I think this is a quirk of strace when invoking it on an already running process. strace relies on the kernel to be notified when a process enters or leaves a syscall. If you run strace on a process that is already in a system call, it may not produce any output until the process is interrupted or returns from the system call.

To implement your hang finder, you might have to use some other mechanism to indicate that a program is attempting to obtain a lock. For instance, one method is to change $0 to reflect the state of the program. On many OS's this will be observable via the ps command, e.g.: 

sub get_lock {
  local($0) = "(acquiring lock)";
  flock(...);
  # returning restores previous value of $0
}

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In reply to Re: Calling strace from a perl program by pc88mxer
in thread Calling strace from a perl program by jason_s

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