Semaphore is a synchronization mechanism. In more words, semaphores are a technique for coordinating or synchronizing activities in which multiple processes compete for the same resources. There are 2 types of semaphores: Binary semaphores & Counting semaphores. But our focus would be on binary semaphore only. That too binary semaphore example between threads in C language specifically. If you are in search of semaphore between processes then see this.

  • As its name suggest binary semaphore can have a value either 0 or 1.
  • It means binary semaphore protect access to a SINGLE shared resource.
  • So the internal counter of the semaphore can only take the values 1 or 0.
  • When a resource is available, the process in charge set the semaphore to 1 else 0.

Example of Binary semaphore example between threads in C using POSIX-semaphore

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

int a, b;
sem_t sem;

void ScanNumbers(void *ptr){
    for (;;){
        printf("%s", (char *)ptr);
        scanf("%d %d", &a, &b);
        sem_post(&sem);
        usleep(100 * 1000);
    }
}

void SumAndPrint(void *ptr){
    for (;;){
        sem_wait(&sem);
        printf("%s %d\n", (char *)ptr, a + b);
    }
}

int main()
{
    pthread_t thread1;
    pthread_t thread2;

    char *Msg1 = "Enter Number Two No\n";
    char *Msg2 = "sum = ";

    /*
    int sem_init(
        sem_t *sem          // pointer to semaphore variable    ,
        int pshared         // If = 0: can be used in threads only, else in process,
        unsigned int value  // initial value of the semaphore counter
    );

    return value 0 on successful & -1 on failure
    */

    sem_init(&sem, 0, 0);   // Can also use `sem = sem_open( "SemaphoreName", O_CREAT, 0777, 0);`

    pthread_create(&thread1, NULL, (void *)ScanNumbers, (void *)Msg1);
    pthread_create(&thread2, NULL, (void *)SumAndPrint, (void *)Msg2);

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

    printf("Wait For Both Thread Finished\n");
    sem_destroy(&sem);      // Can also use `sem_unlink( "SemaphoreName");`

    return 0;
}
  • sem_init() : Initialize semaphore
  • sem_destroy() : releases all resources
  • sem_wait() : Wait for the semaphore to acquire
  • sem_post() : Release semaphore
  • sem_trywait() : Only works when the caller does not have to wait
  • sem_getvalue() : Reads the counter value of the semaphore
  • sem_open() : Connects to, & optionally creates, a named semaphore( like sem_init() )
  • sem_unlink() : Ends connection to an open semaphore & causes the semaphore to be removed when the last process closes it( like sem_destroy())

General pointers

  • Semaphore’s internal implementation is like memory-mapped file(mmap)
  • Two standards of semaphore mechanism
    1. POSIX-semaphore: sem_init(), sem_destroy(), sem_wait(), sem_post(), sem_trywait(), sem_getvalue(), sem_open(),sem_unlink()
    2. System-V-semaphore: semget(), semop(), `semctl()`