System() returning ECHILD error on FreeBSD 7.2

[Oliver Fromme]

Naveen Gujje <gujjenaveen at gmail.com> wrote:
 > signal(SIGCHLD, SigChildHandler);
 > 
 > void
 > SigChildHandler(int sig)
 > {
 >   pid_t pid;
 > 
 >   /* get status of all dead procs */
 >   do {
 >     int procstat;
 >     pid = waitpid(-1, &procstat, WNOHANG);
 >     if (pid < 0) {
 >       if (errno == EINTR)
 >         continue;               /* ignore it */
 >       else {
 >         if (errno != ECHILD)
 >           perror("getting waitpid");
 >         pid = 0;                /* break out */
 >       }
 >     }
 >     else if (pid != 0)
 >       syslog(LOG_INFO, "child process %d completed", (int) pid);
 >   } while (pid);
 > 
 >   signal(SIGCHLD, SigChildHandler);
 > }

There are several problems with your signal handler.

First, the perror() and syslog() functions are not re-entrant,
so they should not be used inside signal handlers.  This can
lead to undefined behaviour.  Please refer to the sigaction(2)
manual page for a list of functions that are considered safe
to be used inside signal handlers.

Second, you are using functions that may change the value of
the global errno variable.  Therefore you must save its value
at the beginning of the signal handler, and restore it at the
end.

Third (not a problem in this particular case, AFAICT, but
still good to know):  Unlike SysV systems, BSD systems do
_not_ automatically reset the signal action when the handler
is called.  Therefore you do not have to call signal() again
in the handler (but it shouldn't hurt either).  Because of
the semantic difference of the signal() function on different
systems, it is preferable to use sigaction(2) instead in
portable code.

 > And, in some other part of the code, we call system() to add an ethernet
 > interface. This system() call is returning -1 with errno set to ECHILD,
 > though the passed command is executed successfully.  I have noticed that,
 > the problem is observed only after we register SigChildHandler. If I have a
 > simple statement like system("ls") before and after the call to
 > signal(SIGCHLD, SigChildHandler), the call before setting signal handler
 > succeeds without errors and the call after setting signal handler returns -1
 > with errno set to ECHILD.
 > 
 > Here, I believe that within the system() call, the child exited before the
 > parent got a chance to call _wait4 and thus resulted in ECHILD error.

I don't think that can happen.

 > But, for the child to exit without notifying the parent, SIGCHLD has to be
 > set to SIG_IGN in the parent and this is not the case, because we are already
 > setting it to SigChildHandler. If I set SIGCHLD to SIG_DFL before calling
 > system() then i don't see this problem.
 > 
 > I would like to know how setting SIGCHLD to SIG_DFL or SigChildHanlder is
 > making the difference.

The system() function temporarily blocks SIGCHLD (i.e. it
adds the signal to the process' signal mask).  However,
blocking is different from ignoring:  The signal is held
as long as it is blocked, and as soon as it is removed
from the mask, it is delivered, i.e. your signal handler
is called right before the system() function returns.

And since you don't save the errno value, your signal
handler overwrites the value returned from the system()
function.  So you get ECHILD.

Best regards
   Oliver

-- 
Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing b. M.
Handelsregister: Registergericht Muenchen, HRA 74606,  Geschäftsfuehrung:
secnetix Verwaltungsgesellsch. mbH, Handelsregister: Registergericht Mün-
chen, HRB 125758,  Geschäftsführer: Maik Bachmann, Olaf Erb, Ralf Gebhart

FreeBSD-Dienstleistungen, -Produkte und mehr:  http://www.secnetix.de/bsd

"What is this talk of 'release'?  We do not make software 'releases'.
Our software 'escapes', leaving a bloody trail of designers and quality
assurance people in its wake."