# \<semaphore.h> Library

I learned about the **\<semaphore.h> library**. A semaphore is similar to a mutex with a few differences. It does not have the inherent concept of ownership, like a mutex, but is used to control access to a pool of resources rather than a single resource. Semaphores are useful if you need multiple threads to acquire and release semaphores concurrently, up to a specified count. A semaphore is typically used to control access to a pool of resources. Threads can acquire a semaphore using the sem\_wait(\&mutex) function, which decrements the semaphore value. This function will wait until the semaphore value becomes greater than zero, allowing the thread to proceed and perform its task. Once the task is completed, the semaphore is incremented using the sem\_post(\&mutex) function to signal that the resource is available for other threads.

***

The **sem\_init()** function is used to initialize a semaphore.

* The first argument is a pointer to a sem\_t variable.
* The second argument is a flag that indicates whether the semaphore is shared between processes.
* The third argument is the initial value of the semaphore.
* The function returns 0 if the semaphore is successfully initialized, otherwise, it returns an error code.

The **sem\_wait()** function is used to acquire a semaphore.

* The first argument is a pointer to a sem\_t variable.
* The function returns 0 if the semaphore is successfully acquired, otherwise, it returns an error code.

The **sem\_post()** function is used to release a semaphore.

* The first argument is a pointer to a sem\_t variable.
* The function returns `0` if the semaphore is successfully released, otherwise, it returns an error code.

***

### Simple implementation of the semaphore library:

```c
#include <semaphore.h>

sem_t mutex;

void *althernateBlink(void *value)
{
  while (true)
  {
    sem_wait(&mutex);
    // do something once the semaphore is greater than 0
  }
  return NULL;
}

void *lightShowLED(void *value)
{
  while (true)
  {
    // do something

    sem_post(&mutex);
  }
  return NULL;
}

int main()
{
  pthread_t thread[4];

  sem_init(&mutex, 0, 0);

  pthread_create(&thread[0], NULL, &lightShowLED, NULL);
  pthread_create(&thread[1], NULL, &althernateBlink, NULL);

  return 0;
}
```

Here is the documentation for the [semaphore.h library](https://pubs.opengroup.org/onlinepubs/7908799/xsh/semaphore.h.html)


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