Mutex variable using pthread in C programming

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

void *functionC();
pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;
int  counter = 0;

main()
{
    int rc1, rc2;
    pthread_t thread1, thread2;

    /* Create independent threads each of which will execute functionC */

    if( (rc1=pthread_create( &thread1, NULL, &functionC, NULL)) )
    {
        printf("Thread creation failed: %d\n", rc1);
    }

    if( (rc2=pthread_create( &thread2, NULL, &functionC, NULL)) )
    {
        printf("Thread creation failed: %d\n", rc2);
    }
    /* Wait till threads are complete before main continues. Unless we  */
    /* wait we run the risk of executing an exit which will terminate   */
    /* the process and all threads before the threads have completed.   */
    pthread_join( thread1, NULL);
    pthread_join( thread2, NULL); 
    exit(0);
}




void *functionC()
{
    pthread_mutex_lock( &mutex1 );
    counter++;
    printf("Counter value: %d\n",counter);
    pthread_mutex_unlock( &mutex1 );
}

OUTPUT

gcc -o mutex mutex.c -lpthread

./mutex


Counter value: 1
Counter value: 2