Question

Can the following code be modified to use a mutex lock rather than a semaphore? Why or why not? (sem_post() is the POSIX form of signal())

sem_t sem;

sem_init(&sem, 0, 1);

sem_wait(&sem);

// critical section

sem_post(&sem);

sem_destroy(&sem);

Answer 1

Critical Section - It is simply defined as an area in the code where shared resources can be accessed. Two or more processes can access the same resource. This creates a problem called the critical section problem when two processes want to access the resource simultaneously. To solve this problem, two mechanisms are used one is a semaphore and the other is a mutex.

Semaphore - A Semaphore is a signaling mechanism that is used in synchronization operations in computer systems, A Semaphore is used to solve critical section problem by using two atomic operations signal and wait.

Mutex - Mutex is a locking mechanism that uses a mutex lock on a resource. The mutex is used to ensure that only one thread is allowed in the critical section and use the resource.

Answer:

The following code cannot be modified to use a mutex lock because the following piece of code uses binary semaphore and a mutex can never act as a semaphore.

Lets break the code to understand better;

sem_t sem; - Here we declare a semaphore where sem_t is the typedef.

sem_init( &sem , 0 , 1 ) - Intialization of the binary semaphore *sem. A binary semaphore only uses 1 and 0 to signal. If 1 then wait operation is implemented and 0 when the signal operation is successful

sem_wait(&sem) - This implements the wait function on the semaphore *sem

critical section - The area of code where shared resources can be accessed.

sem_post(&sem) -   This implements the post function on *sem. On success, it return's 0, and on failure, the return value is -1.

sem_destroy(&sem) - Semaphore *sem is destroyed and memoy is deallocated.