/* Example code using SysV calls for semaphores and shared memory */

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/shm.h>
#include <sys/sem.h>
#include <sys/ipc.h>

/* Union semun */

union semun {

   int val;                  /* value for SETVAL */
   struct semid_ds *buf;     /* buffer for IPC_STAT, IPC_SET */
   unsigned short *array;    /* array for GETALL, SETALL */

};

void free_resources(int shm_id, int sem_id) { 

   /* Delete the shared memory segment */
   shmctl(shm_id,IPC_RMID,NULL);
   
   /* Delete the semaphore */
   semctl(sem_id,0,IPC_RMID,0);

}

/* Semaphore P - down operation, using semop */

int sem_P(int sem_id) {

   struct sembuf sem_d;
   
   sem_d.sem_num = 0;
   sem_d.sem_op = -1;
   sem_d.sem_flg = 0;
   if (semop(sem_id,&sem_d,1) == -1) {
       perror("# Semaphore down (P) operation ");
       return -1;
   }
   return 0;

}

/* Semaphore V - up operation, using semop */

int sem_V(int sem_id) {

   struct sembuf sem_d;
   
   sem_d.sem_num = 0;
   sem_d.sem_op = 1;
   sem_d.sem_flg = 0;
   if (semop(sem_id,&sem_d,1) == -1) {
       perror("# Semaphore up (V) operation ");
       return -1;
   }
   return 0;

}

/* Semaphore Init - set a semaphore's value to val */

int sem_Init(int sem_id, int val) {

   union semun arg;
   
   arg.val = val;
   if (semctl(sem_id,0,SETVAL,arg) == -1) {
       perror("# Semaphore setting value ");
       return -1;
   }
   return 0;

}

int main () { 

   int shm_id;
   int sem_id;
   int t = 0;
   int *sh;
   int pid;
   
   /* Create a new shared memory segment */
   shm_id = shmget(IPC_PRIVATE,sizeof(int),IPC_CREAT | 0660);
   if (shm_id == -1) {
       perror("Shared memory creation");
       exit(EXIT_FAILURE);
   }
   
   /* Create a new semaphore id */
   sem_id = semget(IPC_PRIVATE,1,IPC_CREAT | 0660);
   if (sem_id == -1) {
       perror("Semaphore creation ");
       shmctl(shm_id,IPC_RMID,(struct shmid_ds *)NULL);
       exit(EXIT_FAILURE);
   }
   
   /* Set the value of the semaphore to 1 */
   if (sem_Init(sem_id, 1) == -1) { 
       free_resources(shm_id,sem_id);
       exit(EXIT_FAILURE);
   }
   
   /* Attach the shared memory segment */
   sh = (int *)shmat(shm_id,NULL,0);
   if (sh == NULL) {
       perror("Shared memory attach ");
       free_resources(shm_id,sem_id);
       exit(EXIT_FAILURE);
   }
   /* Setting shared memory to 0 */
   *sh = 0;
   
   /* New process */
   if ((pid = fork()) == -1) { 
       perror("fork");
       free_resources(shm_id,sem_id);
       exit(EXIT_FAILURE);
   }
   
   if (pid == 0) { 
      /* Child process */
      printf("# I am the child process with process id: %d\n", getpid());
   } else {
      /* Parent process */
      printf("# I am the parent process with process id: %d\n", getpid());
   }
   
   printf("(%d): trying to access the critical section\n", getpid());
   sem_P(sem_id);
   printf("(%d): accessed the critical section\n", getpid());
   
   (*sh)++;
   printf("(%d): value of shared memory is now: %d\n", getpid(), *sh);
   
   printf("(%d): getting out of the critical section\n", getpid());
   sem_V(sem_id);
   
   printf("(%d): got out of the critical section\n", getpid());
   
   /* Child process */
   if (!pid)
     exit(EXIT_SUCCESS);
   
   /* Wait for child process */
   wait(NULL);
   
   /* Clear recourses */
   free_resources(shm_id,sem_id);
   
   return 0;
   
}