Phil,
Phil Endecott wrote:
> 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.
You are of course right that the text could be clearer. Thanks for
pointing that out.
For man-pages-2.77, I've changed the text to:
waitpid(): on success, returns the process ID of the
child whose state has changed; if WNOHANG was specified
and one or more child(ren) specified by pid exist, but
have not yet changed state, then 0 is returned. On
error, -1 is returned.
Cheers,
Michael
--
Michael Kerrisk
Maintainer of the Linux man-pages project
http://www.kernel.org/doc/man-pages/
Want to report a man-pages bug? Look here:
http://www.kernel.org/doc/man-pages/reporting_bugs.html
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