posix - 为什么信号量不起作用？

Question

#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; 
}

结果应该是:

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

程序退出，但它从不向信号量发出信号。

Answer 1

来自 sem_init的联机帮助页:

If pshared is nonzero, then the semaphore is shared between processes, and should be located in a region of shared memory (see shm_open(3), mmap(2), and shmget(2)). (Since a child created by fork(2) inherits its parent's memory mappings, it can also access the semaphore.) Any process that can access the shared memory region can operate on the semaphore using sem_post(3), sem_wait(3), etc.

POSIX 信号量是堆栈结构。它们不像文件描述符那样对内核维护的结构进行引用计数引用。如果要与两个进程共享一个 POSIX 信号量，则需要自己处理共享部分。

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:如果您只想要这种行为，那么 waitpid显然是要走的路。我假设您想要的是测试 POSIX 信号量。