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[EDIT] FILE: POSIX.pod

=head1 NAME

POSIX - Perl interface to IEEE Std 1003.1

=head1 SYNOPSIS

use POSIX ();
    use POSIX qw(setsid);
    use POSIX qw(:errno_h :fcntl_h);

printf "EINTR is %d\n", EINTR;

$sess_id = POSIX::setsid();

$fd = POSIX::open($path, O_CREAT|O_EXCL|O_WRONLY, 0644);
	# note: that's a filedescriptor, *NOT* a filehandle

=head1 DESCRIPTION

The POSIX module permits you to access all (or nearly all) the standard
POSIX 1003.1 identifiers.  Many of these identifiers have been given Perl-ish
interfaces.

This document gives a condensed list of the features available in the POSIX
module.  Consult your operating system's manpages for general information on
most features.  Consult L<perlfunc> for functions which are noted as being
identical or almost identical to Perl's builtin functions.

The first section describes POSIX functions from the 1003.1 specification.
The second section describes some classes for signal objects, TTY objects,
and other miscellaneous objects.  The remaining sections list various
constants and macros in an organization which roughly follows IEEE Std
1003.1b-1993.

=head1 CAVEATS

I<Everything is exported by default> (with a handful of exceptions).
This is an unfortunate backwards compatibility feature and its use is
B<strongly L<discouraged|perlpolicy/discouraged>>.
You should either prevent the exporting (by saying S<C<use POSIX ();>>,
as usual) and then use fully qualified names (e.g. C<POSIX::SEEK_END>),
or give an explicit import list.
If you do neither and opt for the default (as in S<C<use POSIX;>>), you
will import I<hundreds and hundreds> of symbols into your namespace.

A few functions are not implemented because they are C specific.  If you
attempt to call these, they will print a message telling you that they
aren't implemented, and suggest using the Perl equivalent, should one
exist.  For example, trying to access the C<setjmp()> call will elicit the
message "C<setjmp() is C-specific: use eval {} instead>".

Furthermore, some evil vendors will claim 1003.1 compliance, but in fact
are not so: they will not pass the PCTS (POSIX Compliance Test Suites).
For example, one vendor may not define C<EDEADLK>, or the semantics of the
errno values set by C<open(2)> might not be quite right.  Perl does not
attempt to verify POSIX compliance.  That means you can currently
successfully say "use POSIX",  and then later in your program you find
that your vendor has been lax and there's no usable C<ICANON> macro after
all.  This could be construed to be a bug.

=head1 FUNCTIONS

=over 8

=item C<_exit>

This is identical to the C function C<_exit()>.  It exits the program
immediately which means among other things buffered I/O is B<not> flushed.

Note that when using threads and in Linux this is B<not> a good way to
exit a thread because in Linux processes and threads are kind of the
same thing (Note: while this is the situation in early 2003 there are
projects under way to have threads with more POSIXly semantics in Linux).
If you want not to return from a thread, detach the thread.

=item C<abort>

This is identical to the C function C<abort()>.  It terminates the
process with a C<SIGABRT> signal unless caught by a signal handler or
if the handler does not return normally (it e.g.  does a C<longjmp>).

=item C<abs>

This is identical to Perl's builtin C<abs()> function, returning the absolute
value of its numerical argument (except that C<POSIX::abs()> must be provided
an explicit value (rather than relying on an implicit C<$_>):

$absolute_value = POSIX::abs(42);   # good

$absolute_value = POSIX::abs();     # throws exception

=item C<access>

Determines the accessibility of a file.

if( POSIX::access( "/", &POSIX::R_OK ) ){
		print "have read permission\n";
	}

Returns C<undef> on failure.  Note: do not use C<access()> for
security purposes.  Between the C<access()> call and the operation
you are preparing for the permissions might change: a classic
I<race condition>.

=item C<acos>

This is identical to the C function C<acos()>, returning
the arcus cosine of its numerical argument.  See also L<Math::Trig>.

=item C<acosh>

This is identical to the C function C<acosh()>, returning the
hyperbolic arcus cosine of its numerical argument [C99].  See also
L<Math::Trig>.

=item C<alarm>

This is identical to Perl's builtin C<alarm()> function, either for arming or
disarming the C<SIGARLM> timer, except that C<POSIX::alarm()> must be provided
an explicit value (rather than relying on an implicit C<$_>):

POSIX::alarm(3)     # good

POSIX::alarm()      # throws exception

=item C<asctime>

This is identical to the C function C<asctime()>.  It returns
a string of the form

"Fri Jun  2 18:22:13 2000\n\0"

and it is called thusly

$asctime = asctime($sec, $min, $hour, $mday, $mon,
			   $year, $wday, $yday, $isdst);

The C<$mon> is zero-based: January equals C<0>.  The C<$year> is
1900-based: 2001 equals C<101>.  C<$wday> and C<$yday> default to zero
(and are usually ignored anyway), and C<$isdst> defaults to -1.

=item C<asin>

This is identical to the C function C<asin()>, returning
the arcus sine of its numerical argument.  See also L<Math::Trig>.

=item C<asinh>

This is identical to the C function C<asinh()>, returning the
hyperbolic arcus sine of its numerical argument [C99].  See also
L<Math::Trig>.

=item C<assert>

Unimplemented, but you can use L<perlfunc/die> and the L<Carp> module
to achieve similar things.

=item C<atan>

This is identical to the C function C<atan()>, returning the
arcus tangent of its numerical argument.  See also L<Math::Trig>.

=item C<atanh>

This is identical to the C function C<atanh()>, returning the
hyperbolic arcus tangent of its numerical argument [C99].  See also
L<Math::Trig>.

=item C<atan2>

This is identical to Perl's builtin C<atan2()> function, returning
the arcus tangent defined by its two numerical arguments, the I<y>
coordinate and the I<x> coordinate.  See also L<Math::Trig>.

=item C<atexit>

Not implemented.  C<atexit()> is C-specific: use C<END {}> instead, see L<perlmod>.

=item C<atof>

Not implemented.  C<atof()> is C-specific.  Perl converts strings to numbers transparently.
If you need to force a scalar to a number, add a zero to it.

=item C<atoi>

Not implemented.  C<atoi()> is C-specific.  Perl converts strings to numbers transparently.
If you need to force a scalar to a number, add a zero to it.
If you need to have just the integer part, see L<perlfunc/int>.

=item C<atol>

Not implemented.  C<atol()> is C-specific.  Perl converts strings to numbers transparently.
If you need to force a scalar to a number, add a zero to it.
If you need to have just the integer part, see L<perlfunc/int>.

=item C<bsearch>

C<bsearch()> not supplied.  For doing binary search on wordlists,
see L<Search::Dict>.

=item C<calloc>

Not implemented.  C<calloc()> is C-specific.  Perl does memory management transparently.

=item C<cbrt>

The cube root [C99].

=item C<ceil>

This is identical to the C function C<ceil()>, returning the smallest
integer value greater than or equal to the given numerical argument.

=item C<chdir>

This is identical to Perl's builtin C<chdir()> function, allowing one to
change the working (default) directory -- see L<perlfunc/chdir> -- with the
exception that C<POSIX::chdir()> must be provided an explicit value (rather
than relying on an implicit C<$_>):

$rv = POSIX::chdir('path/to/dir');      # good

$rv = POSIX::chdir();                   # throws exception

=item C<chmod>

This is identical to Perl's builtin C<chmod()> function, allowing
one to change file and directory permissions -- see L<perlfunc/chmod> -- with
the exception that C<POSIX::chmod()> can only change one file at a time
(rather than a list of files):

$c = chmod 0664, $file1, $file2;          # good

$c = POSIX::chmod 0664, $file1;           # throws exception

$c = POSIX::chmod 0664, $file1, $file2;   # throws exception

As with the built-in C<chmod()>, C<$file> may be a filename or a file
handle.

=item C<chown>

This is identical to Perl's builtin C<chown()> function, allowing one
to change file and directory owners and groups, see L<perlfunc/chown>.

=item C<clearerr>

Not implemented.  Use the method C<IO::Handle::clearerr()> instead, to reset the error
state (if any) and EOF state (if any) of the given stream.

=item C<clock>

This is identical to the C function C<clock()>, returning the
amount of spent processor time in microseconds.

=item C<close>

Close the file.  This uses file descriptors such as those obtained by calling
C<POSIX::open>.

$fd = POSIX::open( "foo", &POSIX::O_RDONLY );
	POSIX::close( $fd );

Returns C<undef> on failure.

See also L</frexp> and L</ldexp>.

=item C<scanf>

Not implemented.  C<scanf()> is C-specific, use E<lt>E<gt> and regular expressions instead,
see L<perlre>.

=item C<setgid>

Sets the real group identifier and the effective group identifier for
this process.  Similar to assigning a value to the Perl's builtin
C<$)> variable, see L<perlvar/$EGID>, except that the latter
will change only the real user identifier, and that the setgid()
uses only a single numeric argument, as opposed to a space-separated
list of numbers.

=item C<setjmp>

Not implemented.  C<setjmp()> is C-specific: use C<eval {}> instead,
see L<perlfunc/eval>.

=item C<setlocale>

WARNING!  Prior to Perl 5.28 or on a system that does not support
thread-safe locale operations, do NOT use this function in a
L<thread|threads>.  The locale will change in all other threads at the
same time, and should your thread get paused by the operating system,
and another started, that thread will not have the locale it is
expecting.  On some platforms, there can be a race leading to segfaults
if two threads call this function nearly simultaneously.  This warning
does not apply on unthreaded builds, or on perls where
C<${^SAFE_LOCALES}> exists and is non-zero; namely Perl 5.28 and later
compiled to be locale-thread-safe.

This function
modifies and queries the program's underlying locale.  Users of this
function should read L<perllocale>, whch provides a comprehensive
discussion of Perl locale handling, knowledge of which is necessary to
properly use this function.  It contains
L<a section devoted to this function|perllocale/The setlocale function>.
The discussion here is merely a summary reference for C<setlocale()>.
Note that Perl itself is almost entirely unaffected by the locale
except within the scope of S<C<"use locale">>.  (Exceptions are listed
in L<perllocale/Not within the scope of "use locale">, and
locale-dependent functions within the POSIX module ARE always affected
by the current locale.)

The following examples assume

use POSIX qw(setlocale LC_ALL LC_CTYPE);

has been issued.

The following will set the traditional UNIX system locale behavior
(the second argument C<"C">).

$loc = setlocale( LC_ALL, "C" );

The following will query the current C<LC_CTYPE> category.  (No second
argument means 'query'.)

$loc = setlocale( LC_CTYPE );

The following will set the C<LC_CTYPE> behaviour according to the locale
environment variables (the second argument C<"">).
Please see your system's C<setlocale(3)> documentation for the locale
environment variables' meaning or consult L<perllocale>.

$loc = setlocale( LC_CTYPE, "" );

The following will set the C<LC_COLLATE> behaviour to Argentinian
Spanish. B<NOTE>: The naming and availability of locales depends on
your operating system. Please consult L<perllocale> for how to find
out which locales are available in your system.

$loc = setlocale( LC_COLLATE, "es_AR.ISO8859-1" );

=item C<setpayload>

use POSIX ':nan_payload';
	setpayload($var, $payload);

Sets the C<NaN> payload of var.

NOTE: the NaN payload APIs are based on the latest (as of June 2015)
proposed ISO C interfaces, but they are not yet a standard.  Things
may change.

See L</nan> for more discussion about C<NaN>.

See also L</setpayloadsig>, L</isnan>, L</getpayload>, and L</issignaling>.

=item C<setpayloadsig>

use POSIX ':nan_payload';
	setpayloadsig($var, $payload);

Like L</setpayload> but also makes the NaN I<signaling>.

Depending on the platform the NaN may or may not behave differently.

Note the API instability warning in L</setpayload>.

Note that because how the floating point formats work out, on the most
common platforms signaling payload of zero is best avoided,
since it might end up being identical to C<+Inf>.

See also L</nan>, L</isnan>, L</getpayload>, and L</issignaling>.

=item C<setpgid>

This is similar to the C function C<setpgid()> for
setting the process group identifier of the current process.

Returns C<undef> on failure.

=item C<setsid>

This is identical to the C function C<setsid()> for
setting the session identifier of the current process.

=item C<setuid>

Sets the real user identifier and the effective user identifier for
this process.  Similar to assigning a value to the Perl's builtin
C<$E<lt>> variable, see L<perlvar/$UID>, except that the latter
will change only the real user identifier.

=item C<sigaction>

Detailed signal management.  This uses C<POSIX::SigAction> objects for
the C<action> and C<oldaction> arguments (the oldaction can also be
just a hash reference).  Consult your system's C<sigaction> manpage
for details, see also C<POSIX::SigRt>.

Synopsis:

sigaction(signal, action, oldaction = 0)

Returns C<undef> on failure.  The C<signal> must be a number (like
C<SIGHUP>), not a string (like C<"SIGHUP">), though Perl does try hard
to understand you.

If you use the C<SA_SIGINFO> flag, the signal handler will in addition to
the first argument, the signal name, also receive a second argument, a
hash reference, inside which are the following keys with the following
semantics, as defined by POSIX/SUSv3:

signo       the signal number
    errno       the error number
    code        if this is zero or less, the signal was sent by
                a user process and the uid and pid make sense,
                otherwise the signal was sent by the kernel

The constants for specific C<code> values can be imported individually
or using the C<:signal_h_si_code> tag.

The following are also defined by POSIX/SUSv3, but unfortunately
not very widely implemented:

pid         the process id generating the signal
    uid         the uid of the process id generating the signal
    status      exit value or signal for SIGCHLD
    band        band event for SIGPOLL
    addr        address of faulting instruction or memory
                reference for SIGILL, SIGFPE, SIGSEGV or SIGBUS

A third argument is also passed to the handler, which contains a copy
of the raw binary contents of the C<siginfo> structure: if a system has
some non-POSIX fields, this third argument is where to C<unpack()> them
from.

Note that not all C<siginfo> values make sense simultaneously (some are
valid only for certain signals, for example), and not all values make
sense from Perl perspective, you should to consult your system's
C<sigaction> and possibly also C<siginfo> documentation.

=item C<siglongjmp>

Not implemented.  C<siglongjmp()> is C-specific: use L<perlfunc/die> instead.

=item C<signbit>

Returns zero for positive arguments, non-zero for negative arguments [C99].

=item C<sigpending>

Examine signals that are blocked and pending.  This uses C<POSIX::SigSet>
objects for the C<sigset> argument.  Consult your system's C<sigpending>
manpage for details.

Synopsis:

sigpending(sigset)

Returns C<undef> on failure.

=item C<sigprocmask>

Change and/or examine calling process's signal mask.  This uses
C<POSIX::SigSet> objects for the C<sigset> and C<oldsigset> arguments.
Consult your system's C<sigprocmask> manpage for details.

Synopsis:

sigprocmask(how, sigset, oldsigset = 0)

Returns C<undef> on failure.

Note that you can't reliably block or unblock a signal from its own signal
handler if you're using safe signals. Other signals can be blocked or unblocked
reliably.

=item C<sigsetjmp>

Not implemented.  C<sigsetjmp()> is C-specific: use C<eval {}> instead,
see L<perlfunc/eval>.

=item C<sigsuspend>

Install a signal mask and suspend process until signal arrives.  This uses
C<POSIX::SigSet> objects for the C<signal_mask> argument.  Consult your
system's C<sigsuspend> manpage for details.

Synopsis:

sigsuspend(signal_mask)

Returns C<undef> on failure.

=item C<sin>

This is identical to Perl's builtin C<sin()> function
for returning the sine of the numerical argument,
see L<perlfunc/sin>.  See also L<Math::Trig>.

=item C<sinh>

This is identical to the C function C<sinh()>
for returning the hyperbolic sine of the numerical argument.
See also L<Math::Trig>.

=item C<sleep>

This is functionally identical to Perl's builtin C<sleep()> function
for suspending the execution of the current for process for certain
number of seconds, see L<perlfunc/sleep>.  There is one significant
difference, however: C<POSIX::sleep()> returns the number of
B<unslept> seconds, while the C<CORE::sleep()> returns the
number of slept seconds.

=item C<sprintf>

This is similar to Perl's builtin C<sprintf()> function
for returning a string that has the arguments formatted as requested,
see L<perlfunc/sprintf>.

=item C<sqrt>

This is identical to Perl's builtin C<sqrt()> function.
for returning the square root of the numerical argument,
see L<perlfunc/sqrt>.

=item C<srand>

Give a seed the pseudorandom number generator, see L<perlfunc/srand>.

=item C<sscanf>

Not implemented.  C<sscanf()> is C-specific, use regular expressions instead,
see L<perlre>.

=item C<stat>

This is identical to Perl's builtin C<stat()> function
for returning information about files and directories.

=item C<strcat>

Not implemented.  C<strcat()> is C-specific, use C<.=> instead, see L<perlop>.

=item C<strchr>

Not implemented.  C<strchr()> is C-specific, see L<perlfunc/index> instead.

=item C<strcmp>

Not implemented.  C<strcmp()> is C-specific, use C<eq> or C<cmp> instead, see L<perlop>.

=item C<strcoll>

This is identical to the C function C<strcoll()>
for collating (comparing) strings transformed using
the C<strxfrm()> function.  Not really needed since
Perl can do this transparently, see L<perllocale>.

Beware that in a UTF-8 locale, anything you pass to this function must
be in UTF-8; and when not in a UTF-8 locale, anything passed must not be
UTF-8 encoded.

=item C<strcpy>

Not implemented.  C<strcpy()> is C-specific, use C<=> instead, see L<perlop>.

=item C<strcspn>

Not implemented.  C<strcspn()> is C-specific, use regular expressions instead,
see L<perlre>.

=item C<strerror>

Returns the error string for the specified errno.
Identical to the string form of C<$!>, see L<perlvar/$ERRNO>.

=item C<strftime>

Convert date and time information to string.  Returns the string.

Synopsis:

strftime(fmt, sec, min, hour, mday, mon, year,
		 wday = -1, yday = -1, isdst = -1)

The month (C<mon>), weekday (C<wday>), and yearday (C<yday>) begin at zero,
I<i.e.>, January is 0, not 1; Sunday is 0, not 1; January 1st is 0, not 1.  The
year (C<year>) is given in years since 1900, I<i.e.>, the year 1995 is 95; the
year 2001 is 101.  Consult your system's C<strftime()> manpage for details
about these and the other arguments.

If you want your code to be portable, your format (C<fmt>) argument
should use only the conversion specifiers defined by the ANSI C
standard (C89, to play safe).  These are C<aAbBcdHIjmMpSUwWxXyYZ%>.
But even then, the B<results> of some of the conversion specifiers are
non-portable.  For example, the specifiers C<aAbBcpZ> change according
to the locale settings of the user, and both how to set locales (the
locale names) and what output to expect are non-standard.
The specifier C<c> changes according to the timezone settings of the
user and the timezone computation rules of the operating system.
The C<Z> specifier is notoriously unportable since the names of
timezones are non-standard. Sticking to the numeric specifiers is the
safest route.

The given arguments are made consistent as though by calling
C<mktime()> before calling your system's C<strftime()> function,
except that the C<isdst> value is not affected.

The string for Tuesday, December 12, 1995.

$str = POSIX::strftime( "%A, %B %d, %Y",
				 0, 0, 0, 12, 11, 95, 2 );
	print "$str\n";

=item C<strlen>

Not implemented.  C<strlen()> is C-specific, use C<length()> instead, see L<perlfunc/length>.

=item C<strncat>

Not implemented.  C<strncat()> is C-specific, use C<.=> instead, see L<perlop>.

=item C<strncmp>

Not implemented.  C<strncmp()> is C-specific, use C<eq> instead, see L<perlop>.

=item C<strncpy>

Not implemented.  C<strncpy()> is C-specific, use C<=> instead, see L<perlop>.

=item C<strpbrk>

Not implemented.  C<strpbrk()> is C-specific, use regular expressions instead,
see L<perlre>.

=item C<strrchr>

Not implemented.  C<strrchr()> is C-specific, see L<perlfunc/rindex> instead.

=item C<strspn>

Not implemented.  C<strspn()> is C-specific, use regular expressions instead,
see L<perlre>.

=item C<strstr>

This is identical to Perl's builtin C<index()> function,
see L<perlfunc/index>.

=item C<strtod>

String to double translation. Returns the parsed number and the number
of characters in the unparsed portion of the string.  Truly
POSIX-compliant systems set C<$!> (C<$ERRNO>) to indicate a translation
error, so clear C<$!> before calling C<strtod>.  However, non-POSIX systems
may not check for overflow, and therefore will never set C<$!>.

C<strtod> respects any POSIX C<setlocale()> C<LC_NUMERIC> settings,
regardless of whether or not it is called from Perl code that is within
the scope of S<C<use locale>>.  Prior to Perl 5.28, or when operating in
a non thread-safe environment, it should not be used in a threaded
application unless it's certain that the underlying locale is C
or POSIX.  This is because it otherwise changes the locale, which
globally affects all threads simultaneously.

To parse a string C<$str> as a floating point number use

$! = 0;
    ($num, $n_unparsed) = POSIX::strtod($str);

The second returned item and C<$!> can be used to check for valid input:

if (($str eq '') || ($n_unparsed != 0) || $!) {
        die "Non-numeric input $str" . ($! ? ": $!\n" : "\n");
    }

When called in a scalar context C<strtod> returns the parsed number.

=item C<strtok>

Not implemented.  C<strtok()> is C-specific, use regular expressions instead, see
L<perlre>, or L<perlfunc/split>.

=item C<strtol>

String to (long) integer translation.  Returns the parsed number and
the number of characters in the unparsed portion of the string.  Truly
POSIX-compliant systems set C<$!> (C<$ERRNO>) to indicate a translation
error, so clear C<$!> before calling C<strtol>.  However, non-POSIX systems
may not check for overflow, and therefore will never set C<$!>.

C<strtol> should respect any POSIX I<setlocale()> settings.

To parse a string C<$str> as a number in some base C<$base> use

$! = 0;
    ($num, $n_unparsed) = POSIX::strtol($str, $base);

The base should be zero or between 2 and 36, inclusive.  When the base
is zero or omitted C<strtol> will use the string itself to determine the
base: a leading "0x" or "0X" means hexadecimal; a leading "0" means
octal; any other leading characters mean decimal.  Thus, "1234" is
parsed as a decimal number, "01234" as an octal number, and "0x1234"
as a hexadecimal number.

The second returned item and C<$!> can be used to check for valid input:

if (($str eq '') || ($n_unparsed != 0) || !$!) {
        die "Non-numeric input $str" . $! ? ": $!\n" : "\n";
    }

When called in a scalar context C<strtol> returns the parsed number.

=item C<strtold>

Like L</strtod> but for long doubles.  Defined only if the
system supports long doubles.

=item C<strtoul>

String to unsigned (long) integer translation.  C<strtoul()> is identical
to C<strtol()> except that C<strtoul()> only parses unsigned integers.  See
L</strtol> for details.

Note: Some vendors supply C<strtod()> and C<strtol()> but not C<strtoul()>.
Other vendors that do supply C<strtoul()> parse "-1" as a valid value.

=item C<strxfrm>

String transformation.  Returns the transformed string.

$dst = POSIX::strxfrm( $src );

Used in conjunction with the C<strcoll()> function, see L</strcoll>.

Not really needed since Perl can do this transparently, see
L<perllocale>.

Beware that in a UTF-8 locale, anything you pass to this function must
be in UTF-8; and when not in a UTF-8 locale, anything passed must not be
UTF-8 encoded.

=item C<sysconf>

Retrieves values of system configurable variables.

The following will get the machine's clock speed.

$clock_ticks = POSIX::sysconf( &POSIX::_SC_CLK_TCK );

Returns C<undef> on failure.

=item C<system>

This is identical to Perl's builtin C<system()> function, see
L<perlfunc/system>.

=item C<tan>

This is identical to the C function C<tan()>, returning the
tangent of the numerical argument.  See also L<Math::Trig>.

=item C<tanh>

This is identical to the C function C<tanh()>, returning the
hyperbolic tangent of the numerical argument.   See also L<Math::Trig>.

=item C<tcdrain>

This is similar to the C function C<tcdrain()> for draining
the output queue of its argument stream.

Returns C<undef> on failure.

=item C<tcflow>

This is similar to the C function C<tcflow()> for controlling
the flow of its argument stream.

Returns C<undef> on failure.

=item C<tcflush>

This is similar to the C function C<tcflush()> for flushing
the I/O buffers of its argument stream.

Returns C<undef> on failure.

=item C<tcgetpgrp>

This is identical to the C function C<tcgetpgrp()> for returning the
process group identifier of the foreground process group of the controlling
terminal.

=item C<tcsendbreak>

This is similar to the C function C<tcsendbreak()> for sending
a break on its argument stream.

Returns C<undef> on failure.

=item C<tcsetpgrp>

This is similar to the C function C<tcsetpgrp()> for setting the
process group identifier of the foreground process group of the controlling
terminal.

Returns C<undef> on failure.

=item C<tgamma>

The Gamma function [C99].