Behavior of waitpid() in multiple threads?

Asked Feb 24 '15 at 17:27

Active Feb 24 '15 at 17:27

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C++ under Linux (CentOS 6.3), using pthreads. gcc 4.7.

I am now maintaining a program where the primary thread does a blocking call to waitpid(-1,...) to reap any and all possible children.

In a thread spawned earlier from the main thread, I have code that does a fork()/execve() call with a waitpid(childpid,...) after the fork (the moral equivalent of system()).

I'm seeing VERY infrequent instances where the child thread's waitpid call just blocks forever.

Am I correct in assuming that there's a race condition if both of these waitpid calls are blocked and the child exits?

asked Feb 24 '15 at 17:27

Michael Kohne

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To head off some of the commentary: No one DESIGNED this app this way. The fork/exec/waitpid calls were added later by people who appear to have been unaware of the main thread's waitpid call. – Michael Kohne Feb 24 '15 at 17:27
You cannot wait for a grandchild. – n. m. could be an AI Feb 24 '15 at 18:11
@n.m. - no one is trying to wait for a grandchild. – Michael Kohne Feb 24 '15 at 18:54
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There would be a race condition. At some point in time the child process must transition from zombie to reaped. However, since you're only waiting for a specific PID in the main thread.. – Mark Nunberg Feb 24 '15 at 19:07
Sorry my mistake, I thought you are forking a grandchild from a child *process* and not from a child thread. You are definitely having a race here. – n. m. could be an AI Feb 24 '15 at 19:11

Behavior of waitpid() in multiple threads?

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