From 5fa5e7dfc2abaaadd377c97cd1ebe78ea844da88 Mon Sep 17 00:00:00 2001 From: Roderick Schertler Date: Tue, 26 Nov 1996 11:42:49 -0500 Subject: [PATCH] Document how to use $SIG{ALRM} and alarm() Subject: Re: reliable signal patch (was Re: Reliable signals) On Tue, 26 Nov 1996 11:14:03 -0500 (EST), Kenneth Albanowski said: > > Thus with restarting being the default, you can restart, if you want, > by returning normally from the signal handler, or interrupt by dying. Thanks, now I get it. This is even inferred in perlipc(1). Here's some more explicit documentation. p5p-msgid: <5898.849026569@eeyore.ibcinc.com> --- pod/perlfunc.pod | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod index df8d23f..f153fe7 100644 --- a/pod/perlfunc.pod +++ b/pod/perlfunc.pod @@ -334,6 +334,23 @@ syscall() interface to access setitimer(2) if your system supports it, or else see L below. It is not advised to intermix alarm() and sleep() calls. +If you want to use alarm() to time out a system call you need to use an +eval/die pair. You can't rely on the alarm causing the system call to +fail with $! set to EINTR because Perl sets up signal handlers to +restart system calls on some systems. Using eval/die always works. + + eval { + local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required + $nread = sysread SOCKET, $buffer, $size; + }; + die if $@ and $@ ne "alarm\n"; # propagate errors + if ($@) { + # timed out + } + else { + # didn't + } + =item atan2 Y,X Returns the arctangent of Y/X in the range -PI to PI. -- 2.7.4