Very Computer

Here's the code, a sample run, and a stack trace
("www.rgee.com" is the first one that fails here):

  % cat try.pl
  #!/usr/bin/perl -w
  use POSIX qw/:signal_h/;

  sub alrm {die "timeout"}
  use constant FLAGS => 0;         # SA_RESTART; #SA_RESETHAND;# ???

  # set up the requisite Perl objects for sigaction a'la POSIX manpage
  $sigset = POSIX::SigSet->new(SIGALRM);
  $sigact = POSIX::SigAction->new("main::alrm", $sigset, FLAGS);

  for (<DATA>) {
    chomp;

    eval {                            # trap the alarm
        sigaction(SIGALRM, $sigact);  # install the signal handler
        alarm 2;
        $addr = gethostbyname $_;     # kaboom
        alarm 0;
    };

    else {
        $addr = $addr ? join '.', unpack('C4', $addr) : "BZZT";
        print "$_ => $addr\n";
    }
  }

  __END__
  www.microsoft.com
  www.aol.com
  www.rgee.com
  www.aol.com
  www.microsoft.com

  % ./try.pl
  www.microsoft.com => 207.46.230.220
  www.aol.com => 64.12.149.24
  www.rgee.com => timeout at /tmp/try.pl line 4, <DATA> line 83.
  www.aol.com => timeout at /tmp/try.pl line 4, <> line 83.
  www.microsoft.com => timeout at /tmp/try.pl line 4, <> line 83.

  % strace ./try.pl
  ...
  send(5, "<\233\1\0\0\1\0\0\0\0\0\0\3www\4rgee\3com\0\0\1\0\1", 30, 0) = 30
  gettimeofday({1003909154, 795857}, NULL) = 0
  poll([{fd=5, events=POLLIN}], 1, 5000)  = -1 EINTR (Interrupted system call)
  --- SIGALRM (Alarm clock) ---
  rt_sigprocmask(SIG_SETMASK, [RT_0], NULL, 8) = 0
  write(1, "www.rgee.com => timeout at /tmp/"...,) = 58
  write(1, "\n", 1
  )                       = 1
  rt_sigaction(SIGALRM, {0x40065f70, [], SA_RESTART|0x4000000},
                        {0x40065f70, [], 0x4000000}, 8) = 0
  rt_sigaction(SIGALRM, {0x40065f70, [], 0x4000000}, NULL, 8) = 0
  alarm(2)                                = 0
  rt_sigprocmask(SIG_SETMASK, NULL, [RT_0], 8) = 0
  rt_sigsuspend([] <unfinished ...>
  --- SIGALRM (Alarm clock) ---
  <... rt_sigsuspend resumed> )           = -1 EINTR (Interrupted system call)
  rt_sigprocmask(SIG_SETMASK, [RT_0], NULL, 8) = 0
  write(1, "www.aol.com => timeout at /tmp/t"...,) = 57
  write(1, "\n", 1
  )                       = 1
  rt_sigaction(SIGALRM, {0x40065f70, [], SA_RESTART|0x4000000},
                        {0x40065f70, [], 0x4000000}, 8) = 0
  rt_sigaction(SIGALRM, {0x40065f70, [], 0x4000000}, NULL, 8) = 0
  alarm(2)                                = 0
  rt_sigprocmask(SIG_SETMASK, NULL, [RT_0], 8) = 0
  rt_sigsuspend([] <unfinished ...>
  --- SIGALRM (Alarm clock) ---
  ...

The code misbehaves as above for Perl versions 5.00503 and up, and
both 2.2 and 2.4 linux kernels.  However, it apparently works fine
on Solaris (the subsequent gethostbyname calls work ok even after the
alarm goes off).

I've tried coding it using various flags for sigaction (SA_RESTART,
SA_RESETHAND, ...) but the behavior remains unchanged. I originally
asked this question a week or so ago in clp.misc, but nobody there
provided an explanation for this behavior. I'm obviously missing
something; can anyone help me out here?

--
Joe Schaefer

Quote:> The code misbehaves as above for Perl versions 5.00503 and up, and
> both 2.2 and 2.4 linux kernels.  However, it apparently works fine
> on Solaris (the subsequent gethostbyname calls work ok even after the
> alarm goes off).

$ uname -a
Linux iljat 2.2.19 #22 Wed Jun 20 18:12:16 PDT 2001 i586 unknown
$ perl -v
This is perl, v5.6.1 built for i386-linux
$ ./r
www.microsoft.com => 207.46.230.219
www.aol.com => 64.12.149.24
www.rgee.com => timeout at ./r line 7, <DATA> line 5.
www.aol.com => 205.188.160.121
www.microsoft.com => 207.46.197.113

Ilja.

 Joe> Here's a Perl script that does 5 hostname lookups via
 Joe> gethostbyname() which is wrapped in a 2 second alarm.  The
 Joe> problem is that once the alarm is triggered, *all* subsequent
 Joe> lookups are failing.

You are interrupting a signal-unsafe function with a signal,
longjmping (via perl's die()) out of the signal handler, and then
calling more signal-unsafe functions. This doesn't even work in C,
let alone Perl.

There is no (reliable|portable) way to impose a timeout on
gethostbyname() using signals.

--
Andrew.

comp.unix.programmer FAQ: see <URL: http://www.erlenstar.demon.co.uk/unix/>
                           or <URL: http://www.whitefang.com/unix/>

--
Joe Schaefer