Hi! I am trying to execute strace from within a perl program. Basically, I want to strace a process, and see if it is blocking while waiting for a ("flock") lock on a file. Since I'm looking for blocking processes, strace will just hang on these -- so I use "alarm(1)" to prevent my script from hanging.

The first step is figuring out how to capture the output of strace, and I'm getting stuck. Here's what I have so far, which doesn't work:
#!/usr/bin/perl -w
use strict;
my $pid=$ARGV[0];
my $trace_out;
my $strace_pid;

eval {
  local $SIG{ALRM} = sub {
    kill 2, $strace_pid || warn "couldn't kill $strace_pid: $!";
    close STRACE or die "couldn't close strace";
    die "exiting eval";
  };
  $strace_pid = open (STRACE,"strace -q -p$pid 2>&1|") or die "couldn'
+t open strace";
  alarm (1);
  undef $/;
  $trace_out=<STRACE> ;
};

print "traceout is $trace_out";

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I have a test script that takes an exclusive lock on a file, and then just goes into an infinite loop: 
#!/usr/bin/perl
use Fcntl ':flock';
open FH, "file.txt" ;
flock FH, LOCK_EX or die "couldn't lock";
while (true) {};

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If I run two instances of this, (the first instance gets the lock, the 2nd blocks while waiting for a lock), and attach strace to them, here's what I see: 
>./locker.pl &
[1] 17289
>./locker.pl &
[2] 17290
>strace -q -p17289
>strace -q -p17290
flock(3, LOCK_EX <unfinished ...>

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As expected, pid 17289 has no strace output, since it's just looping. And 17290 is blocking attempting to take a lock.

But, when I run my script on pid 17290, it's not capturing the "flock(3, LOCK_EX" text: 
./hang_finder 17290
traceout is

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My intention is that the output should be: 
traceout is flock(3, LOCK_EX

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Obviously,I'm doing something wrong, but I can't figure out what . . .

In reply to Calling strace from a perl program by jason_s

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