linux mutex lock: pthread_mutex_t mutex_lock

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

pthread_mutex_t mutex_lock;  
static volatile int count = 0;  

void *test_func(void *arg)  
{  
    int i = 0;  
    for(i = 0; i < 2000000; i++)  
    {  
            pthread_mutex_lock(&mutex_lock); // a thread should get this mutex lock first.
            count++; // at one time, this grobal var can be accessed by only one thread which holds this mutex lock.
            pthread_mutex_unlock(&mutex_lock); // after access, thread should release this lock.
    }  
    return NULL;  
}  

int main(void)
{
	pthread_mutex_init(&mutex_lock, NULL);
	pthread_t thread_ids[10];  
	int i = 0;  
	for(i = 0; i < sizeof(thread_ids)/sizeof(pthread_t); i++)  
	{  
	    pthread_create(&thread_ids[i], NULL, test_func, NULL);  
	}  
	
	for(i = 0; i < sizeof(thread_ids)/sizeof(pthread_t); i++)  
	{  
	    pthread_join(thread_ids[i], NULL);	// wait threads to exit.
	}
	
	printf("count: %d\n", count);
	return 0;
}

the execution result:
count: 20000000	// 2000000 * 10(10 threads)