Ending pthreads with deferred cancelation - c

Question

I'm new to pthreads and signal handling and I am working on a project that will create x amount of pthreads as either a producer or consumer, that execute forever, and I want to end all threads and then the main in a systematic way.
To do this, I am attempting to catch the ^c with a signal handler and then setting some global flag to then end the threads.
I have tried doing the following but I am unsure if it is working or not and I would like advice on my thought process and implementation.
Here is what I am working with, omitting error checking:
#include stdio, pthread, stdlib, semaphore, time, string, unistd, signal
sem_t empty, full;
pthread_mutex_t mutex;
int flag = 0;
void *producer();
void *consumer();
void sig_handler(int sig);
int main(int argc, char *argv[]){
signal(handler(SIGINT, sig_handler);
//init locks and semas
pthread_mutex_init(&mutex, NULL);
sem_init(&empty, 0, SOME_BUFFER_SIZE);
sem_init(&full, 0, 0);
pthread_t prod_threads[5]; //5 for example, can be any amount passed in
pthread_t cons_threads[3];
//start up threads
for(i = 0; i < 5; i++)
pthread_create(&prod_threads[i], NULL, producer, NULL)
for(i = 0; i < 3; i++)
pthread_create(&cons_threads[i], NULL, consumer, NULL)
//join threads at end
for(i = 0; i < PROD; i++)
pthread_join(&prod_threads[i], NULL);
for(i = 0; i < CONS; i++)
pthread_join(&cons_threads[i], NULL);
sleep(4);//could be any amount of time
exit(EXIT_SUCCESS);
}
//not sure if I am going about this the right way
void sig_handler(int sig){
if(sig == SIGINT)
flag = 1;
}
void *producer(){
while(1){
sleep(x);//where x is some random time
//part I am concerned about:
if(flag)
exit(EXIT_SUCCESS);
//get locks and semaphore stuff
//enter crit section
//release locks, semaphore
}
void *consumer(){
while(1){
sleep(x);//sleep some random time
//again not sure if this is right or not
if(flag)
exit(EXIT_SUCCESS);
//get locks
//enter crit
//release locks
}
}
}
My program seems to execute properly, I'm just not sure if I am using pthreads and signals correctly and would like some guidance there. If you need more complete code just let me know
Thanks

Answer 1

You're execution seems appropriate, however using the docs #yano provided, you're pthread_join() is off ever so slightly.
Instead of:
for(i = 0; i < PROD; i++)
pthread_join(&prod_threads[i], NULL);
for(i = 0; i < CONS; i++)
pthread_join(&cons_threads[i], NULL);
You need:
for(i = 0; i < PROD; i++)
pthread_join(prod_threads[i], NULL);
for(i = 0; i < CONS; i++)
pthread_join(cons_threads[i], NULL);
Notice the ampersand. Whether or not this is what you are looking for, I am not sure, but I hope it helps.

Answer 2

I want to end all threads and then the main in a systematic way.
As Martin James suggests, post a special thread message into the queue telling the worker threads to terminate. This message should not be removed from the queue, so that all worker threads have a chance to receive it and terminate.
The other point is that you should not call exit in your worker threads, since this function terminates the entire process, defeating your original goal. Just return from the thread function.
Another point is that in a multi-threaded application it is good practice to handle all signals in one specific thread (often the main thread is the best candidate for that). And block the signals you expect to receive in all other threads. It should be done by blocking the expected signals before creating other threads, so that they inherit the signal mask, and eventually unblocking these signals in the signal handling thread when it is ready to handle them (e.g. when all extra threads have been created). This is because a signal targeted for a process (such as SIGINT) can be delivered to any one of its threads, but we want it to be handled by one specific thread. See Signal Generation and Delivery for more details.