pthread_Join can't return after call pthread_cancel? - pthreads

Question

I used Eclispse Indigo + CDT 8.0.2 + cygwin to develope a multi-thread systerm, the code is below:
pthread_mutex_t mutexCmd = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t signalCmd = PTHREAD_COND_INITIALIZER;
void * Func(void * arg)
{
int iStatus;
while (1)
{
int a = 1;
pthread_cleanup_push(pthread_mutex_unlock, &mutexCmd);
pthread_mutex_lock(&mutexCmd);
iStatus = pthread_cond_wait(&signalCmd, &mutexCmd);
if (iStatus) {
err_abort(iStatus, "signalCmd status error");
}
if(arg->Cmd != navi_Go) //Just a command tag;
{
pthread_mutex_unlock(&(pNaviCtrl->mutexCmd));
continue;
}
//do some work
//.....
pthread_mutex_unlock(&mutexCmd);
pthread_cleanup_pop(1);
}
//pthread_detach(pthread_self());
return NULL;
}
int main()
{
int iStatus = 0;
pthread = tid;
iStatus = pthread_create(&tid;NULL, Func, NULL);
if(iStatus)
{
err_abort(iStatus, "Start pthread error");
}
// do some work
...
//Cancel thread
void * retval;
iStatus = pthread_cancel(tid)
iStatus = pthread_join(tid; &retval);
if(iStatus){
err_abort(iStatus,"Stop thread error");
}
return iStatus;
}
where program run, it stop at "iStatus = pthread_join(tid1; &retval);" couldn't go forward anymore, I think the thread could be happed to deadlock, but can't find the reason. I supposed after call pthread_cancel(), the thread will exit and return to the pthread_join(),
who can tell me what's wrong with my code?

Answer 1

Don't put cleanup_push and _pop inside the while loop. Don't call them more than once. If you look at them, they are macros that wrap the code between them in { }. They setup a longjump that is used when you call pthread_cancel.

Answer 2

pthread_cleanup_pop(1) tells the pthread library to not only pop the cleanup entry off the stack, but to also execute it. So that call will also implicitly call:
pthread_mutex_unlock(&mutexCmd);
Since you've already unlocked the mutex, that call has undefined behavior (assuming the mutex type is PTHREAD_MUTEX_NORMAL). I imagine that call is just never returning or something.
Note that your code has other problems handing the cleanup - if you execute the continue for the loop, you'll call pthread_cleanup_push() a second time (or more), which will add another cleanup context.
There may be other problems (I'm not very familiar with pthread_cancel()).