I looked at the docco on waitpid (at the perl and c lib level) and couldn't find any specification of what waitpid returned if (a) there was a child process (b) it wasn't defunct and (c) you specified WNOHANG.

Huh? from the waitpid section:

waitpid PID,FLAGS

Waits for a particular child process to terminate and returns the pid of the deceased process, or "-1" if there is no such child process. On some systems, a value of 0 indicates that there are processes still running. The status is returned in $?. If you say

  use POSIX ":sys_wait_h";
  #...
  do {
      $kid = waitpid(-1, WNOHANG);
  } until $kid > 0;
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then you can do a non-blocking wait for all pending zombie processes.

Emphasis mine. So, if you happen to be on a system where 0 indicates running processes, your test is wrong. It is also wrong to test for $kid == 0 as waitpid returns the pid of the deceased process. Again in your ordering:

• (a) waitpid returns the PID of the deceased process
• (b) waitpid returns 0 on some systems
• (c) WNOHANG means non-blocking waitpid (it should return if there's no PID immediately reported (hence the loop))

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