试图写一个int在共享内存中（使用mmap）有一个子进程 [英] Trying to write to an int in shared memory (using mmap) with a child process

问题描述

我与一些code，需要家长和派生的子进程之间的通信玩耍。我创建了一个叉前共享内存int，但我似乎做与孩子过程中的任何变化不会当父进程访问的影响INT。这里是一片code的，说明我的问题。

I'm playing around with some code that requires communication between a parent and a forked child process. I've created an int in shared memory before the fork, but any changes I make with the child process don't seem to affect the int when accessed by the parent process. Here's a piece of code that illustrates my problem.

int * shared = mmap(NULL, sizeof(int), PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0); 
pid_t child;
int childstate;
if((child=fork())==0){
      *shared = 1;
      exit(0);
}
waitpid (child, &childstate, 0); 
printf("%d",*shared);

虽然子进程设置的共享为1的值，这个程序输出0。

Although the child process sets the value of 'shared' to 1, this program outputs 0.

在我实际的程序结构将被共享，而不是一个int，但如果我能想出​​这个code片段，我觉得剩下的应该水到渠成。

In my actual program a struct will be shared instead of an int, but if I can figure out this code snippet, I think the rest should fall into place.

我决不是一个有经验的程序员，我有点陌生C.我花了几个小时试图弄清楚这一点，并宣读几十这不能不说应该是一个简单的过程页面。说实话，它一直是一个沉重的打击，我的自尊:)。我敢肯定，我只是缺少一些小细节 - 有谁能够指出来给我吗？在此先感谢您的时间。

I'm by no means an experienced programmer, and I'm a bit unfamiliar with C. I've spent hours trying to figure this out and read dozens of pages which say it should be a simple process. To be honest, it's been a heavy blow to my self esteem :) . I'm sure I'm just missing some small detail - can anybody point it out to me? Thanks in advance for your time.

推荐答案

您的问题是传递到标志的mmap（），你希望 MAP_SHARED 。

Your problem is the flags passed into mmap(), you want MAP_SHARED.

int * shared = mmap(NULL, sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0); 

作品与code低于预期。

Works as expected with the code below

#include <sys/mman.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>

int main(void) {
    int * shared = mmap(NULL, sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0); 
    pid_t child;
    int childstate;
    printf("%d\n", *shared);
    if((child=fork())==0){
            *shared = 1;
            printf("%d\n", *shared);
            exit(0);
    }
    waitpid (child, &childstate, 0); 
    printf("%d\n",*shared);
}

输出：

0
1
1

的mmap手册页可以帮助你，如果你还没有发现它。

mmap man page may help you if you haven't already found it.

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