Semaphores are very useful in process synchronization and multithreading. But how to use one in real life, for example say in C Language?
Well, we have the POSIX semaphore library in Linux systems. Let’s learn how to use it.
The basic code of a semaphore is simple as presented here. But this code cannot be written directly, as the functions require to be atomic and writing code directly would lead to a context switch without function completion and would result in a mess.
The POSIX system in Linux presents its own built-in semaphore library. To use it, we have to :
- Include semaphore.h
- Compile the code by linking with -lpthread -lrt
To lock a semaphore or wait we can use the sem_wait function:
int sem_wait(sem_t *sem);
To release or signal a semaphore, we use the sem_post function:
int sem_post(sem_t *sem);
A semaphore is initialised by using sem_init(for processes or threads) or sem_open (for IPC).
sem_init(sem_t *sem, int pshared, unsigned int value);
- sem : Specifies the semaphore to be initialized.
- pshared : This argument specifies whether or not the newly initialized semaphore is shared between processes or between threads. A non-zero value means the semaphore is shared between processes and a value of zero means it is shared between threads.
- value : Specifies the value to assign to the newly initialized semaphore.
To destroy a semaphore, we can use sem_destroy.
To declare a semaphore, the data type is sem_t.
// C program to demonstrate working of Semaphores
sem_init(&mutex, 0, 1);
Compilation should be done with gcc a.c -lpthread -lrt
2 threads are being created, one 2 seconds after the first one.
But the first thread will sleep for 4 seconds after acquiring the lock.
Thus the second thread will not enter immediately after it is called, it will enter 4 – 2 = 2 secs after it is called.
So the output is:
Entered.. Just Exiting... Entered.. Just Exiting...
Entered.. Entered.. Just Exiting... Just Exiting...
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