如何使用fork和pipe以及c中的execvp将stdout重定向到stdin?

时间:2018-03-21 13:17:47

标签: c pipe fork posix

我需要自己做shell实现,但是我无法正确实现管道。

如果我有ls -l | grep toto这样的命令,我需要将ls -l的输出(stdout)重定向到grep toto的输入(stdin)。

我还希望在父级中显示命令的结果,而不是在execvp的调用中直接显示在子级中(这就是为什么我有2个分叉)。

目前,使用以下代码,我创建一个子节点以使用execvp执行命令,然后将结果写入变量。父母得到它并显示它。 我还引入了另一个fork来正确执行我的管道。 它正在工作(它执行并显示正确的结果),但我的孩子从未完成,execvp在管道后执行第二个命令后被阻止。

通过一些研究我发现这可能是因为我没有关闭我的一个文件描述符,但是我多次检查过,我真的没有看到问题或描述符没有在这里关闭......

任何人都知道我做错了什么?

我在c,forks& amp;管道对我来说真的有很大的魔力,而且我不太清楚它最终是如何运作的...所以如果你有一些建议或者我认为我做的事情不好,请随意在评论中说出来。

这是我的功能:

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<errno.h>
#include<unistd.h>
#include<fcntl.h> // open function
#include<sys/types.h>
#include<sys/stat.h>
#include<getopt.h>
#include<stdbool.h>
#define STDOUT 1
#define STDERR 2
int main(int argc, char** argv)
{

    //PARENT
    pid_t pid, wpid, pid2;
    int status, status2;
    const int MIN_BUFF_SIZE = 1024;

    int fd[2];
    if (pipe(fd) == -1) {
        perror("pipe");
    }
    printf("After creating pipe \n");

    pid = fork();

    if (pid == 0) {
        // First child process (parent of second child)
        printf("First child process before fork again\n");
        pid2 = fork();
        if(pid2 == 0)
        {
            printf("Second child process begin\n");
            //second child we need to execute the left command

            close(fd[0]);
            printf("Second child |Redirect stdout > stdin\n");
            dup2(fd[1], STDOUT_FILENO); // Redirect stdout to stdin

            //test data
            char* test[3];
            test[0] = "ls\0";
            test[1] = "-l\0";
            test[2] = NULL;

            //TODO test to remove
            if (execvp(test[0], test) == -1) {
                perror("shell launch error : error in child process > execvp failed \n");
                exit(errno);
            }
            printf("Second child | After execvp\n");
            exit(errno);

        }else if(pid<0)
        {
            perror("shell launch error : error forking second child");
        }else{
            do {
                wpid = waitpid(pid2, &status2, WUNTRACED);
                printf("Second parent\n");
                //Parent
                close(fd[1]);
                printf("Second parent | Redirect stdout > stdin\n");
                dup2(fd[0], STDIN_FILENO);
                printf("Second parent | After Redirect stdout > stdin\n");

                //test data : grep toto 
                char* test2[3];
                test2[0] = "grep\0";
                test2[1] = "toto\0";
                test2[2] = NULL;
                printf("Second parent | Av dup2 fd stdout\n");
                dup2(fd[1], STDOUT_FILENO);
                close(fd[1]);
                printf("Second parent | Ap dup2 fd stdout\n");
                if (execvp(test2[0], test2) == -1) {
                    perror("shell launch error : error in child process > execvp failed \n");
                    exit(errno);
                }
                exit(errno);
            } while (!WIFEXITED(status2) && !WIFSIGNALED(status2));
        }

    } else if (pid < 0) {
        // Error forking
        perror("shell launch error : error forking");
    } else {
        do {
            //wait child process to finish it execution. So, according to project requirements,
            //we need to print the result of the command after the child process is finished
            wpid = waitpid(pid, &status, WUNTRACED);
            printf("Finished waiting for %d\n", wpid);

            close(fd[1]);  // close the write end of the pipe in the parent

            if(status != 0)
            {
                printf("Status : %d\n", status);

            }else{
                printf("We are in the first parent \n");
            }
        } while (!WIFEXITED(status) && !WIFSIGNALED(status));
    }
    printf("Finish ! \n");
    return 0;
}

以下是执行后的输出:

After creating pipe 
First child process before fork again
Second child process begin
Second child |Redirect stdout > stdin
Second parent
Second parent | Redirect stdout > stdin
Second parent | After Redirect stdout > stdin
Second parent | Av dup2 fd stdout
Second parent | Ap dup2 fd stdout
-rw-r--r-- 1 ubuntu ubuntu    0 Mar  8 08:20 toto.txt
(and here it does not come back to my shell, it is waiting... something...)

我看到很多关于Implementing pipe in cConnecting n commands with pipes in a shell?的主题,但他们没有像我这样的2分叉,这是我的问题。

编辑

我用@William和@Toby建议编辑我的帖子(用评论中给出的模式关闭我的描述符,然后在等待孩子之前关闭祖父进程中管道的写入结束)。我为我添加的所有新行添加了一条评论// new,以帮助其他有相同问题的人看到更改。我的程序不再被execvp阻止了。

我总是有一个问题,如果在我祖父母的过程中我试图读取stdout,它们没有任何内容而不是我的命令,我错过了一次重定向或者我这次关闭了很多描述符?

    //PARENT
    pid_t pid, wpid, pid2;
    int status, status2;
    const int MIN_BUFF_SIZE = 1024;

    int fd[2];
    if (pipe(fd) == -1) {
        perror("pipe");
    }
    printf("After creating pipe \n");

    pid = fork();

    if (pid == 0) {
        // First child process (parent of second child)
        printf("First child process before fork again\n");
        pid2 = fork();
        if(pid2 == 0)
        {
            printf("Second child process begin\n");
            //second child we need to execute the left command

            close(fd[0]);
            printf("Second child |Redirect stdout > stdin\n");
            dup2(fd[1], STDOUT_FILENO); // Redirect stdout to stdin
            close(fd[1]); // NEW                

            //test data
            char* test[3];
            test[0] = "ls\0";
            test[1] = "-l\0";
            test[2] = NULL;

            //TODO test to remove
            if (execvp(test[0], test) == -1) {
                perror("shell launch error : error in child process > execvp failed \n");
                exit(errno);
            }
            printf("Second child | After execvp\n");
            exit(errno);

        }else if(pid<0)
        {
            perror("shell launch error : error forking second child");
        }else{
            do {
                wpid = waitpid(pid2, &status2, WUNTRACED);
                printf("Second parent\n");
                //Parent
                close(fd[1]);
                printf("Second parent | Redirect stdout > stdin\n");
                dup2(fd[0], STDIN_FILENO);
                close(fd[0]); // NEW
                printf("Second parent | After Redirect stdout > stdin\n");

                //test data : grep toto 
                char* test2[3];
                test2[0] = "grep\0";
                test2[1] = "toto\0";
                test2[2] = NULL;
                printf("Second parent | Av dup2 fd stdout\n");

                close(fd[0]); // NEW
                dup2(fd[1], STDOUT_FILENO);
                close(fd[1]);

                printf("Second parent | Ap dup2 fd stdout\n");
                if (execvp(test2[0], test2) == -1) {
                    perror("shell launch error : error in child process > execvp failed \n");
                    exit(errno);
                }
                exit(errno);
            } while (!WIFEXITED(status2) && !WIFSIGNALED(status2));
        }

    } else if (pid < 0) {
        // Error forking
        perror("shell launch error : error forking");
    } else {
        do {
            close(fd[1]);  //NEW close the write end of the pipe in the parent
            //wait child process to finish it execution. So, according to project requirements,
            //we need to print the result of the command after the child process is finished
            wpid = waitpid(pid, &status, WUNTRACED);
            printf("Finished waiting for %d\n", wpid);

            char* line_to_display = malloc(1);
            line_to_display = '\0';

            if(status != 0)
            {
                printf("Status : %d\n", status);

            }else{
                printf("We are in the first parent \n");
                ssize_t bytes_read = 1;
                do {

                    line_to_display = realloc(line_to_display, 1024); 
                    //sizeof(char) = 1 so don't need to do MIN_BUFF_SIZE * sizeof(char)
                    bytes_read = read(fd[0], line_to_display, 1024);

                } while (bytes_read > 0);

                printf("%s\n", line_to_display);
            }
        } while (!WIFEXITED(status) && !WIFSIGNALED(status));
    }
    printf("Finish ! \n");
    return 0;
}

1 个答案:

答案 0 :(得分:2)

管道的写入端仍然在祖父进程中打开。我们可以通过在第一个子节点中创建管道来避免此问题,如下所示:

    pid = fork();

    if (pid == 0) {
        // First child process (parent of second child)

        int fd[2];
        if (pipe(fd) == -1) {
            perror("pipe");
        }
        fprintf(stderr, "After creating pipe \n");

        fprintf(stderr, "First child process before fork again\n");
        pid2 = fork();

然后我们需要删除现在超出范围的close(fd[1]);(依赖于编译器)。

我顺便注意到有一个拼写错误:if(pid<0)而不是pid2<0