waitpid return value when there are no children

["Phil Endecott" <phil_pgkoc_endecott-wZDNlLIRyE5g9hUCZPvPmw@public.gmane.org>]

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.