document the return value of pipe()

[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>

=for Pod::Functions =String

C<chomp>, C<chop>, C<chr>, C<crypt>, C<fc>, 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///>

C<fc> is available only if the C<"fc"> feature is enabled or if it is
prefixed with C<CORE::>.  The C<"fc"> feature is enabled automatically
with a C<use v5.16> (or higher) declaration in the current scope.


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

=for Pod::Functions =Regexp

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

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

=for Pod::Functions =Math

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>

=for Pod::Functions =ARRAY

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

=item Functions for list data
X<list>

=for Pod::Functions =LIST

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

=item Functions for real %HASHes
X<hash>

=for Pod::Functions =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>

=for Pod::Functions =I/O

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<readline> 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>

C<say> is available only if the C<"say"> feature is enabled or if it is
prefixed with C<CORE::>.  The C<"say"> feature is enabled automatically
with a C<use v5.10> (or higher) declaration in the current scope.

=item Functions for fixed-length data or records

=for Pod::Functions =Binary

C<pack>, C<read>, C<syscall>, C<sysread>, C<sysseek>, 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>

=for Pod::Functions =File

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>

=for Pod::Functions =Flow

C<break>, C<caller>, C<continue>, C<die>, C<do>,
C<dump>, C<eval>, C<evalbytes> C<exit>,
C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>

C<break> is available only if you enable the experimental C<"switch">
feature or use the C<CORE::> prefix. The C<"switch"> feature also enables
the C<default>, C<given> and C<when> statements, which are documented in
L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled
automatically with a C<use v5.10> (or higher) declaration in the current
scope. In Perl v5.14 and earlier, C<continue> required the C<"switch">
feature, like the other keywords.

C<evalbytes> is only available with with the C<"evalbytes"> feature (see
L<feature>) or if prefixed with C<CORE::>.  C<__SUB__> is only available
with with the C<"current_sub"> feature or if prefixed with C<CORE::>. Both
the C<"evalbytes"> and C<"current_sub"> features are enabled automatically
with a C<use v5.16> (or higher) declaration in the current scope.

=item Keywords related to scoping

=for Pod::Functions =Namespace

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::>.  The C<"state"> feature is enabled automatically
with a C<use v5.10> (or higher) declaration in the current scope.

=item Miscellaneous functions

=for Pod::Functions =Misc

C<defined>, C<formline>, C<lock>, C<prototype>, C<reset>, C<scalar>, C<undef>

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

=for Pod::Functions =Process

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

=item Keywords related to Perl modules
X<module>

=for Pod::Functions =Modules

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>

=for Pod::Functions =Objects

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>

=for Pod::Functions =Socket

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>

=for Pod::Functions =SysV

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>

=for Pod::Functions =User

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>

=for Pod::Functions =Network

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>

=for Pod::Functions =Time

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

=item Non-function keywords

=for Pod::Functions =!Non-functions

C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>,
C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>,
C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>,
C<while>, C<x>, C<xor>

=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

=for Pod::Functions a file test (-r, -x, etc)

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>.

To avoid confusing would-be users of your code with mysterious
syntax errors, put something like this at the top of your script:

    use 5.010;  # so filetest ops can stack

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

=item abs

=for Pod::Functions absolute value function

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

=item accept NEWSOCKET,GENERICSOCKET
X<accept>

=for Pod::Functions accept an incoming socket connect

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

=for Pod::Functions schedule a SIGALRM

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>

=for Pod::Functions arctangent of Y/X in the range -PI to PI

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>

=for Pod::Functions binds an address to a socket

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

=for Pod::Functions prepare binary files for I/O

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

=for Pod::Functions create an object

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.
SeeL<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

=for Pod::Functions +switch break out of a C<given> block

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

=for Pod::Functions get context of the current subroutine call

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 in
list context, and with an argument, 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

=for Pod::Functions change your current working directory

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>

=for Pod::Functions changes the permissions on a list of files

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

=for Pod::Functions remove a trailing record separator from a string

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

=for Pod::Functions remove the last character from a string

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>

=for Pod::Functions change the ownership on a list of files

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

=for Pod::Functions get character this number represents

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

=for Pod::Functions make directory new root for path lookups

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

=for Pod::Functions close file (or pipe or socket) handle

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>

=for Pod::Functions close directory handle

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

=item connect SOCKET,NAME
X<connect>

=for Pod::Functions connect to a remote socket

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

=for Pod::Functions optional trailing block in a while or foreach

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

=for Pod::Functions cosine function

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>

=for Pod::Functions one-way passwd-style encryption

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>

=for Pod::Functions breaks binding on a tied dbm file

[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>

=for Pod::Functions create binding on a tied dbm file

[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>).  To prevent creation of
the database if it doesn't exist, you may specify a MODE
of 0, and the function will return a false value if it
can't find an existing database.  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 defined EXPR
X<defined> X<undef> X<undefined>

=item defined

=for Pod::Functions test whether a value, variable, or function is 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>

=for Pod::Functions deletes a value from a hash

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>

=for Pod::Functions raise an exception or bail out

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>

=for Pod::Functions turn a BLOCK into a TERM

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

=for Pod::Functions create an immediate core 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

=for Pod::Functions retrieve the next key/value pair from a hash

When called on a hash in list context, returns a 2-element list
consisting of the key and value for the next element of a hash.  In Perl
5.12 and later only, it will also return the index and value for the next
element of an array so that you can iterate over it; older Perls consider
this a syntax error.  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 I<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: In the current implementation, 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) { ... }

To avoid confusing would-be users of your code who are running earlier
versions of Perl with mysterious syntax errors, put this sort of thing at
the top of your file to signal that your code will work I<only> on Perls of
a recent vintage:

    use 5.012;	# so keys/values/each work on arrays
    use 5.014;	# so keys/values/each work on scalars (experimental)

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

=for Pod::Functions test a filehandle for its end

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

=for Pod::Functions catch exceptions or compile and run code

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.

If the C<unicode_eval> feature is enabled (which is the default under a
C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
characters, so C<use utf8> declarations have no effect, and source filters
are forbidden.  In the absence of the C<unicode_eval> feature, the string
will sometimes be treated as characters and sometimes as bytes, depending
on the internal encoding, and source filters activated within the C<eval>
exhibit the erratic, but historical, behaviour of affecting some outer file
scope that is still compiling.  See also the L</evalbytes> keyword, which
always treats its input as a byte stream and works properly with source
filters, and the L<feature> pragma.

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 in list context, and C<$@> is set to the error
message.  (Prior to 5.16, a bug caused C<undef> to be returned
in list context for syntax errors, but not for runtime errors.)
If there was no error, C<$@> is set to the empty string.  A
control flow operator like C<last> or C<goto> can bypass the setting of
C<$@>.  Beware that using C<eval> neither silences Perl from printing
warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
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 localized 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 evalbytes EXPR
X<evalbytes>

=item evalbytes

=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream

This function is like L</eval> with a string argument, except it always
parses its argument, or C<$_> if EXPR is omitted, as a string of bytes.  A
string containing characters whose ordinal value exceeds 255 results in an
error.  Source filters activated within the evaluated code apply to the
code itself.

This function is only available under the C<evalbytes> feature, a
C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix.  See
L<feature> for more information.

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

=item exec PROGRAM LIST

=for Pod::Functions abandon this program to run another

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 C<exec> is called in void context and 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>

=for Pod::Functions test whether a hash key is present

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 most deeply nested array or hash element 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

=for Pod::Functions terminate this program

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

=for Pod::Functions raise I<e> to a power

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

=item fc EXPR
X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>

=item fc

=for Pod::Functions +fc return casefolded version of a string

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

Casefolding is the process of mapping strings to a form where case
differences are erased; comparing two strings in their casefolded
form is effectively a way of asking if two strings are equal,
regardless of case.

Roughly, if you ever found yourself writing this

    lc($this) eq lc($that)  # Wrong!
        # or
    uc($this) eq uc($that)  # Also wrong!
        # or
    $this =~ /\Q$that/i     # Right!

Now you can write

    fc($this) eq fc($that)

And get the correct results.

Perl only implements the full form of casefolding.
For further information on casefolding, refer to
the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
available at L<http://www.unicode.org/versions/latest/>, as well as the
Case Charts available at L<http://www.unicode.org/charts/case/>.

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

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

While the Unicode Standard defines two additional forms of casefolding,
one for Turkic languages and one that never maps one character into multiple
characters, these are not provided by the Perl core; However, the CPAN module
C<Unicode::Casing> may be used to provide an implementation.

This keyword is available only when the C<"fc"> feature is enabled,
or when prefixed with C<CORE::>; See L<feature>. Alternately,
include a C<use v5.16> or later to the current scope.

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

=for Pod::Functions file control system call

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 __FILE__
X<__FILE__>

=for Pod::Functions the name of the current source file

A special token that returns the name of the file in which it occurs.

=item fileno FILEHANDLE
X<fileno>

=for Pod::Functions return file descriptor from filehandle

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>

=for Pod::Functions lock an entire file with an advisory lock

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>

=for Pod::Functions create a new process just like this one

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, at the level of the Perl program,
to 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>

=for Pod::Functions declare a picture format with use by the write() function

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>

=for Pod::Functions internal function used for formats

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

=for Pod::Functions get the next character from the filehandle

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>

=for Pod::Functions return who logged in at this tty

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>

=for Pod::Functions find the other end of a socket connection

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>

=for Pod::Functions get process 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>

=for Pod::Functions get parent process ID

Returns the process id of the parent process.

Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
around non-POSIX thread semantics the minority of Linux systems (and
Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
has since been removed. See the documentation for L<$$|perlvar/$$> for
details.

Portability issues: L<perlport/getppid>.

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

=for Pod::Functions get current nice value

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> 

=for Pod::Functions get passwd record given user login name

=item getgrnam NAME

=for Pod::Functions get group record given group name

=item gethostbyname NAME

=for Pod::Functions get host record given name

=item getnetbyname NAME

=for Pod::Functions get networks record given name

=item getprotobyname NAME

=for Pod::Functions get protocol record given name

=item getpwuid UID

=for Pod::Functions get passwd record given user ID

=item getgrgid GID

=for Pod::Functions get group record given group user ID

=item getservbyname NAME,PROTO

=for Pod::Functions get services record given its name

=item gethostbyaddr ADDR,ADDRTYPE

=for Pod::Functions get host record given its address

=item getnetbyaddr ADDR,ADDRTYPE

=for Pod::Functions get network record given its address

=item getprotobynumber NUMBER

=for Pod::Functions get protocol record numeric protocol

=item getservbyport PORT,PROTO

=for Pod::Functions get services record given numeric port

=item getpwent

=for Pod::Functions get next passwd record

=item getgrent

=for Pod::Functions get next group record

=item gethostent

=for Pod::Functions get next hosts record

=item getnetent

=for Pod::Functions get next networks record

=item getprotoent

=for Pod::Functions get next protocols record

=item getservent

=for Pod::Functions get next services record

=item setpwent

=for Pod::Functions prepare passwd file for use

=item setgrent

=for Pod::Functions prepare group file for use

=item sethostent STAYOPEN

=for Pod::Functions prepare hosts file for use

=item setnetent STAYOPEN

=for Pod::Functions prepare networks file for use

=item setprotoent STAYOPEN

=for Pod::Functions prepare protocols file for use

=item setservent STAYOPEN

=for Pod::Functions prepare services file for use

=item endpwent

=for Pod::Functions be done using passwd file

=item endgrent

=for Pod::Functions be done using group file

=item endhostent

=for Pod::Functions be done using hosts file

=item endnetent

=for Pod::Functions be done using networks file

=item endprotoent

=for Pod::Functions be done using protocols file

=item endservent

=for Pod::Functions be done using services file

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

The C<getprotobynumber> function, even though it only takes one argument,
has the precedence of a list operator, so beware:

    getprotobynumber $number eq 'icmp'   # WRONG
    getprotobynumber($number eq 'icmp')  # actually means this
    getprotobynumber($number) eq 'icmp'  # better this way

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>

=for Pod::Functions retrieve the sockaddr for a given socket

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>

=for Pod::Functions get socket options on a given socket

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

=item glob

=for Pod::Functions expand filenames using wildcards

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 you want to glob filenames that might contain whitespace, you'll
have to use extra quotes around the spacey filename to protect it.
For example, to glob filenames that have an C<e> followed by a space
followed by an C<f>, use either of:

    @spacies = <"*e f*">;
    @spacies = glob '"*e f*"';
    @spacies = glob q("*e f*");

If you had to get a variable through, you could do this:

    @spacies = glob "'*${var}e f*'";
    @spacies = glob qq("*${var}e f*");

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

=for Pod::Functions convert UNIX time into record or string using Greenwich time

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

=for Pod::Functions create spaghetti code

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

=for Pod::Functions locate elements in a list test true against a given criterion

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

=for Pod::Functions convert a string to a hexadecimal number

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>

=for Pod::Functions patch a module's namespace into your own

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

=for Pod::Functions find a substring within a string

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 zero.
If the substring is not found, C<index> returns -1.

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

=item int

=for Pod::Functions get the integer portion of a number

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>

=for Pod::Functions system-dependent device control system call

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>

=for Pod::Functions join a list into a string using a separator

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