pthreads and concurrency

Question

I have the following code:

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

#define LOOPS 10000

void *run(void *arg)
{
    int id = strtol(arg,NULL,0);
    int i;
    for(i=0; i<LOOPS; i++)
    {
        printf("In %d.\n",id);
    }
}

int main()
{
    pthread_t p1,p2;
    void *res;

    pthread_create(&p1,NULL,run,"1");
    pthread_create(&p2,NULL,run,"2");
    pthread_join(p1,&res);
    pthread_join(p2,&res);
    return 0;
}

When I run this, either the string "In 1" displays 10000 times consecutively then "In 2" displays 10000 times consecutively or vice versa. Shouldn't the strings be alternating and not displaying consecutively as they are here?

Answer 1

The order in which threads are interleaved by the scheduler is not deterministic (...well it is but only from the scheduler's/kernel's point of view). You should not make assumptions about the order.

In this situation you experience that either one of the threads is allowed to complete their whole work before the scheduler ->preempts it and allows the other thread to run.