personal implementation of semaphores for the C programming language

semaphore.c

#include <stdlib.h>
#include <pthread.h>
#include <stdbool.h>

// canonical macro aliases from `semaphore.h` 
#define P(sem) sem_wait(sem)
#define V(sem) sem_post(sem)

#define UP(sem) sem_wait(sem)
#define DOWN(sem) sem_post(sem)

typedef struct semaphore {
  int permit_counter;
  pthread_cond_t cv;
  pthread_mutex_t mutex; /* Controls access to `permit_counter` */
} semaphore_t;

semaphore_t* semaphore_renew(void);

semaphore_t* semaphore_init(semaphore_t* sem, int count) {
  sem->permit_counter = count;
  pthread_cond_init(&sem->cv, NULL);
  pthread_mutex_init(&sem->mutex, NULL);
}

semaphore_t* semaphore_wait(semaphore_t* sem) {
  pthread_mutex_lock(&sem->mutex);
  sem->permit_counter--;

  // a thread was blocked
  if (sem->permit_counter > 0) {
    pthread_cond_wait(&sem->cv, &sem->mutex);
  }

  pthread_mutex_unlock(&sem->mutex);
}

semaphore_t* semaphore_post(semaphore_t* sem) {
  bool has_waiting_thread;
  pthread_mutex_lock(&sem->mutex);

  has_waiting_thread = sem->permit_counter < 0 ? true : false;
  sem->permit_counter++;

  if (has_waiting_thread) {
    pthread_cond_signal(&sem->cv);
  }

  pthread_mutex_unlock(&sem->mutex);
}

semaphore_t* semaphore_destroy(semaphore_t* sem) {
  sem->permit_counter = 0;
  pthread_mutex_unlock(&sem->mutex);
  pthread_cond_destroy(&sem->cv);
  pthread_mutex_destroy(&sem->mutex);  
}

int semaphore_reveal(semaphore_t* sem) {
  return sem->permit_counter;
}