c - 如何使用管道将 C 中的数据从两个 child 发送到 parent ？

Question

我有这个任务，我必须使用 fork 创建两个 child 。 parent 必须向这些 child 发送一些信件并收到一个号码。必须使用管道进行通信。我不明白的是，为什么我的代码设法将信件发送给了 children ，但随后却什么都不做就挂起了。起初我以为是因为调用了 waitpid() 但事实并非如此。让我感到困惑的是，如果我停止读取其中一个子进程并只发送一个随机数，其他一切都会按预期工作。

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

#define PIPE_1 0 // write to 1st child
#define PIPE_2 1 // write to parent from 1st child
#define PIPE_3 2 // write to 2nd child
#define PIPE_4 3 // write to parent from 2nd child

int main()
{
    pid_t p1, p2;
    int pipes[4][2];
    FILE *read[4], *write[4];

    int k;
    for (k = 0; k < 4; k++){
        if (-1 == pipe(pipes[k]) ){
            perror("Error creating pipe");
            exit(k+1);
        }
        read[k] = fdopen(pipes[k][0], "r");
        write[k] = fdopen(pipes[k][1], "w");
    }

    if (-1 == (p1 = fork()) ){
        perror("Error, failed to fork first child");
        exit(5);
    }

    if (p1 == 0){
        // 1st child
        fclose(write[PIPE_1]);
        int caseChange = 0;
        char c;
        while( fscanf(read[PIPE_1], "%c", &c) != EOF){
            if (c >= 97 && c <= 122){
                    c -= 32;
                    caseChange++;
            }
            printf("[Process (%d)]: %c\n", getpid(), c);
        }
        fclose(read[PIPE_1]);

        fclose(read[PIPE_2]);
        fprintf(write[PIPE_2],"%d",caseChange);
        fflush(write[PIPE_2]);
        fclose(write[PIPE_2]);  
    }
    else{
        if (-1 == (p2 = fork()) ){
            perror("Error, failed to fork second child");
            exit(6);
        }

        if (p2 == 0){
            // 2nd child
            fclose(write[PIPE_3]);
            int caseChange = 0;
            char c;
            while( fscanf(read[PIPE_3], "%c", &c) != EOF){
                if (c >= 97 && c <= 122){
                    c -= 32;
                    caseChange++;
                }
                printf("[Process (%d)]: %c\n", getpid(), c);
            }
            fclose(read[PIPE_3]);

            fclose(read[PIPE_4]);
            fprintf(write[PIPE_4],"%d",caseChange);
            fflush(write[PIPE_4]);
            fclose(write[PIPE_4]);  
        }
        else{
            // Parent
            //char *string = (char*) malloc(100);
            char string[100];
            scanf("%s", string);
            int i;

            int readCaseChange, caseChange = 0;

            fclose(read[PIPE_1]);
            for (i = 0; i < strlen(string); i+=2){
                fprintf(write[PIPE_1],"%c",string[i]);
                fflush(write[PIPE_1]);
            }
            fclose(write[PIPE_1]);

            fclose(read[PIPE_3]);
            for (i = 1; i < strlen(string); i+=2){
                fprintf(write[PIPE_3],"%c",string[i]);
                fflush(write[PIPE_3]);
            }
            fclose(write[PIPE_3]);

            waitpid(p1, NULL, 0);
            fclose(write[PIPE_2]);
            fscanf(read[PIPE_2], "%d", &readCaseChange);
            fclose(read[PIPE_2]);
            caseChange += readCaseChange;

            waitpid(p2, NULL, 0);
            fclose(write[PIPE_4]);
            fscanf(read[PIPE_4], "%d", &readCaseChange);
            fclose(read[PIPE_4]);
            caseChange += readCaseChange;

            printf("%d\n", caseChange);
            fflush(stdout);
        }
    }
    return 0;
}

Answer 1

您想关闭所有未在每个子进程中使用的东西，尤其是父进程的编写器端:

// 1st child
fclose(write[PIPE_1]);
fclose(write[PIPE_3]);

// 2nd child
fclose(write[PIPE_1]);
fclose(write[PIPE_3]);

否则，子进程将保持写入端打开并阻止管道完全关闭，从而使您永远不会在子进程中获得 EOF。你部分地这样做了，但由于第二个 child 已经写 [PIPE_1] 打开，PIPE_1 从未完全关闭。