了解posix进程间信号量 [英] Understanding the posix interprocess semaphore

问题描述

根据我的理解，信号量应该可以在相关进程中使用，而不必放置在共享内存中.如果是这样，为什么以下代码会死锁?

According to my understanding, a semaphore should be usable across related processes without it being placed in shared memory. If so, why does the following code deadlock?

#include <iostream>
#include <semaphore.h>
#include <sys/wait.h>

using namespace std;

static int MAX = 100;

int main(int argc, char* argv[]) {
  int retval;
  sem_t mutex;

  cout << sem_init(&mutex, 1, 0) << endl;

  pid_t pid = fork();

  if (0 == pid) {
    //     sem_wait(&mutex);
    cout << endl;
    for (int i = 0; i < MAX; i++) {
      cout << i << ",";
    }
    cout << endl;
    sem_post(&mutex);

  } else if(pid > 0) {
    sem_wait(&mutex);
    cout << endl;
    for (int i = 0; i < MAX; i++) {
      cout << i << ",";
    }
    cout << endl;
    //     sem_post(&mutex);
    wait(&retval);

  } else {
    cerr << "fork error" << endl;
    return 1;
  }

//   sem_destroy(&mutex);

  return 0;
}

当我在Gentoo/Ubuntu Linux上运行它时，父级挂起.显然，它没有收到孩子的职位.取消注释sem_destroy不会有任何好处.我想念什么吗?

When I run this on Gentoo/Ubuntu Linux, the parent hangs. Apparently, it did not receive the post by child. Uncommenting sem_destroy won't do any good. Am I missing something?

更新1: 该代码有效

mutex = (sem_t *) mmap(NULL, sizeof(sem_t), PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED, 0, 0);
if (!mutex) {
  perror("out of memory\n");
  exit(1);
}

谢谢， 尼罗什.

推荐答案

如果pshared不为零，则信号在进程之间共享， 和应位于共享内存区域.

If pshared is nonzero, then the semaphore is shared between processes, and should be located in a region of shared memory.

由于fork(2)创建的子项继承了其父项的内存 映射，它也可以访问信号量.

Since a child created by fork(2) inherits its parent's memory mappings, it can also access the semaphore.

是，但它仍必须位于共享区域中.否则，只需使用通常的 CoW 复制内存即可.

Yes, but it still has to be in a shared region. Otherwise the memory simply gets copied with the usual CoW and that's that.

您可以通过至少两种方式解决此问题:

You can solve this in at least two ways:

• 使用sem_open("my_sem", ...)
• 使用shm_open和mmap创建共享区域

• Use sem_open("my_sem", ...)
• Use shm_open and mmap to create a shared region

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