How can I use wait() as a non-blocking command in Unix?

Question

The wait() system call suspends execution of the calling thread until one of its children terminates.

Regarding why to use wait(), it says

In the case of a terminated child, performing a wait allows the system to release the resources associated with the child; if a wait is not performed, then the terminated child remains in a "zombie" state

So, it is a good practice to use wait() & wait() is blocking command. That is what I derive from the man page.

How to use wait() but in a non-blocking way so that the calling thread can go about its business & when child state changes, it gets notified.

From wait man page "WNOHANG return immediately if no child has exited." (use waitpid(), wait() is obsolete) — Stargateur
– Stargateur, Commented Jun 29, 2018 at 5:47
@Stargateur if I use wait() it will always be blocking but if I use waitpid() I have the option of making it non-blocking right? — KharoBangdo
– KharoBangdo, Commented Jun 29, 2018 at 5:53
"when child state changes, it gets notified" – That's what SIGCHLD is for. — Martin R
– Martin R, Commented Jun 29, 2018 at 6:45
@MartinR so, you are saying I should use SIGCHLD to ignore all children so that I don't have to wait(). If my parent has a loop of instructions it's supposed to perform, then call wait() at the end to see if any child is dead & if yes than release resource of that child. Is that correct? — KharoBangdo
– KharoBangdo, Commented Jun 29, 2018 at 7:02
What I (roughly) mean is that you register a signal handler for SIGCHLD, and set a flag if it is called. In your main event loop check if the flag is set – then you can safely call wait(). — Martin R
– Martin R, Commented Jun 29, 2018 at 7:07

Krishna Kanth Yenumula · Accepted Answer · 2019-10-15 07:11:07Z

2

wait() is always blocking.
waitpid() can be used for blocking or non-blocking.

we can use waitpid() as a non-blocking system call with the following format:

int pid = waitpid(child_pid, &status, WNOHANG);

WNOHANG-->Returns immediately regardless of the child’s status.

Reference:https://www-users.cs.umn.edu/~kauffman/4061/04-making-processes.pdf
Page no:13 (Non-Blocking waitpid()).

answered Oct 15, 2019 at 7:11

Krishna Kanth Yenumula

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Krishna Kanth Yenumula Over a year ago

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