7-32 セマフォの初期化

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/types.h>
#include <sys/wait.h>

#define MYIPCKEY 0xdeadbeef
#define NSEMS 16

int
my_sem_init()
{
  int semid;
  unsigned short semun_array[NSEMS];
  struct sembuf sb[NSEMS];
  int i;

  semid = semget(MYIPCKEY, NSEMS, 0600 | IPC_CREAT | IPC_EXCL);
  if (semid >= 0) {
    for (i = 0; i < NSEMS; i++) {
      semun_array[i] = 1;
    }

    if (semctl(semid, NSEMS, SETALL, &semun_array) != 0) {
      perror("semctl");
      return 1;
    }

    printf("[pid:%d] new semaphore set, semid=%d\n", getpid(), semid);

    for (i = 0; i < NSEMS; i++) {
      sb[i].sem_num = i;
      sb[i].sem_op = -1;
      sb[i].sem_flg = SEM_UNDO;
    }

    printf("[pid:%d] before semop()\n", getpid());
    printf("[pid:%d] press enter to start semop()\n", getpid());

    getchar();

    if (semop(semid, sb, NSEMS) != 0) {
      perror("semop");
      return 1;
    }

    printf("[pid:%d] press enter to exit this process\n", getpid());

    getchar();

    exit(0);

  } else {

    if (errno != EEXIST) {
      perror("semget");
      return 1;
    } else {
      struct semid_ds sds;

      semid = semget(MYIPCKEY, NSEMS, 0600);
      if (semid < 0) {
        perror("semget");
        return 1;
      }

      printf("[pid:%d] before semctl()\n", getpid());

      for (;;) {
        if (semctl(semid, 0, IPC_STAT, &sds) != 0) {
          perror("semctl");
          return 1;
        }

        if (sds.sem_otime != 0) {
          break;
        }

        printf("[pid:%d] waiting otime change...\n", getpid());
        sleep(2);
      }

      sb[0].sem_num = 0;
      sb[0].sem_op = -1;
      sb[0].sem_flg = SEM_UNDO;

      printf("[pid:%d] before semop()\n", getpid());

      if (semop(semid, sb, 1) != 0) {
        perror("semop");
        return 1;
      }

      printf("[pid:%d] after semop()\n", getpid());
    }
  }

  return 0;
}

int
main()
{
  pid_t pid;

  pid = fork();

  if (my_sem_init() < 0) {
    printf("[pid:%d] my_sem_init() failed\n", getpid());
  }

  return 0;
}

拙著「Linuxネットワークプログラミング」の紹介