Threads synchronization using Mutex variable

1. To initialize a mutex, use below API:

int pthread_mutex_init (pthread_mutex_t *mutex, const pthread_mutexattr_t *mutexattr);

attributes for the mutex can be given through mutexattr, use NULL to use default attributes.

2. To lock a critical section use the below API. This is a blocking call. It means, it will block the calling thread until the lock is acquired.

int pthread_mutex_lock(pthread_mutex_t *mutex)

3. To acquire lock on critical section without blocking use below API:

int pthread_mutex_trylock(pthread_mutex_t *mutex);

4. To unlock the mutex use below API:

int pthread_mutex_unlock(pthread_mutex_t *mutex);

Before we use mutex, let us see the output of below function without thread synchronization:

Example 1:

In the below program I have created 2 threads, both of the threads try to increment the counter. This is called as race condition. It means, 2 threads are racing together to increment the count.

The section where the race condition occurs is called as critical section.

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <pthread.h>

//for more tutorials on C, C++, STL, DS, Linux visit www.ProDeveloperTutorial.com

int  counter = 0;


void *myFun()
{
   counter++;
   printf("Counter value: %d\n",counter);
}


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

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

   if( (rc2=pthread_create( &thread2, NULL, &myFun, NULL)) )
   {
      printf("Thread creation failed: %d\n", rc2);
   }

   pthread_join( thread1, NULL);
   pthread_join( thread2, NULL); 

return 0;
}

Output:

Counter value: 1
Counter value: 1

So as you see from the output, the counter value in both of the cases is 1.

Hence to avoid these kind of errors, we use mutex lock and unlock.

Example 2:

In the below example, I have used mutex lock and mutex unlock.

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <pthread.h>

//for more tutorials on C, C++, STL, DS, Linux visit www.ProDeveloperTutorial.com

int  counter = 0;

pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;

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


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

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

   if( (rc2=pthread_create( &thread2, NULL, &myFun, NULL)) )
   {
      printf("Thread creation failed: %d\n", rc2);
   }

   pthread_join( thread1, NULL);
   pthread_join( thread2, NULL); 

return 0;
}

Output:

Counter value: 1
Counter value: 2

As you can see from the output above, the value will always return 2.