/* After the #includes, the function prototypes and the global variable, we come to the main function. There the semaphore is created with a call to semget, which returns the semaphore ID. If the program is the first to be called (i.e. it's called with a parameter and argc > 1), a call is made to set_semvalue to initialize the semaphore and op_char is set to X. */

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
//#include "semun1.h"

static int set_semvalue(void);
static void del_semvalue(void);
static int semaphore_p(void);
static int semaphore_v(void);
static int sem_id;

int main(int argc, char *argv[])
{
    int i;
    int pause_time;
    char op_char = 'O';
    srand((unsigned int)getpid());
    sem_id = semget((key_t)1234, 1, 0666 | IPC_CREAT);

    if (argc > 1) {
        if (!set_semvalue())
        {
            fprintf(stderr, "Failed to initialize semaphore\n");
            exit(EXIT_FAILURE);
        }

        op_char = 'X';
        sleep(2);

    }


    /* Then we have a loop which enters and leaves the critical section ten times. There, we first make a call
     to semaphore_p which sets the semaphore to wait, as this program is about to enter the critical section.
     */

for(i = 0; i < 10; i++) {
    if (!semaphore_p()) exit(EXIT_FAILURE);
    printf("%c", op_char);
    fflush(stdout);
    pause_time = rand() % 3;
    sleep(pause_time);
    printf("%c", op_char); fflush(stdout);


    /* After the critical section, we call semaphore_v, setting the semaphore available, before going through the for loop again after a random wait. After the loop, the call to del_semvalue is made to clean up the code. */


    if (!semaphore_v()) exit(EXIT_FAILURE);

    pause_time = rand() % 2;
    sleep(pause_time);

}

printf("\n%d - finished\n", getpid());

    if (argc > 1)
    {
        sleep(10);
        del_semvalue();
    }

    exit(EXIT_SUCCESS);
}



/* The function set_semvalue initializes the semaphore using the SETVAL command in a
 semctl call. We need to do this before we can use the semaphore. */


static int set_semvalue(void)
{
    union semun sem_union;

    sem_union.val = 1;
    if (semctl(sem_id, 0, SETVAL, sem_union) == -1) return(0);
    return(1);
}


/* The del_semvalue function has almost the same form, except the call to semctl uses
 the command IPC_RMID to remove the semaphore's ID. */

static void del_semvalue(void)
{
    union semun sem_union;

    if(semctl(sem_id,0, IPC_RMID, sem_union) == -1)
       fprintf(stderr, "Failed to delete semaphore\n");
}

       static int semaphore_p(void) {
           struct sembuf sem_b;


           sem_b.sem_num = 0;
           sem_b.sem_op = -1; /* P() */
           sem_b.sem_flg = SEM_UNDO;

           if (semop(sem_id, &sem_b, 1) == -1)
           {
               fprintf(stderr, "semaphore_p failed\n");
               return(0);
           }
           return(1);
       }


static int semaphore_v(void)
       {
           struct sembuf sem_b;

           sem_b.sem_num = 0;

           sem_b.sem_op = 1; /* V() */
           sem_b.sem_flg = SEM_UNDO;
           if (semop(sem_id, &sem_b, 1) == -1)
           {
               fprintf(stderr, "semaphore_v failed\n");
               return(0);
           }
           return(1);
       }