2

我一直很难理解命令 pipe() 在 C 中是如何工作的。据我所知，匿名管道用于在两个“相关”进程之间发送/接收信息。
我发现读取和写入/写入相应文件描述符的条件非常令人困惑，并且不明白为什么必须关闭读取 fd 才能写入，反之亦然，以及如何使用它来阻止进程，直到另一个人读过或写过。

管道如何用于同步进程？例如，如何使用 pipe() 实现以下流程层次结构？

A / B ----D\
  \ C ------\
             E


-A 必须在创建 B 和 C 之前完成。
-B 必须在创建 D 之前完成。
-C 和 C 必须在 E 创建之前完成。
-B 和 C 可以按任何顺序完成

非常感谢您的帮助

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2 回答 2

2

我已经模拟了您提到的流程同步并提供了评论。希望这个概念现在很清楚。

注意这里没有通过管道读取数据，它仅用于实现同步，通过使用读取功能的阻塞特性，当管道的写入端仍然打开时。

#include<stdio.h>
#include<stdlib.h>
#include<sys/types.h>
#include<unistd.h>
#include<sys/wait.h>
#define MAXBUF 1

int main(){

    int ret, i, fd[2];
    char buf[MAXBUF];

    //Create a Pipe
    pipe(fd);

    ret = fork();//Creating process A
    if (0 == ret)
    {
            printf("Process - A created\n");
            exit(0);
    }
    //I need to wait till Process - A completes
    //Let me close my write end else I will be blocked for ever in the loop below
    close(fd[1]);

    //Waiting over reading the pipe -> The loop will come out when Process A exits
    //Why ->since all the write ends of the pipe are closed, so read returns 0
    while ((ret = read(fd[0], buf, MAXBUF)) > 0)
    {
            //Just Wait, the read blocks till A exits and then read returns 0
    }
    //Again a pipe to synchronise B and C
    pipe(fd);

    //Lets Create Now  B and C

    ret = fork();
    if (0 == ret)
    {
            printf("Process - B created\n");
            exit(0);
    }
    ret = fork();
    if (0 == ret)
    {
            printf("Process - C created\n");
            exit(0);

    }
    //Let me close my Write End of pipe
    close(fd[1]);

    //Here Waiting for Termination of both B and C who can terminate in any order.
    while ((ret = read(fd[0], buf, MAXBUF)) > 0)
    {
            //This loop will turn false only if both B & C have exited.
            //Since both the write ends of the pipe that
            //were there in B and C will no longer be available
            //after they exit and thus the read will return 0
            //Just Wait ..Do nothing
    }
    //Now all A, B, C are finished.
    //Create D and E
    ret = fork();
    if (0 == ret)
    {
            printf("Process - D created\n");
            exit(0);
    }
    ret = fork();
    if (0 == ret)
    {
            printf("Process - E created\n");
            exit(0);

    }
//Here let the main process wait for all the 5 Child to complete
    //This is not needed, but I gave given so that the Shell
    //Prompt is shown after the code exits..
    //If this is deleted then you need to press return
    //to get back the shell prompt after the code completes execution.
    for (i = 0; i < 5; i++)
    {
            wait(NULL);
    }

}
于 2012-08-19T19:30:45.030 回答
1

Pipes are not used for process synchronization, not directly anyway. Instead it's a way to communicate.

For example, a process creates a pair of pipes (one write-end and one read-end), then forks so a new child process is created. The parent can then send data to the child process through the pipe.

于 2012-08-19T17:43:01.927 回答