>From: Mark Heath <[email protected]>
>
>I assume I'd need to use glib but it appears to be broken on my system.
>I also tried the gcc atomic operations but I've only got gcc 4.0.1.
>
>I've also sig_atomic_t but I've got no idea if it's reading and
>incrementing as an atomic action. The read and increment is such a
>short duration I cannot get it to break even without locking.
I think this talk of threaded applications says it's time for
a reposting of my caution and plea for portability...
Threaded programming is hard. PORTABLE threaded programming is
very hard.
Two things to keep in mind: 1) Check ALL return values and status
indications (I've seen too many programs *assume* that
pthred_mutex_unlock()
can not/will not fail) and 2) all the world is NOT linux (as much as
some might think otherwise ;)).
With those two cautions here's what I've posted several times over
the years (and is what mjpegtools uses in mpeg2enc). Happy reading :)
With linux a mutex can, according to the manpages, be of type 'fast',
'recursive' or 'error checking'. The 'fast' type has no concept of
thread "ownership" and any thread can unlock any mutex *even if it is
not the thread that locked it*. On the other hand the 'error checking'
type does enforce mutex "ownership" and only the thread that locked
a mutex may unlock it, attempting to unlock a mutex not owned by the
thread returns an error (EPERM) - thus checking the status returned
by pthread_mutex_lock() and pthread_mutex_unlock() is a good idea.
FreeBSD, Solaris and BSD/OS all use 'error checking' mutexes (I did
verify this with a simple test program). MAC OS/X says unlocking a
mutex not owned by the calling thread is undefined. The default linux
mutex attribute is, for some unknown reason, 'fast'.
As you can imagine a program that relies on 'fast' mutex behaviour
will not function correctly on a system which uses 'error checking'
(i.e. ownership) mutexes (the reverse case might work - I haven't
tried it ;)).
The workaround is to override the brain damaged default with:
#ifdef __linux__
pthread_mutexattr_t mu_attr;
pthread_mutexattr_t *p_attr = &mu_attr;
pthread_mutexattr_init(&mu_attr);
pthread_mutexattr_settype( &mu_attr, PTHREAD_MUTEX_ERRORCHECK );
#else
pthread_mutexattr_t *p_attr = NULL;
#endif
pthread_mutex_init( &frame_buffer_lock, p_attr);
Cheers,
Steven Schultz
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