在Linux中使用父进程和子进程使用管道 [英] Using pipe in linux using parent and child process

问题描述

我正在尝试使用C实现此linux命令. ls -l |切-b 1

i am trying to implement this linux command using C. ls -l | cut -b 1

我试图做到的方式是

在父进程中调用ls -l 将ls -l的输出放入文件(写入文件)

calling ls -l in parent process putting output of ls -l in a file(writing to a file)

在子进程中调用剪切 读取文件(在父进程中编写的文件) 将剪切应用于文件 打印输出

calling cut in child process reading the file (the one written in the parent process) applying cut to the file printing the output

到目前为止，这是我所做的

this is by far what i have done

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

void main()
{
   int filedes[2];
   int p;
   pid_t pid, pid1;
   p=pipe(filedes);
   FILE *stream;
   char buff[20];

   printf("pipe command returns %d, %d ,%d\n",p, filedes[0],filedes[1]);

   if(pipe(filedes) == -1) /* Create the pipe */
      printf("error pipe");
      pid1=fork();
      pid=getpid();
      switch (pid1) { /* Create a child process */
      case -1:
         printf("error fork");
      case 0: /* Child */
      /* Close unused write end */
      /* Child can now read from pipe */
         if (close(filedes[1]) == -1)
            printf("error close");
         printf("I am a child process pid %d, and will read from pipe\n",pid);

         while (read(filedes[0], &buff, 1) > 0)
            write(STDOUT_FILENO, &buff, 1);

         write(STDOUT_FILENO, "\n", 1);
         close(filedes[0]);
         _exit(EXIT_SUCCESS);

         break;

         default: /* Parent */
         /* Close unused read end */
         /* Parent can now write to pipe */
         if (close(filedes[0]) == -1)
            printf("error close");
         printf("I am the parent process pid %d, and will write to pipe\n", pid );
         stream = fdopen(filedes[1], "w");
         strcpy(buff, "This is a test\n");
         write(filedes[1], buff, strlen(buff));

         char *args[80];
         args[0] = "ls";
         args[1] = "-l";
         args[2] = NULL;
         execvp(args[0],args);

         int bak, new;
         bak = dup(1);
         new = open("/home/urwa/abc.txt", O_WRONLY);
         dup2(new, 1);
         close(new);



         close(filedes[1]);          /* Reader will see EOF */
         wait(NULL);                /* Wait for child */
         exit(EXIT_SUCCESS);

         break;
   }
}

这段代码可以很好地工作.并在测试台输出中打印测试说明.以及ls -l输出.但文件为空.我究竟做错了什么. 我也尝试了如下的freopen ..仍然为空文件. :/

this piece of code works perfectly fine. and prints at stand output the test statement. as well as the ls -l output. but the file is empty. what am i doing wrong. I also tried freopen as follow.. still empty file. :/

 FILE *fp;
 fp = freopen ("/temp/abc.txt", "a+", stdout);

推荐答案

您没有在子对象中调用cut，而且文件描述符在这里也被弄乱了.

You didn't called the cut in the child and also the file descriptors are messed up here.

要执行此任务，您必须关闭父级的标准输出，并在execvp 之前在父级中设置写入结束标准输出.在孩子中，您必须关闭孩子的stdin ，并在执行vpv之前以您的孩子的stdin结尾.这样，your parent's stdout is stdin of your child(创建两个黑白的管道).

For performing the task you have to close the stdout of parent and make the write end stdout in parent before execvp. In child you have to close stdin of child and make read end as stdin to your child before execvp. In that way your parent's stdout is stdin of your child(creating the pipe b/w two).

int main()
{
   int filedes[2];
   int p;
   pid_t pid = 0, pid1 = 0;
   p=pipe(filedes);
   FILE *stream;
   char buff[20];
   char *args[80];

   printf("pipe command returns %d, %d ,%d\n",p, filedes[0],filedes[1]);

   if(pipe(filedes) == -1) /* Create the pipe */
      printf("error pipe");
      pid1=fork();
      pid=getpid();
      switch (pid1) { /* Create a child process */
      case -1:
         printf("error fork"); break;
      case 0: /* Child */
      /* Close unused write end */
      /* Child can now read from pipe */
         if (close(filedes[1]) == -1)
            printf("error close");
         printf("I am a child process pid %d, and will read from pipe\n",pid);

         close(0); //close stdin of child
         dup(filedes[0]); //make pipes read end stdin of child

         args[0] = "cut";
         args[1] = "-b";
         args[2] = "1";
         args[3] = NULL;
         execvp(args[0],args);
         break;

         default: /* Parent */
         /* Close unused read end */
         /* Parent can now write to pipe */
         if (close(filedes[0]) == -1)
            printf("error close");
         printf("I am the parent process pid %d, and will write to pipe\n", pid );

         close(1); //close stdout
         dup(filedes[1]); //make write end of pipe stdout of parent
         args[0] = "ls";
         args[1] = "-l";
         args[2] = NULL;
         execvp(args[0],args);
         break;
   }
}

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