sem1.c

#include<unistd.h>
#include<stdlib.h>
#include<stdio.h>

#include<sys/sem.h>
/*#include "semun.h"*/


union semun {
  int val;
  struct semid_ds *buf;
  unsigned short int *array;
  struct seminfo *__buf;
};

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='Y';

  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);
  }

  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);

    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);
}

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);
}

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);
}