pthread_kill in multithread'd app causes segmentation fault - linux

Question

I managed to find several references to this problem which suggested pthread_kill was de-referencing the pthread_t structure which was causing some problem however other articles said this is not a problem as long as the pthread_t staructure is created via pthread_create.
Then I found a multi thread example of how to do this correctly :
How to send a signal to a process in C?
However I am still getting seg faults so here is my code example:
static pthread_t GPUthread;
static void GPUsigHandler(int signo)
{
fprintf(stderr, "Queue waking\n");
}
void StartGPUQueue()
{
sigset_t sigmask;
pthread_attr_t attr_obj; /* a thread attribute variable */
struct sigaction action;
/* set up signal mask to block all in main thread */
sigfillset(&sigmask); /* to turn on all bits */
pthread_sigmask(SIG_BLOCK, &sigmask, (sigset_t *)0);
/* set up signal handlers for SIGINT & SIGUSR1 */
action.sa_flags = 0;
action.sa_handler = GPUsigHandler;
sigaction(SIGUSR1, &action, (struct sigaction *)0);
pthread_attr_init(&attr_obj); /* init it to default */
pthread_attr_setdetachstate(&attr_obj, PTHREAD_CREATE_DETACHED);
GPUthread = pthread_create(&GPUthread, &attr_obj, ProcessGPUqueue, NULL);
if (GPUthread != 0)
{
fprintf(stderr, "Cannot start GPU thread\n");
}
}
void ProcessGPUqueue(void *ptr)
{
int sigdummy;
sigset_t sigmask;
sigfillset(&sigmask); /* will unblock all signals */
pthread_sigmask(SIG_UNBLOCK, &sigmask, (sigset_t *)0);
fprintf(stderr, "GPU queue alive\n");
while(queueActive)
{
fprintf(stderr, "Processing GPU queue\n");
while(GPUqueue != NULL)
{
// process stuff
}
sigwait(&sigmask, &sigdummy);
}
}
void QueueGPUrequest(unsigned char cmd, unsigned short p1, unsigned short p2, unsigned short p3, unsigned short p4)
{
// Add request to queue logic ...
fprintf(stderr, "About to Wake GPU queue\n");
pthread_kill(GPUthread, SIGUSR1);// Earth shattering KA-BOOM!!!
}