使用信号量在三个进程之间共享内存

Question

进程1 fork进程2和3，每个进程向共享内存写入一个与其编号相等的字符。最后一个进程应该读取内存

输出应该看起来像 "read: 123"，但我得到 "read: 1"（只有最后一个进程号）

我不知道如何在多个进程中使用共享内存，有什么帮助吗？

#include <stdio.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <zconf.h>
#include <sys/sem.h>
#include <wait.h>

int main(){
    int pid1, pid2, pid3, semid, shmid, n=3;
    semid = semget(1, 1, 0);
    shmid = shmget(2,1024,0666|IPC_CREAT);
    char *number = (char *) shmat(shmid, (void *) 0, 0);
    pid1=getpid();
    pid2=fork();
    pid3=fork();

    if(pid3==0) {
        printf("Process 3 is writing...\n");
        gets(number);
    }
    else if(pid2==0){
        wait(NULL);
        printf("Process 2 is writing...\n");
        gets(number);
    }
    else{
        wait(NULL);
        printf("Process 1 is writing...\n");
        gets(number);
        printf("read: %s\n", number);
    }

}

我只是想得到一些关于多进程内存写入的建议。

Answer 1

这是一个固定的答案，带有经过修改和简化的程序和解释。

创建 child 进程时，parent 进程使用的大部分环境都是继承的：共享内存段是自动继承的 - 您不需要运行任何 shmat 、shmget 或 shmctl 例程。

要读取和写入共享内存段，child 进程需要知道相应的地址：它已经有了它，因为在您的代码中它是数字变量。

我已经在我的代码中简化了你的例子：

只有 2 个进程：parent 和 child
在进程中没有使用gets：进程只是写一个字母在共享内存：parent写'P'，child写'C'；在 child 之后退出，parent 再次读取共享内存以检查该值是否为 child 写了什么
我加了很多printf来显示谁是当前进程（parent或child）以及进程在做什么
就像在您的代码中一样，parent 和 child 之间没有同步来读取和写入共享内存：这只是一个简化的示例（但在实际程序中应该有一些同步信号量或互斥量）。

代码：

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

int main(){
    pid_t pid;
    int shmid;
    char *addr;

    shmid = shmget(2,1024,0666|IPC_CREAT);
    addr = (char *) shmat(shmid, (void *) 0, 0);
    printf("In parent => addr: %p \n", (void *)addr);
    printf("In parent  =>  writing *addr ... \n");
    *addr = 'P';
    printf("In parent  =>  ... writing *addr: done \n");
    printf("In parent  => reading *addr = %c \n", *addr);

    pid = fork();
    if(pid == 0) 
    {
        printf("In child  => PID: %d PPID: %d\n", getpid(), getppid());
        printf("In child  => addr: %p \n", (void *)addr);
        printf("In child  => reading *addr = %c \n", *addr);
        printf("In child  => writing *addr ... \n");
        *addr = 'C';
        printf("In child  => ... writing *addr: done \n");
        printf("In child  => reading *addr = %c \n", *addr);
        printf("In child  => exiting \n");
        exit(EXIT_SUCCESS);
    }
    else if(pid > 0) 
    {
        printf("In parent => PID: %d\n", getpid());
        printf("In parent => waiting for child process to finish ...\n");
        wait(NULL);
        printf("In parent => child process finished.\n");
        printf("In parent  => reading *addr = %c \n", *addr);
    }
    else {
        printf("In parent => unable to create child process.\n");
    }

   printf("In parent => exiting.\n"); 
  return EXIT_SUCCESS;
}

输出示例：

In parent => addr: 0x7fcef5100000 
In parent  =>  writing *addr ... 
In parent  =>  ... writing *addr: done 
In parent  => reading *addr = P 
In parent => PID: 22027
In parent => waiting for child process to finish ...
In child  => PID: 22028 PPID: 22027
In child  => addr: 0x7fcef5100000 
In child  => reading *addr = P 
In child  => writing *addr ... 
In child  => ... writing *addr: done 
In child  => reading *addr = C 
In child  => exiting 
In parent => child process finished.
In parent  => reading *addr = C 
In parent => exiting.

使用信号量在三个进程之间共享内存

Sharing memory between three processes with semaphores

c

unix

process

semaphore