SystemV IPC信号量

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semget

http://man7.org/linux/man-pages/man2/semget.2.html
semctl

http://man7.org/linux/man-pages/man2/semctl.2.html

semop

http://man7.org/linux/man-pages/man2/semop.2.html

ftok

http://man7.org/linux/man-pages/man3/ftok.3.html

=============================================================================

程序示例：

struct semid_ds   my_semds;
struct sembuf     my_sembuf[1];
union semun       my_semarg;
const char *pathname = "xxx";
  
key_t key = ftok(pathname, 1);

/* Create the semaphore */
  semid = semget(key, 1, IPC_CREAT|IPC_EXCL|0666);
  if (semid >= 0)
  {
    /* The semaphore was created first time.  Initialize with value 1 */
    my_semarg.val = 1;
    rc = semctl(semid, 0, SETVAL, my_semarg);
    if (rc == -1)
    {
        fprintf(stderr, "semctl failed, errno=%d\n", errno);
        exit(1);
    }
  }
  else
  {
    if (errno != EEXIST)
    {
        fprintf(stderr, "semget failed, errno=%d\n", errno);
        exit(1);
    }

    /* The semaphore already exists. get its id. */
    semid = semget(key, 1, 0);
	
	/* otime 0 indicates there may be another process creating the sempahore
    ** at the same time. 
    */
    my_semarg.buf = &my_semds;
    rc = semctl(semid, 0, IPC_STAT, my_semarg);
    if (rc == -1)
    {
        fprintf(stderr, "semctl(IPC_STAT) failed, errno=%d\n", errno);
        exit(1);
    }

    if (my_semds.sem_otime == 0)
    {
        /* Another process is running
        */
        fprintf(stderr, "race condition detected.\n");
        exit(1);
    }
  }

  /* The IPC_NOWAIT flag is set for none blocking
  ** The SEM_UNDO is used for releasing semaphore when process exit
  */
  my_sembuf[0].sem_num = 0; /* semaphore number, 0 is the first one */
  my_sembuf[0].sem_op = -1; /* semaphore operation, -1 is subtract 1 from current value*/
  my_sembuf[0].sem_flg = SEM_UNDO | IPC_NOWAIT;

  rc = semop(semid, my_sembuf, 1);
  if (rc == -1)
  {
    if (errno == EAGAIN)
    {
        fprintf(stderr, "Semaphore is hold by another process.\n");
        exit(2);
    }
    else
    {
        fprintf(stderr, "semop failed, errno=%d\n");
        exit(1);
    }
  } 
  
  //Lock the semaphore successfully, continue other works

参考文献：

https://docs.oracle.com/cd/E19120-01/open.solaris/817-4415/svipc-65382/index.html