C＆＃x2B;＆＃x2B;管道，dup2，execlp悬挂

Question

因此，我试图以一种顺序运行3个命令。

ls -l / | grep a | sort -r

但是，我的代码似乎挂起了过程。我知道这通常表明没有手持管道。但是我似乎找不到问题，我已经关闭了我能找到的所有可能的文件描述符。

代码可能具有一些毫无意义的行，例如关闭相同的FD两次，我在尝试修复它时添加了

#include <iostream>
#include<unistd.h>
#include <sys/wait.h>
using namespace std;
    
#define pipeWrite 1
#define pipeRead 0
    
// Main Function
int main() {
    //Command to run: ls -l / | grep a | sort -r | wc > count.txt
    // Total 4 commands to run, so 4 child processes.
    
    pid_t c1,c2,c3,c4;
    // File Descriptors
    int pipeFD_1[2];
    int pipeFD_2[2];
    
    pipe(pipeFD_1);
    
    // To execute ls -l
    c1 = fork();
    
    // Child
    if (c1 == 0) {
        // Closing input end
        close(1);
        dup(pipeFD_1[1]);
    
        close(pipeFD_1[0]);
        close(pipeFD_1[1]);
        // Running command
        execl("/bin/ls", "ls", "-l", "/",NULL);
    }
    
    pipe(pipeFD_2);
    c2 = fork();
    
    if (c2 == 0) {
        // Closing output end
        close(0);
        dup(pipeFD_1[0]);
    
        close(1);
        dup(pipeFD_2[1]);
               
        close(pipeFD_1[0]); 
        close(pipeFD_1[1]);
        close(pipeFD_2[0]); 
        close(pipeFD_2[1]);
        // Running command
        execl("/bin/grep", "grep", "a", NULL);
    }
    
    //Closing pipe 1.
    close(pipeFD_1[0]);
    close(pipeFD_1[1]);
    
    c3 = fork();
    
    if (c3 == 0) {
        // Closing out end
        close(0);
        dup(pipeFD_2[0]);
                
                
        close(pipeFD_2[0]);
        close(pipeFD_2[1]);
        // Running command
        execl("/bin/sort", "sort", "-r", NULL);
    }
    
    // Since exec auto exits the child process. No need to put else.
    // Closing output end
    waitpid(-1, NULL, 0);
    waitpid(-1, NULL, 0);
    waitpid(-1, NULL, 0);
    exit(0);
}

编辑：通过在第三叉之后添加2行修复的情况下添加了这些fd，请关闭Pipe2（propent）的FDS。

Answer 1

谢谢 barmar 和 ishoney 您的宝贵讨论。将它们发布为帮助其他社区成员的答案。

通过在第三叉之后添加2条线修复，关闭pipe2的FD（对于父母）。

#include <iostream>
#include<unistd.h>
#include <sys/wait.h>
using namespace std;
    
#define pipeWrite 1
#define pipeRead 0
    
// Main Function
int main() {
    //Command to run: ls -l / | grep a | sort -r | wc > count.txt
    // Total 4 commands to run, so 4 child processes.
    
    pid_t c1,c2,c3,c4;
    // File Descriptors
    int pipeFD_1[2];
    int pipeFD_2[2];
    
    pipe(pipeFD_1);
    
    // To execute ls -l
    c1 = fork();
    
    // Child
    if (c1 == 0) {
        // Closing input end
        close(1);
        dup(pipeFD_1[1]);
    
        close(pipeFD_1[0]);
        close(pipeFD_1[1]);
        // Running command
        execl("/bin/ls", "ls", "-l", "/",NULL);
    }
    
    pipe(pipeFD_2);
    c2 = fork();
    
    if (c2 == 0) {
        // Closing output end
        close(0);
        dup(pipeFD_1[0]);
    
        close(1);
        dup(pipeFD_2[1]);
               
        close(pipeFD_1[0]); 
        close(pipeFD_1[1]);
        close(pipeFD_2[0]); 
        close(pipeFD_2[1]);
        // Running command
        execl("/bin/grep", "grep", "a", NULL);
    }
    
    //Closing pipe 1.
    close(pipeFD_1[0]);
    close(pipeFD_1[1]);
    
    c3 = fork();
    
    if (c3 == 0) {
        // Closing out end
        close(0);
        dup(pipeFD_2[0]);
                
                
        close(pipeFD_2[0]);
        close(pipeFD_2[1]);
        // Running command
        execl("/bin/sort", "sort", "-r", NULL);
    }
    
    // Since exec auto exits the child process. No need to put else.
    // Closing output end
    waitpid(-1, NULL, 0);
    waitpid(-1, NULL, 0);
    waitpid(-1, NULL, 0);
    exit(0);
}