multithreads_mutex.c

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

/* number of threads */
#define N	3

/* global variable incremented by threads */
uint32_t g = 0;

/* global mutex protecting g */
pthread_mutex_t mutex;

/* function to be executed by all threads */
void *my_thread(void *vargp)
{
    uint32_t my_id = (uintptr_t)vargp;
    uint32_t iterations = 10000;

    while (iterations--)
    {
        uint32_t rounds = 10000, sum = 0, inc;

        /* Increment global variable g with an atomic operation */
        pthread_mutex_lock(&mutex);
        inc = ++g;
        /* sleep 0.1 milliseconds */
        usleep(100);
        pthread_mutex_unlock(&mutex);

        while (rounds--)
        {
            sum += inc;
        }

        /* sleep 0.9 millisecond */
        usleep(900);

        printf("Thread %u: inc = %5u, sum = %9u\n", my_id, inc, sum);
    }
}

int main()
{
    pthread_t tid[N];
    uint32_t i, id;

    /* init mutex */
    pthread_mutex_init(&mutex, NULL);

    /* create N threads */
    for (i = 0; i < N; i++)
    {
        id = i + 2;
        pthread_create(&tid[i], NULL, my_thread, (void*)(uintptr_t)id);
        printf("Started thread %d: %p\n", id, (void *)tid[i]);
    }

    /* wait for threads to terminate */
    for (i = 0; i < N; i++)
    {
        pthread_join(tid[i], NULL);
    }

    /* deinit mutex */
    pthread_mutex_destroy(&mutex);

    exit(0);
}