Segmentation fault (exit code 139) when using pthread_join

I’m doing some first steps with threads on Linux systems, and i have this error which occures on the base of a program that gets some n argument and creates n number of threads.

Here is the important part of the code:

The function the threads should run:

void* function(void* arg){
    int id = (long)arg;

    printf("Thread #%ld created!n",id);

    pthread_exit(0);
}

Important part of code in the main function

int main(int argc, char **argv){

    if(argc != 3){
        printf("Usage: %s <num> <exp>n",argv[0]);
        exit(-1);
    }

    int num = atoi(argv[1]), exp = atoi(argv[2]);
    long i;
    pthread_t threads[num];
    pthread_attr_t attr;

    printf("Creating %d threads n",num);

    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr,PTHREAD_CREATE_JOINABLE);

    for (i = 0; i < num; i++) {
        pthread_create(&threads[i],&attr,function,(void*)i);
    }

    pthread_attr_destroy(&attr);

    for (i = 0; i < num; i++) {
        printf("Thread #%d is %ldn", i, threads[i]);
    }

    for (i = 0; i < num; i++) {
        pthread_join(threads[num], NULL);
    }

}

Notice the last for loop with the pthread_join function, when i comment it out the program ends fine (exit code 0), but the output is obviously wrong since not all of the threads runs function before the main process exits.

When i don’t comment it out i get segmentation fault (core dumped) when trying to run using the terminal in my Linux OS and Process finished with exit code 139 when running in my IDE (CLion).

I couldn’t find anything i’ve done wrong as it’s a very basic program that shouldn’t have anything hard to find, what is the problem that causes the error?

Answer

You should have used pthread_join(threads[i], NULL); instead of pthread_join(threads[num], NULL);. Suppose, the num is set to 4 via argument. Then threads[num] points to 5th element of array which does not exist as you have allocated an array of only 4 elements.