Question

我的作业有问题我上课了。我必须创建一个读/写程序，将文本文件读入其中并将内容写入新的文本文件。问题是，我必须使用父/子进程和管道。我必须将内容传递给具有一个子节点的管道，并使用另一个子节点从管道读取数据并将其写入新文件。

我有三个文件：parent.c，read.c和write.c。该程序在大多数情况下工作正常！它甚至可以完美地将数据从一个文件传输到另一个文件。我遇到的问题是write.c进程永远不会完成。我认为它可能与管道读取有关（不会返回0或EOF）。这是我的源代码：

parent.c

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

#define BUFF_SIZE 255

int main(int ac, char* av[]) 
{   
    if(ac <3)
    {
        printf("Please enter all required arguments!\n");   
        exit(0);    
    }

    int pfd[2];
    int pipeCreated;

    char readFile[50];
    char writePipe[20];

    pid_t child_pid_read;
    pid_t child_pid_write;

    pipeCreated = pipe(pfd);

    if(pipeCreated == -1)
    {
        printf("An error occurred when trying to create a pipe\n");
        exit(0);
    }

    strcpy(readFile, av[1]);
    sprintf(writePipe,"%d", pfd[1]);

    child_pid_read = fork();

    char writeFile[50];
    char readPipe[20];

    //Handling the read()
    switch(child_pid_read) 
    {
        //Error in case forfk() failed
        case -1:
            perror("fork failed");
            return 1;

            //Handle child processes
        case 0:
             if(close(pfd[0]) == -1)
             {
                 printf("An error occurred while closing the pipe\n");
                 exit(0);
             }
             if(execle("./read.out", "./read.out", readFile, writePipe, (char*)0, NULL) == -1)
             {
                 printf("Child: Error creating read.\n");
                 exit(0);
             }

        default:
            wait(&child_pid_read);

            strcpy(writeFile, av[2]);
            sprintf(readPipe,"%d", pfd[0]);

            child_pid_write = fork();
            break;
    }

    //Handling the write
    switch(child_pid_write) 
    {
        //Error in case fork() failed
        case -1:
            perror("fork failed");
            return 1;

        //Handle child processes
        case 0:
            if(close(pfd[1]) == -1)
            {
                printf("An error occurred while closing the pipe\n");
                exit(0);
            }

            if(execle("./write.out", "./write.out", writeFile, readPipe, (char*)0, NULL) == -1)
            {
                printf("Child: Error creating read.\n");
                exit(-1);
            }
            break;

        default:
            wait(&child_pid_write);
            break;
    }
    printf("Write completed!");
    return 0;
}

read.c：

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

#define BUFF_SIZE 16

int main(int ac, char* av[]) 
{   
    char buffer[BUFF_SIZE];
    int fd;
    int pid;

    if(ac > 1)
    {
        fd = open(av[1], O_RDONLY);
        if(fd == -1)
        {
            printf("error: Could Not Open File\n");
            exit(0);
        }

        pid = atoi(av[2]);

    }

    int num_read = 1;

    while(1)
    {
        num_read = read(fd, buffer, BUFF_SIZE);

        if(num_read == -1)
        {
            printf("Error reading file\n");
            exit(0);
        }

        if(num_read == 0)
        {
            break;
        }

        if(write(pid, buffer, num_read) != num_read)
        {
            printf("Error writing to pipe\n");
            break;
        }
    }
    close(fd);
    return 1;
}

为write.c

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

#define BUFF_SIZE 1

int main(int ac, char* av[]) 
{
    char buffer[BUFF_SIZE];

        int fd = open(av[1], O_WRONLY | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);

        int pid = atoi(av[2]);

        int num_read = 1;

    while(1)
    {
            num_read = read(pid, buffer, BUFF_SIZE);

            printf("num_read: %d\n", num_read);

            if(num_read == -1)
            {
                printf("Error reading pipe\n");
                break;
            }

            if(write(fd, buffer, num_read) != num_read)
            {
                printf("Error writing to file\n");
                break;
            }

            if(num_read == EOF)
            {
                break;
            }
    }

        close(fd);
        return 1;
}

请查看我的代码并提出更正建议。我通过终端（./parent.out，oldFile.txt，newFile.txt）传递了文本文件的名称。

Answer 1

两个问题：

在读取进程的wait（）返回之后，您才会执行写入过程。如果读取进程尝试写入的数据多于管道缓冲区中的数据，则它将阻塞并永不退出。您需要允许两个进程同时运行以避免此死锁。它将使用一个小文件，但如果文件大于4KB，它将挂起。

在分配写入过程后，父进程必须关闭pfd[0]。在所有具有写入结束的进程打开之前，管道的读取器不会获得EOF。它应该是：

default:
    if(close(pfd[0]) == -1)
     {
         printf("An error occurred while closing the pipe\n");
         exit(0);
     }
    wait(&child_pid_write);
    break;

Answer 2

您的孩子想要读取数据，为什么要关闭fd [0]，从管道返回指示fd [0]用于读取和fd [1]用于写入。由于我无法添加注释，我必须在这里发表评论....

为什么read（）从管道读取时不会返回零

parent.c

read.c：

为write.c

2 个答案: