为什么信号灯不起作用?

问题描述

#include <stdio.h>
#include <sys/types.h>
#include <iostream>
#include <unistd.h>
#include <fstream>
#include <string>
#include <semaphore.h>

using namespace std;

int main(int argc, char *argv[]){
  int pshared = 1;
  unsigned int value = 0;
  sem_t sem_name;
  sem_init(&sem_name, pshared, value);

  int parentpid = getpid();
  pid_t  pid = fork();

  if (parentpid == getpid()){
    cout << "parent id= " << getpid() << endl;
    sem_wait(&sem_name);
    cout << "child is done." << endl;
  }

  if (parentpid != getpid()){
    cout << "child id= " << getpid() << endl;
    for (int i = 0; i < 10; i++)
      cout << i << endl;

    sem_post(&sem_name);
}
  sleep(4);
  return 0;
}

结果应为:

parent id 123456.
child id 123457.
0
1
2
3
4
5
6
7
8
9
child is done.

程序退出，但从不发出信号.

Program exits, but instead it never signals the semaphore.

推荐答案

来自sem_init的联机帮助页:

POSIX信号量是堆栈结构.它们不是像文件描述符那样对内核维护的结构进行引用计数的引用.如果要通过两个过程共享POSIX信号量，则需要自己进行共享.

POSIX semaphores are on-the-stack structs. They aren't reference-counted references to a kernel-maintained struct like filedescriptors are. If you want to share a POSIX semaphore with two processes, you need to take care of the sharing part yourself.

这应该有效:

#include <fstream>
#include <iostream>
#include <semaphore.h>
#include <stdio.h>
#include <string>
#include <sysexits.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <unistd.h>


int main(int argc, char *argv[]){
  using namespace std;
  sem_t* semp = (sem_t*)mmap(0, sizeof(sem_t), PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_SHARED, 0, 0 );
  if ((void*)semp == MAP_FAILED) { perror("mmap");  exit(EX_OSERR); }

  sem_init(semp, 1 /*shared*/, 0 /*value*/);

  pid_t  pid = fork();
  if(pid < 0) { perror("fork");  exit(EX_OSERR); }

  if (pid==0){ //parent
    cout << "parent id= " << getpid() << endl;
    sem_wait(semp);
    cout << "child is done." << endl;
  }else { //child
    cout << "child id= " << getpid() << endl;
    for (int i = 0; i < 10; i++)
      cout << i << endl;
    sem_post(semp);
  }
  return 0;
}

注意::如果您只想要这种行为，那么waitpid显然是可行的方法.我假设您要测试POSIX信号量.

Note: If you want just this behavior, then waitpid is obviously the way to go. I'm assuming what you want is to test out POSIX semaphores.

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