On Wed, Feb 11, 2009 at 9:26 PM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> 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?
No (as you eventually realized, there is the comment in the code).
> 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.
Nowadays, signal(7) at keast documents the functions that POSIX
reuires to be async-signal-safe.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git
man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
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