System() returning ECHILD error on FreeBSD 7.2

[Naveen Gujje]

On Thu, Feb 11, 2010 at 3:06 AM, Jilles Tjoelker <jilles at stack.nl> wrote:

> On Wed, Feb 10, 2010 at 12:44:57PM +0530, Naveen Gujje wrote:
> > [SIGCHLD handler that calls waitpid()]
>
> > 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.
> 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.
>
> I think your process is multi-threaded. In a single-threaded process,
> system()'s signal masking will ensure it will reap the zombie, leaving
> the signal handler with nothing (in fact, as of FreeBSD 7.0 it will not
> be called at all unless there are other child processes).
>
> In a multi-threaded process, each thread has its own signal mask and
> system() can only affect its own thread's signal mask. If another thread
> has SIGCHLD unblocked, the signal handler will race with system() trying
> to call waitpid() first.
>
> This makes sense. Bear with my ignorance, but will the signal handlers be
common for all or will each thread has its own signal handler?

Suppose, initially I set SIGCHLD to SIG_DFL, and created some pthreads, and
from the main process, I set SIGCHLD to my_handler, will it result in
invoking
my_handler for signals generated from within pthreads?

And, can a pthread generate a SIGCHLD signal on termination? If not, what
other
possibilities are there for a pthread to generate SIGCHLD signal?


> Possible fixes are using siginfo_t information to only waitpid() child
> processes you know about, setting up the signal masks so the bad
> situation does not occur (note that the signal mask is inherited across
> pthread_create()) and calling fork/execve and managing the child process
> exit yourself.
>
> Note that POSIX does not require system() to be thread-safe.
>
> --
> Jilles Tjoelker
>