Creating and passing multiple pthreads in C-CodePudding

I'm trying to get pthread to output a variable declared in a for loop:

pthread_t pthread[10];

void * count(void* argv){
    int index = *(int*)argv;
    printf("%d", index);
    pthread_exit(NULL);
}

int main() {
     for (int i = 0; i < 10;   i) {
        pthread_create(&pthread[i], NULL, count,(void*)&i);
    }
    return 0;
}

Output I thought:

0123456789 (or not in order, but all 10 number)

What I got:

123456789

Why 0 is not in here?

CodePudding user response：

One problem is that your main() thread exits without waiting for the child threads to finish, which means the child threads may not have time to finish (or potentially even begin) their execution before the process is terminated.

To avoid that problem, you need to call pthread_join() on all of your threads before main() exits, like this:

int main() {
     for (int i = 0; i < 10;   i) {
        pthread_create(&pthread[i], NULL, count,(void*)&i);
     }
     for (int i = 0; i < 10;   i) {
        pthread_join(pthread[i], NULL);  // won't return until thread has exited
     }
    return 0;
}

The other problem, as mentioned by 500 in the comments, is that you are passing a pointer to i to the child threads, which they then dereference, and since i is being modified in the main thread's loop, it's undefined behavior what value the child threads will read from that pointer. One way to avoid this is to give each thread its own separate (non-changing) integer to read:

 int values[10];

 for (int i = 0; i < 10;   i) {
    values[i] = i;
    pthread_create(&pthread[i], NULL, count,(void*)&values[i]);
 }