sysv semaphore set, increment, decrement, blocking and reading current value.

semaphore.c

#include <stdio.h>                                                                                                                                                                                                                              
#include <stdlib.h>                                                                                                                                                                                                                             
#include <sys/stat.h>
#include <sys/sem.h>
#include <unistd.h>

union semun {                   /* Used in calls to semctl() */
  int                 val;
  struct semid_ds *   buf;
  unsigned short *    array;
  struct seminfo *    __buf;
};


void errExit(const char* msg) {
  perror(msg);
  exit(1);
}

int main(int argc, char* argv[]) {
  if (argc < 2 || argc > 3) {
    fprintf(stderr, "Invalid arguments: Usage \n  %s init-val\n  %s semid increment-val\n", argv[0], argv[0]);
    exit(1);
  }

  if (argc == 2) { // Setting initial value
    key_t key = ftok(".", 'A');
    if (key == -1) {
      errExit("ftok");
    }

    int semid = semget(key, 1, IPC_CREAT | S_IRUSR | S_IWUSR);
    if (semid == -1) {
      errExit("semget");
    }

    union semun arg;
    arg.val = atoi(argv[1]);
    printf("%d, about to set semaphore with id=%d to %d\n", getpid(), semid, arg.val);
    if (semctl(semid, 0, SETVAL, arg) == -1) {
      errExit("semctl");
    }
    printf("%d, done settting semaphore with id=%d to %d\n", getpid(), semid, arg.val);
  }
  else {
    int semid = atoi(argv[1]);
    int inc_val = atoi(argv[2]);
    printf("%d, about to change semaphore id=%d by %d\n", getpid(), semid, inc_val);

    struct sembuf o;
    o.sem_num = 0;
    o.sem_op = inc_val;
    o.sem_flg = 0;

    if (semop(semid, &o, 1) == -1) {
      errExit("semop");
    }

    printf("%d, done setting semaphore with id=%d to %d\n", getpid(), semid, inc_val);

    int semval = semctl(semid, 0, GETVAL);
    if (semval == -1) {
      errExit("semctl");
    }
    printf("%d, current value of semaphore with id=%d = %d\n", getpid(), semid, semval);
  }
}