Re: For review: timer_create.2

[Peter Zijlstra <a.p.zijlstra-/NLkJaSkS4VmR6Xm/wNWPw@public.gmane.org>]

On Tue, 2009-02-10 at 14:53 +1300, Michael Kerrisk wrote:
>        static void
>        handler(int sig, siginfo_t *si, void *uc)
>        {
>            /* Note: calling printf() from a signal handler is not
>               strictly correct, since printf() is not async-signal-safe;
>               see signal(7) */
> 
>            printf("Caught signal %d\n", sig);
>            print_siginfo(si);
>            signal(SIG, SIG_IGN);
>        }

Is that actually safe to do?

I can remember getting into deadlocks by doing this some few years ago.

The problem was that glibc uses a mutex to guard its memory area for the
malloc memory pool, and signals can come in while the main thread of
execution is within that mutex, if at that point the signal handler also
tries to use anything using malloc (here, I was thinking printf()
might), you've got a deadlock.

One thing I always enjoyed about the QNX man-pages was that it
explicitly states which functions are and are _NOT_ signal safe, iow
which functions can be reliably used from signal handler context.



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