c++进程在两个子进程之间进行管道连接时挂起

Question

我正在尝试将数据从一个子进程传输到另一个子进程。当我运行它时，它挂起。如果我不让它等待第一个子进程，它会回到循环顶部提示命令而不给出预期的输出，当我提示它退出时，它会转储我期望的所有输出。我让它只与一个子进程一起工作，但随后第二个 execvp 杀死了父进程，我没有回到循环的顶部提示更多命令。

#include <iostream>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <sys/wait.h>

using namespace std;

int main(int argc, const char * argv[]) {

bool quit=0;
char quitArray[] = "quit";
int pipeReturnValue, fork1ReturnValue, fork2ReturnValue, pipefd[2], checkForQuit;

//Enter a loop where each iteration prompts for two single-line inputs
while (!quit) {

    //Get command 1
    char command1[128];
    printf("Enter command 1: ");
    fgets(command1,128,stdin);
    command1[strlen(command1) -1] = 0;

    //Exit if user enters quit
    checkForQuit = strncmp(command1, quitArray, 4);
    if (checkForQuit == 0) {
        exit(0);
    }

    //Get command 2
    char command2[128];
    printf("Enter command 2: ");
    fgets(command2,128,stdin);
    command2[strlen(command2) -1] = 0;

    //Exit if user enters quit
    checkForQuit = strncmp(command2, quitArray, 4);
    if (checkForQuit == 0) {
        exit(0);
    }

    //Open pipe
    pipeReturnValue = pipe(pipefd);
    if (pipeReturnValue < 0) {
        perror("Pipe failed");
        exit(1);
    }

    //Fork 1
    fork1ReturnValue = fork();
    if(fork1ReturnValue < 0) {
        perror("Fork failed");
        exit(1);
    }

    else if (fork1ReturnValue == 0) {

        //Fork 2
        fork2ReturnValue = fork();
        if (fork2ReturnValue < 0) {
            perror("Fork 2 failed");
        }
        else if (fork2ReturnValue == 0) {
            //close read end of pipe
            close(pipefd[0]);
            //parse command 1 arguments
            //store tokens in array
            char *arguments[6] = {};
            arguments[0] = strtok(command1, " ");
            int tokenCounter = 0;
            while (arguments[tokenCounter] != NULL) {
                tokenCounter++;
                arguments[tokenCounter] = strtok(NULL, " ");
            }
            //dup stdo to pipe
            dup2(pipefd[1], 1);
            //execute arguments
            execvp(arguments[0], arguments);
        }
        else {
            wait(&fork2ReturnValue);
            //close write end of pipe
            close(pipefd[1]);
            //parse command 2 arguments
            //store tokens in array
            char *arguments[6] = {};
            arguments[0] = strtok(command2, " ");
            int tokenCounter = 0;
            while (arguments[tokenCounter] != NULL) {
                tokenCounter++;
                arguments[tokenCounter] = strtok(NULL, " ");
            }
            //dup stdin to pipe
            dup2(pipefd[0], 0);
            //exec
            execvp(arguments[0], arguments);
        }
    }
    else {
        wait(&fork1ReturnValue);
    }
}
return 0;

Answer 1

我终于明白了。我需要在第一次 fork 之后而不是之前打开管道。

#include <iostream>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <sys/wait.h>

using namespace std;

int main(int argc, const char * argv[]) {

    bool quit=0;
    char command1[128],
         command2[128],
         quitArray[] = "quit";
    int pipeReturnValue,
        fork1ReturnValue,
        fork2ReturnValue,
        checkForQuit,
        pipefd[2];

    //Loop where each iteration prompts for two single-line inputs
    while (!quit) {

        //Get command 1
        printf("Enter command 1: ");
        fgets(command1,128,stdin);
        command1[strlen(command1) -1] = 0;

        //Exit if user enters quit
        checkForQuit = strncmp(command1, quitArray, 4);
        if (checkForQuit == 0) {
            quit = 1;
            exit(0);
        }

        //Get command 2 and trim trailing new line character
        printf("Enter command 2: ");
        fgets(command2,128,stdin);
        command2[strlen(command2) -1] = 0;

        //Exit if user enters quit
        checkForQuit = strncmp(command2, quitArray, 4);
        if (checkForQuit == 0) {
            quit = 1;
            exit(0);
        }

        //Fork to create 1st child process, return error if fork fails
        fork1ReturnValue = fork();
        if(fork1ReturnValue < 0) {
            perror("Fork 1 failed");
            exit(1);
        }

        //Open pipe, return error if fork fails
        pipeReturnValue = pipe(pipefd);
        if (pipeReturnValue < 0) {
            perror("Pipe failed");
            exit(1);
        }

        //First child process
        else if (fork1ReturnValue == 0) {

            //Fork to create 2nd child process, return error if fork fails
            fork2ReturnValue = fork();
            if (fork2ReturnValue < 0) {
                perror("Fork 2 failed");
            }

            //Second child process
            else if (fork2ReturnValue == 0) {

                //close read end of pipe
                close(pipefd[0]);

                //Parse command 1 arguments, store tokens in an array
                char *arguments[6] = {};
                arguments[0] = strtok(command1, " ");
                int tokenCounter = 0;
                while (arguments[tokenCounter] != NULL) {
                    tokenCounter++;
                    arguments[tokenCounter] = strtok(NULL, " ");
                }

                //Dup standard output to write side of pipe
                dup2(pipefd[1], 1);

                //Execute arguments from command 1
                execvp(arguments[0], arguments);
            }

            //First child code continued
            else {

                //Wait for child 2 to to terminate
                wait(&fork2ReturnValue);

                //Close write end of pipe
                close(pipefd[1]);

                //Parse command 2 arguments, store tokens in array
                char *arguments[6] = {};
                arguments[0] = strtok(command2, " ");
                int tokenCounter = 0;
                while (arguments[tokenCounter] != NULL) {
                    tokenCounter++;
                    arguments[tokenCounter] = strtok(NULL, " ");
                }

                //dup standard input to read side of pipe
                dup2(pipefd[0], 0);

                //Execute arguments from command 2
                execvp(arguments[0], arguments);
            }
        }

        //Parent process continued
        else {

            //Wait for child 1 to terminate
            wait(&fork1ReturnValue);
        }

        //return to top of loop
    }
    return 0;
}