perlipc.1

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.\" ========================================================================
.\"
.IX Title "PERLIPC 1"
.TH PERLIPC 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.SH "NAME"
perlipc \- Perl interprocess communication (signals, fifos, pipes, safe subprocesses, sockets, and semaphores)
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The basic \s-1IPC\s0 facilities of Perl are built out of the good old Unix
signals, named pipes, pipe opens, the Berkeley socket routines, and SysV
\&\s-1IPC\s0 calls.  Each is used in slightly different situations.
.SH "Signals"
.IX Header "Signals"
Perl uses a simple signal handling model: the \f(CW%SIG\fR hash contains names
or references of user-installed signal handlers.  These handlers will
be called with an argument which is the name of the signal that
triggered it.  A signal may be generated intentionally from a
particular keyboard sequence like control-C or control-Z, sent to you
from another process, or triggered automatically by the kernel when
special events transpire, like a child process exiting, your process
running out of stack space, or hitting file size limit.
.PP
For example, to trap an interrupt signal, set up a handler like this:
.PP
.Vb 7
\&    sub catch_zap {
\&        my $signame = shift;
\&        $shucks++;
\&        die "Somebody sent me a SIG$signame";
\&    }
\&    $SIG{INT} = \*(Aqcatch_zap\*(Aq;  # could fail in modules
\&    $SIG{INT} = \e&catch_zap;  # best strategy
.Ve
.PP
Prior to Perl 5.7.3 it was necessary to do as little as you possibly
could in your handler; notice how all we do is set a global variable
and then raise an exception.  That's because on most systems,
libraries are not re-entrant; particularly, memory allocation and I/O
routines are not.  That meant that doing nearly \fIanything\fR in your
handler could in theory trigger a memory fault and subsequent core
dump \- see \*(L"Deferred Signals (Safe Signals)\*(R" below.
.PP
The names of the signals are the ones listed out by \f(CW\*(C`kill \-l\*(C'\fR on your
system, or you can retrieve them from the Config module.  Set up an
\&\f(CW@signame\fR list indexed by number to get the name and a \f(CW%signo\fR table
indexed by name to get the number:
.PP
.Vb 7
\&    use Config;
\&    defined $Config{sig_name} || die "No sigs?";
\&    foreach $name (split(\*(Aq \*(Aq, $Config{sig_name})) {
\&        $signo{$name} = $i;
\&        $signame[$i] = $name;
\&        $i++;
\&    }
.Ve
.PP
So to check whether signal 17 and \s-1SIGALRM\s0 were the same, do just this:
.PP
.Vb 4
\&    print "signal #17 = $signame[17]\en";
\&    if ($signo{ALRM}) {
\&        print "SIGALRM is $signo{ALRM}\en";
\&    }
.Ve
.PP
You may also choose to assign the strings \f(CW\*(AqIGNORE\*(Aq\fR or \f(CW\*(AqDEFAULT\*(Aq\fR as
the handler, in which case Perl will try to discard the signal or do the
default thing.
.PP
On most Unix platforms, the \f(CW\*(C`CHLD\*(C'\fR (sometimes also known as \f(CW\*(C`CLD\*(C'\fR) signal
has special behavior with respect to a value of \f(CW\*(AqIGNORE\*(Aq\fR.
Setting \f(CW$SIG{CHLD}\fR to \f(CW\*(AqIGNORE\*(Aq\fR on such a platform has the effect of
not creating zombie processes when the parent process fails to \f(CW\*(C`wait()\*(C'\fR
on its child processes (i.e. child processes are automatically reaped).
Calling \f(CW\*(C`wait()\*(C'\fR with \f(CW$SIG{CHLD}\fR set to \f(CW\*(AqIGNORE\*(Aq\fR usually returns
\&\f(CW\*(C`\-1\*(C'\fR on such platforms.
.PP
Some signals can be neither trapped nor ignored, such as
the \s-1KILL\s0 and \s-1STOP\s0 (but not the \s-1TSTP\s0) signals.  One strategy for
temporarily ignoring signals is to use a \fIlocal()\fR statement, which will be
automatically restored once your block is exited.  (Remember that \fIlocal()\fR
values are \*(L"inherited\*(R" by functions called from within that block.)
.PP
.Vb 7
\&    sub precious {
\&        local $SIG{INT} = \*(AqIGNORE\*(Aq;
\&        &more_functions;
\&    }
\&    sub more_functions {
\&        # interrupts still ignored, for now...
\&    }
.Ve
.PP
Sending a signal to a negative process \s-1ID\s0 means that you send the signal
to the entire Unix process-group.  This code sends a hang-up signal to all
processes in the current process group (and sets \f(CW$SIG\fR{\s-1HUP\s0} to \s-1IGNORE\s0 so
it doesn't kill itself):
.PP
.Vb 5
\&    {
\&        local $SIG{HUP} = \*(AqIGNORE\*(Aq;
\&        kill HUP => \-$$;
\&        # snazzy writing of: kill(\*(AqHUP\*(Aq, \-$$)
\&    }
.Ve
.PP
Another interesting signal to send is signal number zero.  This doesn't
actually affect a child process, but instead checks whether it's alive
or has changed its \s-1UID\s0.
.PP
.Vb 3
\&    unless (kill 0 => $kid_pid) {
\&        warn "something wicked happened to $kid_pid";
\&    }
.Ve
.PP
When directed at a process whose \s-1UID\s0 is not identical to that
of the sending process, signal number zero may fail because
you lack permission to send the signal, even though the process is alive.
You may be able to determine the cause of failure using \f(CW\*(C`%!\*(C'\fR.
.PP
.Vb 3
\&    unless (kill 0 => $pid or $!{EPERM}) {
\&        warn "$pid looks dead";
\&    }
.Ve
.PP
You might also want to employ anonymous functions for simple signal
handlers:
.PP
.Vb 1
\&    $SIG{INT} = sub { die "\enOutta here!\en" };
.Ve
.PP
But that will be problematic for the more complicated handlers that need
to reinstall themselves.  Because Perl's signal mechanism is currently
based on the \fIsignal\fR\|(3) function from the C library, you may sometimes be so
unfortunate as to run on systems where that function is \*(L"broken\*(R", that
is, it behaves in the old unreliable SysV way rather than the newer, more
reasonable \s-1BSD\s0 and \s-1POSIX\s0 fashion.  So you'll see defensive people writing
signal handlers like this:
.PP
.Vb 8
\&    sub REAPER {
\&        $waitedpid = wait;
\&        # loathe sysV: it makes us not only reinstate
\&        # the handler, but place it after the wait
\&        $SIG{CHLD} = \e&REAPER;
\&    }
\&    $SIG{CHLD} = \e&REAPER;
\&    # now do something that forks...
.Ve
.PP
or better still:
.PP
.Vb 10
\&    use POSIX ":sys_wait_h";
\&    sub REAPER {
\&        my $child;
\&        # If a second child dies while in the signal handler caused by the
\&        # first death, we won\*(Aqt get another signal. So must loop here else
\&        # we will leave the unreaped child as a zombie. And the next time
\&        # two children die we get another zombie. And so on.
\&        while (($child = waitpid(\-1,WNOHANG)) > 0) {
\&            $Kid_Status{$child} = $?;
\&        }
\&        $SIG{CHLD} = \e&REAPER;  # still loathe sysV
\&    }
\&    $SIG{CHLD} = \e&REAPER;
\&    # do something that forks...
.Ve
.PP
Signal handling is also used for timeouts in Unix,   While safely
protected within an \f(CW\*(C`eval{}\*(C'\fR block, you set a signal handler to trap
alarm signals and then schedule to have one delivered to you in some
number of seconds.  Then try your blocking operation, clearing the alarm
when it's done but not before you've exited your \f(CW\*(C`eval{}\*(C'\fR block.  If it
goes off, you'll use \fIdie()\fR to jump out of the block, much as you might
using \fIlongjmp()\fR or \fIthrow()\fR in other languages.
.PP
Here's an example:
.PP
.Vb 7
\&    eval {
\&        local $SIG{ALRM} = sub { die "alarm clock restart" };
\&        alarm 10;
\&        flock(FH, 2);   # blocking write lock
\&        alarm 0;