您面临的问题是对sem_init()功能的误解。阅读手册页时， 您将看到以下内容：

pshared参数指示此信号量是在进程的线程之间还是在进程之间共享。

如果您到此为止都读完了，您将认为pshared的非零值将使信号量成为进程间信号量。但是，这是错误的。您应该继续阅读，您将了解到必须在共享内存区域中找到信号灯。为此，可以使用几个功能，如下所示：

如果pshared为非零，则信号量在进程之间共享，并且应位于共享内存的区域中（请参见shm_open（3），mmap（2）和shmget（2））。（由于fork（2）创建的子级继承了其父级的内存映射，因此它也可以访问该信号量。）任何可以访问共享内存区域的进程都可以使用sem_post（3），sem_wait（3）等对该信号量进行操作。
。

我发现这种方法比其他方法更为复杂，因此我想鼓励人们使用sem_open()而不是sem_init()。

在下面您可以看到一个完整的程序，说明了以下内容：

• 如何在派生的进程之间分配共享内存和使用共享变量。
• 如何在共享内存区域中初始化信号量，并由多个进程使用。

• 如何派生多个进程并使父级等待所有子级退出。

  include / printf() /

  include / exit(), malloc(), free() /

  include / key_t, sem_t, pid_t /

  include / shmat(), IPC_RMID /

  include / errno, ECHILD /

  include / sem_open(), sem_destroy(), sem_wait().. /

  include / O_CREAT, O_EXEC /

  int main (int argc, char argv){
  int i; / loop variables /
  key_t shmkey; / shared memory key /
  int shmid; / shared memory id /
  sem_t sem; / synch semaphore //shared /
  pid_t pid; / fork pid /
  int p; / shared variable //shared /
  unsigned int n; / fork count /
  unsigned int value; / semaphore value /

  /* initialize a shared variable in shared memory */
  shmkey = ftok ("/dev/null", 5);       /* valid directory name and a number */
  printf ("shmkey for p = %d\n", shmkey);
  shmid = shmget (shmkey, sizeof (int), 0644 | IPC_CREAT);
  if (shmid < 0){                           /* shared memory error check */
      perror ("shmget\n");
      exit (1);
  }
  
  p = (int *) shmat (shmid, NULL, 0);   /* attach p to shared memory */
  *p = 0;
  printf ("p=%d is allocated in shared memory.\n\n", *p);
  
  /********************************************************/
  
  printf ("How many children do you want to fork?\n");
  printf ("Fork count: ");
  scanf ("%u", &n);
  
  printf ("What do you want the semaphore value to be?\n");
  printf ("Semaphore value: ");
  scanf ("%u", &value);
  
  /* initialize semaphores for shared processes */
  sem = sem_open ("pSem", O_CREAT | O_EXCL, 0644, value); 
  /* name of semaphore is "pSem", semaphore is reached using this name */
  
  printf ("semaphores initialized.\n\n");
  
  /* fork child processes */
  for (i = 0; i < n; i++){
      pid = fork ();
      if (pid < 0) {
      /* check for error      */
          sem_unlink ("pSem");   
          sem_close(sem);  
          /* unlink prevents the semaphore existing forever */
          /* if a crash occurs during the execution         */
          printf ("Fork error.\n");
      }
      else if (pid == 0)
          break;                  /* child processes */
  }
  
  /******************************************************/
  /******************   PARENT PROCESS   ****************/
  /******************************************************/
  if (pid != 0){
      /* wait for all children to exit */
      while (pid = waitpid (-1, NULL, 0)){
          if (errno == ECHILD)
              break;
      }
  
      printf ("\nParent: All children have exited.\n");
  
      /* shared memory detach */
      shmdt (p);
      shmctl (shmid, IPC_RMID, 0);
  
      /* cleanup semaphores */
      sem_unlink ("pSem");   
      sem_close(sem);  
      /* unlink prevents the semaphore existing forever */
      /* if a crash occurs during the execution         */
      exit (0);
  }
  
  /******************************************************/
  /******************   CHILD PROCESS   *****************/
  /******************************************************/
  else{
      sem_wait (sem);           /* P operation */
      printf ("  Child(%d) is in critical section.\n", i);
      sleep (1);
      *p += i % 3;              /* increment *p by 0, 1 or 2 based on i */
      printf ("  Child(%d) new value of *p=%d.\n", i, *p);
      sem_post (sem);           /* V operation */
      exit (0);
  }
  

  }

输出值

./a.out 
shmkey for p = 84214791
p=0 is allocated in shared memory.

How many children do you want to fork?
Fork count: 6 
What do you want the semaphore value to be?
Semaphore value: 2
semaphores initialized.

  Child(0) is in critical section.
  Child(1) is in critical section.
  Child(0) new value of *p=0.
  Child(1) new value of *p=1.
  Child(2) is in critical section.
  Child(3) is in critical section.
  Child(2) new value of *p=3.
  Child(3) new value of *p=3.
  Child(4) is in critical section.
  Child(5) is in critical section.
  Child(4) new value of *p=4.
  Child(5) new value of *p=6.

Parent: All children have exited.

检查还不错，shmkey因为ftok()失败时返回-1。但是，如果您有多个共享变量，并且ftok()函数多次失败，则具有shmkeywith值的共享变量-1将驻留在共享内存的同一区域中，从而导致一个更改影响另一个。因此，程序执行将变得混乱。为了避免这种情况，最好检查是否ftok()
返回-1（最好是签入源代码，而不是像我一样打印到屏幕上，尽管我想向您显示键值以防发生冲突）。

请注意如何声明和初始化信号量。它与您在问题（sem_t semvs sem_t* sem）中所做的不同。此外，您应该使用它们在本示例中显示的形式。您不能在中定义sem_t*和使用它sem_init()。