=over =item alarm SECONDS =item alarm Arranges to have a SIGALRM delivered to this process after the specified number of seconds have elapsed. If SECONDS is not specified, the value stored in C<$_> is used. (On some machines, unfortunately, the elapsed time may be up to one second less than you specified because of how seconds are counted.) Only one timer may be counting at once. Each call disables the previous timer, and an argument of C<0> may be supplied to cancel the previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer. For delays of finer granularity than one second, you may use Perl's four-argument version of select() leaving the first three arguments undefined, or you might be able to use the C interface to access setitimer(2) if your system supports it. The Time::HiRes module from CPAN may also prove useful. It is usually a mistake to intermix C and C calls. (C may be internally implemented in your system with C) If you want to use C to time out a system call you need to use an C/C pair. You can't rely on the alarm causing the system call to fail with C<$!> set to C because Perl sets up signal handlers to restart system calls on some systems. Using C/C always works, modulo the caveats given in L. 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 } =back