使用两个管道在父进程和子进程之间进行通信

Question

问题

我只在终端输出中得到这个。我相信该程序在fork() 调用时卡住了，但我不知道具体原因。

程序名称是q9:

prompt>$ ./q9 inputString
Parent: writing to pipe 'inputString'

任务

• 从终端读取输入到父子管道。
• fork() 创建子进程。
• 从父子管道读取输入。
• 将一些其他字符串连接到从管道读取的字符串。
• 将新连接的字符串写入子到父管道。
• 在父级中，从子级到父级管道读取，并将从管道读取的输出打印到终端。

尝试

我已尝试通过以下方式解决此问题：

• 正在尝试关闭不同位置的管道。我以为我可能遗漏了一些东西或留下了一些东西，但我不这么认为。
• 在父级中放置一个 wait()，因为它可能没有让子级完全运行
• 尝试打印连接字符串的输出，以防万一打印结果混乱。

代码

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <sys/wait.h>

int main (int argc, char *argv[]) {
    // parent RUN
    if(argc == 1) {
        printf("usage: q9 <string>\n");
        return 0;
    }

    // create two way pipes
    int parent_fds[2], child_fds[2];

    // create strings to save too
    char fromParent[100];
    char fromChild[100];

    // read:[0] - write:[1]
    if (pipe(parent_fds) != 0 && pipe(child_fds) != 0) {
        fprintf(stderr, "pipes failed!\n");
    }

    // close unused pipe end by parent
    close(parent_fds[0]);
    close(child_fds[1]);
    close(child_fds[0]);

    // write from terminal to parent pipe FOR child to read
    printf("Parent: writing to pipe '%s'\n", argv[1]);
    write(parent_fds[1], argv[1], strlen(argv[1]));
    close(parent_fds[1]);

    // fork() child process
    int child = fork();

    // NEVER GETS PASSED HERE :(

    if (child < 0) {
        fprintf(stderr, "fork failed!");
        exit(1);
    } else if (child == 0) {
        printf("I reached the child :)");
        // close unwanted pipe ends by child
        close(child_fds[0]);
        close(parent_fds[1]);

        // read from parent pipe
        int n = read(parent_fds[0], fromParent, 100);
        fromParent[n] = 0;
        printf("Child: reading from parent pipe '%s'\n", fromParent);
        close(parent_fds[0]);

        // Concatinate to what was read in
        const char myText[14] = " (added this.)";
        strcat(fromParent, myText);

        write(child_fds[1], fromParent, strlen(fromParent));
        close(child_fds[1]);
        printf("Child: writing to pipe - '%s'\n", fromParent);
    } else {
        // read from child pipe
        int n = read(child_fds[0], fromChild, 100);
        fromChild[n] = 0;
        printf("Parent: reading from pipe - '%s'\n", fromChild);
    }

    return 0;
}

出了什么问题？

Answer 1

有几个问题，您的诊断消息不能保证会出现。确保以换行符结束消息。

1. 您只创建了一个管道，因为您使用了&& 而不是||。
2. 您在创建子代之前“为父代”关闭了管道（kaylum 在 comment 中也注明了这一点）。

下面的代码中有多个其他清理。代码（仍然）不能确保写入管道操作成功（它们之前失败）。它确实确保从管道读取的字符串不长于放置它们的缓冲区；它不能确保有足够的空间在孩子中附加额外的信息。显示的代码在退出之前等待任何子进程完成。孩子执行wait() 调用但它立即失败（并且孩子不打印任何东西）并退出。父母等待孩子完成并在退出之前报告它这样做。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
    if (argc == 1)
    {
        fprintf(stderr, "Usage: %s <string>\n", argv[0]);
        return EXIT_FAILURE;
    }

    // create two pipes:
    // - parent_fds used by parent to write to child
    // - child_fds used by child to write to parent
    int parent_fds[2], child_fds[2];

    // read:[0] - write:[1]
    if (pipe(parent_fds) != 0 || pipe(child_fds) != 0)  /* || not && */
    {
        fprintf(stderr, "pipes failed!\n");
        return EXIT_FAILURE;
    }

    // fork() child process
    int child = fork();

    if (child < 0)
    {
        fprintf(stderr, "fork failed!");
        return EXIT_FAILURE;
    }
    else if (child == 0)
    {
        printf("%d: I reached the child :)\n", (int)getpid());
        // close unwanted pipe ends by child
        close(child_fds[0]);
        close(parent_fds[1]);

        // read from parent pipe
        char fromParent[100];
        int n = read(parent_fds[0], fromParent, sizeof(fromParent) - 1);
        fromParent[n] = '\0';
        printf("%d: Child: read from parent pipe '%s'\n", (int)getpid(), fromParent);
        close(parent_fds[0]);

        // Append to what was read in
        strcat(fromParent, " (added this.)");

        write(child_fds[1], fromParent, strlen(fromParent));
        close(child_fds[1]);
        printf("%d: Child: writing to pipe - '%s'\n", (int)getpid(), fromParent);
    }
    else
    {
        // close unwanted pipe ends by parent
        close(parent_fds[0]);
        close(child_fds[1]);

        // write from terminal to parent pipe FOR child to read
        printf("%d: Parent: writing to pipe '%s'\n", (int)getpid(), argv[1]);
        write(parent_fds[1], argv[1], strlen(argv[1]));
        close(parent_fds[1]);
        // read from child pipe
        char fromChild[100];
        int n = read(child_fds[0], fromChild, sizeof(fromChild) - 1);
        fromChild[n] = '\0';
        close(child_fds[0]);
        printf("%d: Parent: read from pipe - '%s'\n", (int)getpid(), fromChild);
    }

    int corpse;
    int status;
    while ((corpse = wait(&status)) > 0)
        printf("%d: child PID %d exited with status 0x%.4X\n", (int)getpid(), corpse, status);

    return EXIT_SUCCESS;
}

示例输出（源pipe43.c，程序pipe43）：

$ pipe43 'Nobody expects the Spanish Inquisition!'
84543: Parent: writing to pipe 'Nobody expects the Spanish Inquisition!'
84544: I reached the child :)
84544: Child: read from parent pipe 'Nobody expects the Spanish Inquisition!'
84544: Child: writing to pipe - 'Nobody expects the Spanish Inquisition! (added this.)'
84543: Parent: read from pipe - 'Nobody expects the Spanish Inquisition! (added this.)'
84543: child PID 84544 exited with status 0x0000
$