hi Ken,

the solution you have suggested looks like my last example, except that the presence of the child process is verified with kill(0, $pid), not waitpid($pid, WNOHANG). The use of kill() involves one caveat, though -- it merely checks that the process with the given PID exists, but that does not mean this process is a child. In order to illustrate this, suppose you are logged in as root and execute the following commandline: 

perl -e 'kill(0, 1) && print "Init is our child\n"'

[download]

On my Linux laptop, this commandline always prints out the message "Init is our child", although this is not true. This caveat can lead to the following race condition -- if the signal handler gets triggered after waitpid($pid, 0) has returned *and* the OS has had enough time to start a new process with the same PID, the new process can mistakenly get the TERM signal. I acknowledge that modern operating systems attempt not to reuse the same PID immediately, but I wouldn't like to rely on that assumption in the code, especially because it will be executing with root privileges and can thus kill any process in the system.

In contrast, waitpid($pid, WNOHANG) returns 0 for running child processes only, and a new unrelated process with the same PID is not reported as a running child. That's why I was asking if it is OK to call waitpid() again from the signal handler, if the blocking waitpid() call was interrupted by the signal.

regards, risto

In reply to Re^2: using waitpid() with signals by ristov
in thread using waitpid() with signals by ristov

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