by A Mutex is basically a lock that is locked before accessing any critical section of code to ensure only one thread at a time can access it. Mutex is unlocked when the thread is done processing. Thus, mutex can also be thought as “Mutually exclusive flag” to ensure all other threads are blocked to a certain section of code when one thread is already accessing it.
Important Points
- Mutex must be initialized before use.
- Mutex locking function is a blocking call. Hence should be used judiciously.
- Putting everything in mutex lock is not useful, it will defeat the benefits of multi-threading. Instead, only critical sections must be protected with mutexes.
- Mutexes are only applicable between threads of a process. It’s not applicable between threads of different processes.
| Functions | Usage |
|---|---|
| pthread_mutex_init | Function to initialize the mutex. |
| PTHREAD_MUTEX_INITIALIZER | Mutex initializer applicable to statically allocated mutexes. |
| pthread_mutex_lock | • Function to take the lock. • Blocking Call • Returns 0 if success else error code. |
| pthread_mutex_trylock | • Function to take the lock. • Non-Blocking call. • Returns 0 if success else error code. |
| pthread_mutex_unlock | • Function to release the lock • Returns 0 if success else error code. |
| pthread_mutex_timedlock | • Function to take the lock. • Blocking call with a timeout value. • Returns 0 if success else error code. |
Example
Let’s have a look into sample program to illustrate mutex functionality.
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
pthread_mutex_t lock;
int counter = 0;
void* threadFun(void* arg)
{
printf("Inside Thread function \n");
pthread_mutex_lock(&lock);
printf("Got the mutex Counter now: %d \n", counter);
counter += 20;
pthread_mutex_unlock(&lock);
return NULL;
}
int main()
{
pthread_t tid[2];
if(pthread_mutex_init(&lock, NULL) != 0)
{
printf("Mutex initialization failed \n");
}
int i = 0;
while (i < 2)
{
int error = pthread_create(&tid[i], NULL, &threadFun, NULL);
if (error != 0)
{
printf("Thread Creation failed error code %d \n", error);
}
i++;
}
pthread_join(tid[0], NULL);
pthread_join(tid[1], NULL);
pthread_mutex_destroy(&lock);
return 0;
}
Let’s check the output of above sample program.
Inside Thread function
Got the mutex Counter now: 0
Inside Thread function
Got the mutex Counter now: 20
