counter.c

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

pthread_mutex_t lock;
int counter = 0;

#define MAX_ITER 100000

void* writer(void *arg) {
    for(int i=0; i < MAX_ITER; i++)
    {
        pthread_mutex_lock(&lock);
        counter += 1;
        pthread_mutex_unlock(&lock);
    }

    return NULL;
}

int main() {
    int writers_num = 8;

    pthread_t tid_writers[writers_num];

    if (pthread_mutex_init(&lock, NULL) != 0)
    {
        printf("\n mutex init failed\n");
        return 1;
    }

	for (int i=0; i < writers_num; i++)
	{
		if (pthread_create(&(tid_writers[i]), NULL, &writer, &i)!=0)
		{
			printf("\ncannot create writer thread\n");
			exit(0);
		}
	}

	for(int i=0; i < writers_num; i++)
	{
		pthread_join(tid_writers[i], NULL);
	}

	pthread_mutex_destroy(&lock);

    assert((MAX_ITER*writers_num==counter) && "counter failed");
}