c - 如何在 Linux 中使用信号量与 C 同步两个进程的输出？

Question

我有以下代码:

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

#define SEMAPHORE 0

int main(int argc, char *argv[])
{
    int semid, pid, j = 10;
    struct sembuf operation;
    key_t key;

    key = ftok(argv[0], 'U');

    if ((semid = semget(key, 1, IPC_CREAT | 0600)) == -1)
    {
        perror("Error at semget");
        exit(-1);
    }
    semctl(semid, SEMAPHORE, SETVAL, 1);

    /* Child is created  */
    if ((pid = fork()) == -1)
    {
        perror("Error at fork");
        exit(-1);
    }
    else if (pid == 0)
    {
        /* Code corresponding to the child process  */
        while (j)
        {
            /* DOWN operation is performed at the semaphore  */
            operation.sem_flg = 0;
            operation.sem_op = -1;
            operation.sem_num = SEMAPHORE;

            semop(semid, &operation, 1);

            printf("I AM THE CHILD PROCESS, IMPRESSION: %d\n", j--);

            /* UP operation is performed at the semaphore  */
            operation.sem_op = 1;
            operation.sem_num = SEMAPHORE;
            semop(semid, &operation, 1);
        }
    }
    else
    {
        /* Code corresponding to the parent process */
        while (j)
        {
            /* DOWN operation is performed at the semaphore */
            operation.sem_flg = 0;
            operation.sem_op = -1;
            operation.sem_num = SEMAPHORE;

            semop(semid, &operation, 1);

            printf("I AM THE FATHER PROCESS, IMPRESSION: %d\n", j--);

            /* UP operation is performed at the semaphore */
            operation.sem_op = 1;
            operation.sem_num = SEMAPHORE;
            semop(semid, &operation, 1);
        }
    }
}

所以我可以期待以下输出:

I AM THE FATHER PROCESS, IMPRESSION: 10
I AM THE CHILD PROCESS, IMPRESSION: 10
I AM THE FATHER PROCESS, IMPRESSION: 9
I AM THE CHILD PROCESS, IMPRESSION: 9
I AM THE FATHER PROCESS, IMPRESSION: 8
I AM THE CHILD PROCESS, IMPRESSION: 8
I AM THE FATHER PROCESS, IMPRESSION: 7
I AM THE CHILD PROCESS, IMPRESSION: 7
I AM THE FATHER PROCESS, IMPRESSION: 6
I AM THE CHILD PROCESS, IMPRESSION: 6
I AM THE FATHER PROCESS, IMPRESSION: 5
I AM THE CHILD PROCESS, IMPRESSION: 5
I AM THE FATHER PROCESS, IMPRESSION: 4
I AM THE CHILD PROCESS, IMPRESSION: 4
I AM THE FATHER PROCESS, IMPRESSION: 3
I AM THE CHILD PROCESS, IMPRESSION: 3
I AM THE FATHER PROCESS, IMPRESSION: 2
I AM THE CHILD PROCESS, IMPRESSION: 2
I AM THE FATHER PROCESS, IMPRESSION: 1
I AM THE CHILD PROCESS, IMPRESSION: 1

或者先是 child ，然后是父亲。但是，我运行它，它产生的输出如下:

I AM THE FATHER PROCESS, IMPRESSION: 10
I AM THE FATHER PROCESS, IMPRESSION: 9
I AM THE FATHER PROCESS, IMPRESSION: 8
I AM THE FATHER PROCESS, IMPRESSION: 7
I AM THE FATHER PROCESS, IMPRESSION: 6
I AM THE FATHER PROCESS, IMPRESSION: 5
I AM THE FATHER PROCESS, IMPRESSION: 4
I AM THE FATHER PROCESS, IMPRESSION: 3
I AM THE FATHER PROCESS, IMPRESSION: 2
I AM THE FATHER PROCESS, IMPRESSION: 1
I AM THE CHILD PROCESS, IMPRESSION: 10
I AM THE CHILD PROCESS, IMPRESSION: 9
I AM THE CHILD PROCESS, IMPRESSION: 8
I AM THE CHILD PROCESS, IMPRESSION: 7
I AM THE CHILD PROCESS, IMPRESSION: 6
I AM THE CHILD PROCESS, IMPRESSION: 5
I AM THE CHILD PROCESS, IMPRESSION: 4
I AM THE CHILD PROCESS, IMPRESSION: 3
I AM THE CHILD PROCESS, IMPRESSION: 2
I AM THE CHILD PROCESS, IMPRESSION: 1

或者有时候这个印象出来了，但是我没有得到预期的印象:

I AM THE FATHER PROCESS, IMPRESSION: 10
I AM THE FATHER PROCESS, IMPRESSION: 9
I AM THE CHILD PROCESS, IMPRESSION: 10
I AM THE CHILD PROCESS, IMPRESSION: 9
I AM THE CHILD PROCESS, IMPRESSION: 8
I AM THE CHILD PROCESS, IMPRESSION: 7
I AM THE CHILD PROCESS, IMPRESSION: 6
I AM THE CHILD PROCESS, IMPRESSION: 5
I AM THE FATHER PROCESS, IMPRESSION: 8
I AM THE FATHER PROCESS, IMPRESSION: 7
I AM THE FATHER PROCESS, IMPRESSION: 6
I AM THE CHILD PROCESS, IMPRESSION: 4
I AM THE CHILD PROCESS, IMPRESSION: 3
I AM THE CHILD PROCESS, IMPRESSION: 2
I AM THE CHILD PROCESS, IMPRESSION: 1
I AM THE FATHER PROCESS, IMPRESSION: 5
I AM THE FATHER PROCESS, IMPRESSION: 4
I AM THE FATHER PROCESS, IMPRESSION: 3
I AM THE FATHER PROCESS, IMPRESSION: 2
I AM THE FATHER PROCESS, IMPRESSION: 1

我明白进程的执行顺序直接取决于操作系统，但不是应该当两个进程中的一个执行UP时，另一个应该进入运行吗？

所以我的问题是，这里到底发生了什么？

Answer 1

令 P (sem) 表示信号量 sem 的向下操作，V (sem) 表示信号量 sem 的向上操作。

目前，您的代码是这样的，

// semaphore s1
s1 = 1;
// parent process
P (s1);  
 // work
V (s1)

// child process
P (s1)
  // work
V (s1)

所以这里所说的是，一次只有一个进程可以完成“工作”，而内核会紧随其后。

要同步两个进程的输出，需要做以下事情。

// semaphores, sem-parent, sem-child
sem-parent = 1; sem-child = 0;

// parent process
P (sem-parent);
   // work
V (sem-child);

// child process
P (sem-child);
   // work
V (sem-parent);