Linux Pthread argument

Question

This is my code. It's very simple.

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

void *func(void *arg)
{
    printf("ID=%d\n", *(int*)arg);
    pthread_exit(NULL);
}

int main()
{
    pthread_t pt[4];
    int i;

    for (i = 0; i < 4; i++)
    {
        int temp = i;
        pthread_create(&pt[i], NULL, func, (void*)&temp);
    }
    sleep(1);
    return 0;
}

I compiled it:

gcc p_test.c -lpthread

I ran it. It printed 2 2 3 3. I ran it again. It printed 2 3 3 2.

My problem is:

Why was 2 or 3 printed twice?

Why didn't it print 1 3 2 0 or any other results?

Answer 1

The major problem here is that you're taking the address of the local variable temp, and then using that pointer outside the scope of the variable - as soon as you exit one iteration of the loop, your pointer to temp becomes invalid and you must not dereference it.