C: Indexing pthreads with numbers 0 through n -1

I've got a set of n threads that need to be numbered 0 to n - 1.

This does not work right.

#include <pthread.h>
#include <stdio.h>

#define MY_THREAD_COUNT 6

void *runner(void *param) {
	int id = *(int*)param;
	printf("Thread %d\n", id);
}

int main() {
	pthread_t thread_arr[MY_THREAD_COUNT];
	for (int i = 0; i < MY_THREAD_COUNT; i++)
		pthread_create(&thread_arr[i], NULL, runner, &i);
	for (int i = 0; i < MY_THREAD_COUNT; i++)
		pthread_join(thread_arr[i], NULL);
}

It results in output such as:

> Thread 2<br>Thread 2<br>Thread 3<br>Thread 4<br>Thread 5<br>Thread 6

Creating a pointer for each thread's ID number does work right:

	for (int i = 0; i < MY_THREAD_COUNT; i++) {
		int *tmp = malloc(sizeof(int));
		*tmp = i;
		pthread_create(&thread_arr[i], NULL, runner, tmp);
	}

It results in output such as:

> Thread 0<br>Thread 1<br>Thread 2<br>Thread 4<br>Thread 5<br>Thread 3

However, since it entails mallocing, leaking, yada yada, that's not my ideal solution.

Is there any way to get the id number from a function called inside of runner()? Something like pthread_self() but that would give serial numbers, 0 through n - 1?

Or — to open the question up more generally — what's the overall cleanest way to do this? If any approach is on the table, what do you think is best?

The results you see are because you have no guarantees when the created threads will run in relation to the main thread. You pass the address of i to runner. That's the same address each time, so whatever value is in i is what runner will see when it runs. Even worse, the for loop could terminate before the thread runs (this explains your 6) .. you're just getting lucky with that 6, runner is accessing memory out of scope when that happens.

One possible solution:

#define MY_THREAD_COUNT 6

void *runner(void *param) {
    int id = *(int*)param;
    printf("Thread %d\n", id);
}

int main() {
    pthread_t thread_arr[MY_THREAD_COUNT];
    int threadArgs[MY_THREAD_COUNT];

    for (int i = 0; i < MY_THREAD_COUNT; i++)
    {
        threadArgs[i] = i;
        pthread_create(&thread_arr[i], NULL, runner, &(threadArgs[i]));
    }
    for (int i = 0; i < MY_THREAD_COUNT; i++)
        pthread_join(thread_arr[i], NULL);
}

This will keep the arguments in scope for duration of run time.

You could do:

#define MY_THREAD_COUNT 6

void *runner(void *param) {
    int id = (int) param;
    printf("Thread %d\n", id);
    return NULL;
}

int main() {
    pthread_t thread_arr[MY_THREAD_COUNT];
    for (int i = 0; i < MY_THREAD_COUNT; i++)
        pthread_create(&thread_arr[i], NULL, runner, (void *) i);
    for (int i = 0; i < MY_THREAD_COUNT; i++)
        pthread_join(thread_arr[i], NULL);
}

And then you have no lifetime issues with the parameter passed to the child thread.