Hi Michael,
man waitpid says under RETURN VALUE:
waitpid(): on success, returns the process ID of the child whose
state has changed; on
error, -1 is returned; if WNOHANG was specified and no
child(ren) specified by pid has
yet changed state, then 0 is returned.
There is a subtle case when the child has no children at all and you
call waitpid(-1,NULL,WNOHANG). To me, the existing wording implies
that it will return 0. In fact it returns -1 and ECHILD, as I have
discovered the hard way :-(. The SUS documentation is more explicit
about this, e.g. http://www.opengroup.org/onlinepubs/007908799/xsh/wait.html:
"If waitpid() was invoked with WNOHANG set in options, ****it has at
least one child process specified by pid for which status is not
available****, and status is not available for any process specified by
pid, 0 will be returned. Otherwise, (pid_t)-1 will be returned, and
errno will be set to indicate the error."
I suggest the following, which is based on your existing text:
waitpid(): on success, returns the process ID of the child whose state
has changed;
else if WNOHANG was specified and child(ren) specified by pid exist but
have not yet changed state, then 0 is returned;
else an error has occurred and -1 is returned.
Regards,
Phil.
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