waitpid for child process not succeeding

问题

I am starting a process using execv and letting it write to a file. I start a thread simultaneously that monitors the file so that it's size does not exceed a certain limit using stat.st_size. Now, when the limit is hit, I waitpid for the child process, but this throws an error and the process I start in the background becomes a zombie. When I do the stop using the same waitpid from the main thread, the process is killed without becoming a zombie. Any ideas?

Edit: The errno is 10 and waitpid returns -1. This is on a linux platform.

回答1:

This is difficult to debug without code, but errno 10 is ECHILD.

Per the man page, this is returned as follows:

ECHILD (for waitpid() or waitid()) The process specified by pid (waitpid()) or idtype and id (waitid()) does not exist or is not a child of the calling process. (This can happen for one's own child if the action for SIGCHLD is set to SIG_IGN. See also the Linux Notes section about threads.)

In short, the pid you are specifying is not a child of the process calling waitpid() (or is no longer, perhaps because it has terminated).

Note the parenthetical section:

• "This can happen for one's own child if the action for SIGCHLD is set to SIG_IGN" - if you've set up a signal handler for SIGCHLD to be SIG_IGN, the wait is effectively done automatically, and therefore waitpid won't work as the child will have already terminated (will not go through zombie state).

• "See also the Linux Notes section about threads." - In Linux, threads are essentially processes. Modern linux will allow one thread to wait for children of other threads (provided they are in the same thread group - broadly parent process). If you are using Linux prior to 2.4, this is not the case. See the documentation on __WNOTHREAD for details.

I'm guessing the thread thing is a red herring, and the problem is actually the signal handler, as this accords with your statement 'the process is killed without becoming a zombie.'

来源：https://stackoverflow.com/questions/30032775/waitpid-for-child-process-not-succeeding