.

When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
trailing null byte, so the resulting string is always one byte longer than
the byte length of the item itself.

When used with C<@>, the repeat count represents an offset from the start
of the innermost C<()> group.

When used with C<.>, the repeat count determines the starting position to
calculate the value offset as follows:

=over 

=item *

If the repeat count is C<0>, it's relative to the current position.

=item *

If the repeat count is C<*>, the offset is relative to the start of the
packed string.

=item *

And if it's an integer I<n>, the offset is relative to the start of the
I<n>th innermost C<( )> group, or to the start of the string if I<n> is
bigger then the group level.

=back

The repeat count for C<u> is interpreted as the maximal number of bytes
to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat 
count should not be more than 65.

=item *

The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
string of length count, padding with nulls or spaces as needed.  When
unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
after the first null, and C<a> returns data with no stripping at all.

If the value to pack is too long, the result is truncated.  If it's too
long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
followed by a null byte.  Thus C<Z> always packs a trailing null, except
when the count is 0.

=item *

Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
Each such format generates 1 bit of the result.  These are typically followed
by a repeat count like C<B8> or C<B64>.

Each result bit is based on the least-significant bit of the corresponding
input character, i.e., on C<ord($char)%2>.  In particular, characters C<"0">
and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.

Starting from the beginning of the input string, each 8-tuple
of characters is converted to 1 character of output.  With format C<b>,
the first character of the 8-tuple determines the least-significant bit of a
character; with format C<B>, it determines the most-significant bit of
a character.

If the length of the input string is not evenly divisible by 8, the
remainder is packed as if the input string were padded by null characters
at the end.  Similarly during unpacking, "extra" bits are ignored.

If the input string is longer than needed, remaining characters are ignored.

A C<*> for the repeat count uses all characters of the input field.  
On unpacking, bits are converted to a string of C<0>s and C<1>s.

=item *

The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.

For each such format, pack() generates 4 bits of result.
With non-alphabetical characters, the result is based on the 4 least-significant
bits of the input character, i.e., on C<ord($char)%16>.  In particular,
characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
C<"\000"> and C<"\001">.  For characters C<"a".."f"> and C<"A".."F">, the result
is compatible with the usual hexadecimal digits, so that C<"a"> and
C<"A"> both generate the nybble C<0xA==10>.  Use only these specific hex 
characters with this format.

Starting from the beginning of the template to pack(), each pair
of characters is converted to 1 character of output.  With format C<h>, the
first character of the pair determines the least-significant nybble of the
output character; with format C<H>, it determines the most-significant
nybble.

If the length of the input string is not even, it behaves as if padded by
a null character at the end.  Similarly, "extra" nybbles are ignored during
unpacking.

If the input string is longer than needed, extra characters are ignored.

A C<*> for the repeat count uses all characters of the input field.  For
unpack(), nybbles are converted to a string of hexadecimal digits.

=item *

The C<p> format packs a pointer to a null-terminated string.  You are
responsible for ensuring that the string is not a temporary value, as that
could potentially get deallocated before you got around to using the packed
result.  The C<P> format packs a pointer to a structure of the size indicated
by the length.  A null pointer is created if the corresponding value for
C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
unpacks into C<undef>.

If your system has a strange pointer size--meaning a pointer is neither as
big as an int nor as big as a long--it may not be possible to pack or
unpack pointers in big- or little-endian byte order.  Attempting to do
so raises an exception.

=item *

The C</> template character allows packing and unpacking of a sequence of
items where the packed structure contains a packed item count followed by
the packed items themselves.  This is useful when the structure you're
unpacking has encoded the sizes or repeat counts for some of its fields
within the structure itself as separate fields.

For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
I<length-item> describes how the length value is packed. Formats likely
to be of most use are integer-packing ones like C<n> for Java strings,
C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.

For C<pack>, I<sequence-item> may have a repeat count, in which case
the minimum of that and the number of available items is used as the argument
for I<length-item>. If it has no repeat count or uses a '*', the number
of available items is used.

For C<unpack>, an internal stack of integer arguments unpacked so far is
used. You write C</>I<sequence-item> and the repeat count is obtained by
popping off the last element from the stack. The I<sequence-item> must not
have a repeat count.

If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
the I<length-item> is the string length, not the number of strings.  With
an explicit repeat count for pack, the packed string is adjusted to that
length.  For example:

    unpack("W/a", "\004Gurusamy")           gives ("Guru")
    unpack("a3/A A*", "007 Bond  J ")       gives (" Bond", "J")
    unpack("a3 x2 /A A*", "007: Bond, J.")  gives ("Bond, J", ".")

    pack("n/a* w/a","hello,","world")       gives "\000\006hello,\005world"
    pack("a/W2", ord("a") .. ord("z"))      gives "2ab"

The I<length-item> is not returned explicitly from C<unpack>.

Supplying a count to the I<length-item> format letter is only useful with
C<A>, C<a>, or C<Z>.  Packing with a I<length-item> of C<a> or C<Z> may
introduce C<"\000"> characters, which Perl does not regard as legal in
numeric strings.

=item *

The integer types C<s>, C<S>, C<l>, and C<L> may be
followed by a C<!> modifier to specify native shorts or
longs.  As shown in the example above, a bare C<l> means
exactly 32 bits, although the native C<long> as seen by the local C compiler
may be larger.  This is mainly an issue on 64-bit platforms.  You can
see whether using C<!> makes any difference this way:

    printf "format s is %d, s! is %d\n", 
        length pack("s"), length pack("s!");

    printf "format l is %d, l! is %d\n", 
        length pack("l"), length pack("l!");


C<i!> and C<I!> are also allowed, but only for completeness' sake:
they are identical to C<i> and C<I>.

The actual sizes (in bytes) of native shorts, ints, longs, and long
longs on the platform where Perl was built are also available from
the command line:

    $ perl -V:{short,int,long{,long}}size
    shortsize='2';
    intsize='4';
    longsize='4';
    longlongsize='8';

or programmatically via the C<Config> module:

       use Config;
       print $Config{shortsize},    "\n";
       print $Config{intsize},      "\n";
       print $Config{longsize},     "\n";
       print $Config{longlongsize}, "\n";

C<$Config{longlongsize}> is undefined on systems without 
long long support.

=item *

The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
inherently non-portable between processors and operating systems because
they obey native byteorder and endianness.  For example, a 4-byte integer
0x12345678 (305419896 decimal) would be ordered natively (arranged in and
handled by the CPU registers) into bytes as

    0x12 0x34 0x56 0x78  # big-endian
    0x78 0x56 0x34 0x12  # little-endian

Basically, Intel and VAX CPUs are little-endian, while everybody else,
including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
big-endian.  Alpha and MIPS can be either: Digital/Compaq uses (well, used) 
them in little-endian mode, but SGI/Cray uses them in big-endian mode.

The names I<big-endian> and I<little-endian> are comic references to the
egg-eating habits of the little-endian Lilliputians and the big-endian
Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
This entered computer lingo via the paper "On Holy Wars and a Plea for
Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.

Some systems may have even weirder byte orders such as

   0x56 0x78 0x12 0x34
   0x34 0x12 0x78 0x56

You can determine your system endianness with this incantation:

   printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678); 

The byteorder on the platform where Perl was built is also available
via L<Config>:

    use Config;
    print "$Config{byteorder}\n";

or from the command line:

    $ perl -V:byteorder

Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
and C<"87654321"> are big-endian.

For portably packed integers, either use the formats C<n>, C<N>, C<v>, 
and C<V> or else use the C<< > >> and C<< < >> modifiers described
immediately below.  See also L<perlport>.

=item *

Starting with Perl 5.9.2, integer and floating-point formats, along with
the C<p> and C<P> formats and C<()> groups, may all be followed by the 
C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
or little-endian byte-order.  These modifiers are especially useful 
given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers, 
64-bit integers, or floating-point values.

Here are some concerns to keep in mind when using an endianness modifier:

=over

=item * 

Exchanging signed integers between different platforms works only 
when all platforms store them in the same format.  Most platforms store
signed integers in two's-complement notation, so usually this is not an issue.

=item * 

The C<< > >> or C<< < >> modifiers can only be used on floating-point
formats on big- or little-endian machines.  Otherwise, attempting to
use them raises an exception.

=item * 

Forcing big- or little-endian byte-order on floating-point values for
data exchange can work only if all platforms use the same
binary representation such as IEEE floating-point.  Even if all
platforms are using IEEE, there may still be subtle differences.  Being able
to use C<< > >> or C<< < >> on floating-point values can be useful,
but also dangerous if you don't know exactly what you're doing.
It is not a general way to portably store floating-point values.

=item * 

When using C<< > >> or C<< < >> on a C<()> group, this affects
all types inside the group that accept byte-order modifiers,
including all subgroups.  It is silently ignored for all other
types.  You are not allowed to override the byte-order within a group
that already has a byte-order modifier suffix.

=back

=item *

Real numbers (floats and doubles) are in native machine format only.
Due to the multiplicity of floating-point formats and the lack of a
standard "network" representation for them, no facility for interchange has been
made.  This means that packed floating-point data written on one machine
may not be readable on another, even if both use IEEE floating-point
arithmetic (because the endianness of the memory representation is not part
of the IEEE spec).  See also L<perlport>.

If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
modifiers to force big- or little-endian byte-order on floating-point values.

Because Perl uses doubles (or long doubles, if configured) internally for
all numeric calculation, converting from double into float and thence 
to double again loses precision, so C<unpack("f", pack("f", $foo)>)
will not in general equal $foo.

=item *

Pack and unpack can operate in two modes: character mode (C<C0> mode) where
the packed string is processed per character, and UTF-8 mode (C<U0> mode)
where the packed string is processed in its UTF-8-encoded Unicode form on
a byte-by-byte basis. Character mode is the default unless the format string 
starts with C<U>. You can always switch mode mid-format with an explicit 
C<C0> or C<U0> in the format.  This mode remains in effect until the next 
mode change, or until the end of the C<()> group it (directly) applies to.

Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode 
bytes is not necessarily obvious.   Probably only the first of these
is what you want:

    $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | 
      perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
    03B1.03C9
    $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | 
      perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
    CE.B1.CF.89
    $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | 
      perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
    CE.B1.CF.89
    $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | 
      perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
    C3.8E.C2.B1.C3.8F.C2.89

Those examples also illustrate that you should not try to use
C<pack>/C<unpack> as a substitute for the L<Encode> module.

=item *

You must yourself do any alignment or padding by inserting, for example,
enough C<"x">es while packing.  There is no way for pack() and unpack()
to know where characters are going to or coming from, so they 
handle their output and input as flat sequences of characters.

=item *

A C<()> group is a sub-TEMPLATE enclosed in parentheses.  A group may
take a repeat count either as postfix, or for unpack(), also via the C</>
template character.  Within each repetition of a group, positioning with
C<@> starts over at 0. Therefore, the result of

    pack("@1A((@2A)@3A)", qw[X Y Z])

is the string C<"\0X\0\0YZ">.

=item *

C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
jump forward or back to the closest position aligned at a multiple of C<count>
characters. For example, to pack() or unpack() a C structure like

    struct {
        char   c;    /* one signed, 8-bit character */
        double d; 
        char   cc[2];
    }

one may need to use the template C<c x![d] d c[2]>.  This assumes that
doubles must be aligned to the size of double.

For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
both are no-ops.

=item *

C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
represent signed 16-/32-bit integers in big-/little-endian order.
This is portable only when all platforms sharing packed data use the
same binary representation for signed integers; for example, when all
platforms use two's-complement representation.

=item *

Comments can be embedded in a TEMPLATE using C<#> through the end of line.
White space can separate pack codes from each other, but modifiers and
repeat counts must follow immediately.  Breaking complex templates into
individual line-by-line components, suitably annotated, can do as much to
improve legibility and maintainability of pack/unpack formats as C</x> can
for complicated pattern matches.

=item *

If TEMPLATE requires more arguments than pack() is given, pack()
assumes additional C<""> arguments.  If TEMPLATE requires fewer arguments
than given, extra arguments are ignored.

=back

Examples:

    $foo = pack("WWWW",65,66,67,68);
    # foo eq "ABCD"
    $foo = pack("W4",65,66,67,68);
    # same thing
    $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
    # same thing with Unicode circled letters.
    $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
    # same thing with Unicode circled letters. You don't get the UTF-8
    # bytes because the U at the start of the format caused a switch to
    # U0-mode, so the UTF-8 bytes get joined into characters
    $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
    # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
    # This is the UTF-8 encoding of the string in the previous example

    $foo = pack("ccxxcc",65,66,67,68);
    # foo eq "AB\0\0CD"

    # NOTE: The examples above featuring "W" and "c" are true
    # only on ASCII and ASCII-derived systems such as ISO Latin 1
    # and UTF-8.  On EBCDIC systems, the first example would be
    #      $foo = pack("WWWW",193,194,195,196);

    $foo = pack("s2",1,2);
    # "\001\000\002\000" on little-endian
    # "\000\001\000\002" on big-endian

    $foo = pack("a4","abcd","x","y","z");
    # "abcd"

    $foo = pack("aaaa","abcd","x","y","z");
    # "axyz"

    $foo = pack("a14","abcdefg");
    # "abcdefg\0\0\0\0\0\0\0"

    $foo = pack("i9pl", gmtime);
    # a real struct tm (on my system anyway)

    $utmp_template = "Z8 Z8 Z16 L";
    $utmp = pack($utmp_template, @utmp1);
    # a struct utmp (BSDish)

    @utmp2 = unpack($utmp_template, $utmp);
    # "@utmp1" eq "@utmp2"

    sub bintodec {
        unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
    }

    $foo = pack('sx2l', 12, 34);
    # short 12, two zero bytes padding, long 34
    $bar = pack('s@4l', 12, 34);
    # short 12, zero fill to position 4, long 34
    # $foo eq $bar
    $baz = pack('s.l', 12, 4, 34);
    # short 12, zero fill to position 4, long 34

    $foo = pack('nN', 42, 4711);
    # pack big-endian 16- and 32-bit unsigned integers
    $foo = pack('S>L>', 42, 4711);
    # exactly the same
    $foo = pack('s<l<', -42, 4711);
    # pack little-endian 16- and 32-bit signed integers
    $foo = pack('(sl)<', -42, 4711);
    # exactly the same

The same template may generally also be used in unpack().

=item package NAMESPACE

=item package NAMESPACE VERSION
X<package> X<module> X<namespace> X<version>

=item package NAMESPACE BLOCK

=item package NAMESPACE VERSION BLOCK
X<package> X<module> X<namespace> X<version>

Declares the BLOCK or the rest of the compilation unit as being in the
given namespace.  The scope of the package declaration is either the
supplied code BLOCK or, in the absence of a BLOCK, from the declaration
itself through the end of current scope (the enclosing block, file, or
C<eval>).  That is, the forms without a BLOCK are operative through the end
of the current scope, just like the C<my>, C<state>, and C<our> operators.
All unqualified dynamic identifiers in this scope will be in the given
namespace, except where overridden by another C<package> declaration or
when they're one of the special identifiers that qualify into C<main::>,
like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.

A package statement affects dynamic variables only, including those
you've used C<local> on, but I<not> lexical variables, which are created
with C<my>, C<state>, or C<our>.  Typically it would be the first 
declaration in a file included by C<require> or C<use>.  You can switch into a
package in more than one place, since this only determines which default 
symbol table the compiler uses for the rest of that block.  You can refer to
identifiers in other packages than the current one by prefixing the identifier
with the package name and a double colon, as in C<$SomePack::var>
or C<ThatPack::INPUT_HANDLE>.  If package name is omitted, the C<main>
package as assumed.  That is, C<$::sail> is equivalent to
C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
code, mostly from Perl 4).

If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
namespace to a L<version> object with the VERSION provided.  VERSION must be a
"strict" style version number as defined by the L<version> module: a positive
decimal number (integer or decimal-fraction) without exponentiation or else a
dotted-decimal v-string with a leading 'v' character and at least three
components.  You should set C<$VERSION> only once per package.

See L<perlmod/"Packages"> for more information about packages, modules,
and classes.  See L<perlsub> for other scoping issues.

=item pipe READHANDLE,WRITEHANDLE
X<pipe>

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
IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
after each command, depending on the application.

See L<IPC::Open2>, L<IPC::Open3>, and
L<perlipc/"Bidirectional Communication with Another Process">
for examples of such things.

On systems that support a close-on-exec flag on files, that flag is set
on all newly opened file descriptors whose C<fileno>s are I<higher> than 
the current value of $^F (by default 2 for C<STDERR>).  See L<perlvar/$^F>.

=item pop ARRAY
X<pop> X<stack>

=item pop EXPR

=item pop

Pops and returns the last value of the array, shortening the array by
one element.

Returns the undefined value if the array is empty, although this may also
happen at other times.  If ARRAY is omitted, pops the C<@ARGV> array in the
main program, but the C<@_> array in subroutines, just like C<shift>.

Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a
reference to an unblessed array.  The argument will be dereferenced
automatically.  This aspect of C<pop> is considered highly experimental.
The exact behaviour may change in a future version of Perl.

=item pos SCALAR
X<pos> X<match, position>

=item pos

Returns the offset of where the last C<m//g> search left off for the
variable in question (C<$_> is used when the variable is not
specified). Note that 0 is a valid match offset. C<undef> indicates
that the search position is reset (usually due to match failure, but
can also be because no match has yet been run on the scalar).

C<pos> directly accesses the location used by the regexp engine to
store the offset, so assigning to C<pos> will change that offset, and
so will also influence the C<\G> zero-width assertion in regular
expressions. Both of these effects take place for the next match, so
you can't affect the position with C<pos> during the current match,
such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.

Setting C<pos> also resets the I<matched with zero-length> flag, described
under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.

Because a failed C<m//gc> match doesn't reset the offset, the return
from C<pos> won't change either in this case.  See L<perlre> and
L<perlop>.

=item print FILEHANDLE LIST
X<print>

=item print FILEHANDLE

=item print LIST

=item print

Prints a string or a list of strings.  Returns true if successful.
FILEHANDLE may be a scalar variable containing the name of or a reference
to the filehandle, thus introducing one level of indirection.  (NOTE: If
FILEHANDLE is a variable and the next token is a term, it may be
misinterpreted as an operator unless you interpose a C<+> or put
parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
last selected (see L</select>) output handle.  If LIST is omitted, prints
C<$_> to the currently selected output handle.  To use FILEHANDLE alone to
print the content of C<$_> to it, you must use a real filehandle like
C<FH>, not an indirect one like C<$fh>.  To set the default output handle
to something other than STDOUT, use the select operation.

The current value of C<$,> (if any) is printed between each LIST item.  The
current value of C<$\> (if any) is printed after the entire LIST has been
printed.  Because print takes a LIST, anything in the LIST is evaluated in
list context, including any subroutines whose return lists you pass to
C<print>.  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; put parentheses around all arguments
(or interpose a C<+>, but that doesn't look as good).

If you're storing handles in an array or hash, or in general whenever
you're using any expression more complex than a bareword handle or a plain,
unsubscripted scalar variable to retrieve it, you will have to use a block
returning the filehandle value instead, in which case the LIST may not be
omitted:

    print { $files[$i] } "stuff\n";
    print { $OK ? STDOUT : STDERR } "stuff\n";

Printing to a closed pipe or socket will generate a SIGPIPE signal.  See
L<perlipc> for more on signal handling.

=item printf FILEHANDLE FORMAT, LIST
X<printf>

=item printf FILEHANDLE

=item printf FORMAT, LIST

=item printf

Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
(the output record separator) is not appended.  The first argument of the
list will be interpreted as the C<printf> format. See
L<sprintf|/sprintf FORMAT, LIST> for an
explanation of the format argument.    If you omit the LIST, C<$_> is used;