您面临的问题是对

sem_init()

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()

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 /
  

  shmkey = ftok ("/dev/null", 5);       printf ("shmkey for p = %dn", shmkey);shmid = shmget (shmkey, sizeof (int), 0644 | IPC_CREAT);if (shmid < 0){         perror ("shmgetn");    exit (1);}p = (int *) shmat (shmid, NULL, 0);   *p = 0;printf ("p=%d is allocated in shared memory.nn", *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);sem = sem_open ("pSem", O_CREAT | O_EXCL, 0644, value); printf ("semaphores initialized.nn");for (i = 0; i < n; i++){    pid = fork ();    if (pid < 0) {            sem_unlink ("pSem");sem_close(sem);                          printf ("Fork error.n");    }    else if (pid == 0)        break;       }if (pid != 0){        while (pid = waitpid (-1, NULL, 0)){        if (errno == ECHILD) break;    }    printf ("nParent: All children have exited.n");        shmdt (p);    shmctl (shmid, IPC_RMID, 0);        sem_unlink ("pSem");       sem_close(sem);              exit (0);}else{    sem_wait (sem);    printf ("  Child(%d) is in critical section.n", i);    sleep (1);    *p += i % 3;       printf ("  Child(%d) new value of *p=%d.n", i, *p);    sem_post (sem);    exit (0);}

  }

输出值

./a.out shmkey for p = 84214791p=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: 2semaphores 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

ftok()

ftok()

shmkey

-1

ftok()

请注意如何声明和初始化信号量。它与您在问题（

sem_t sem

sem_t*sem

sem_t*

sem_init()