使用父进程和子进程在linux中使用管道

时间:2013-05-06 20:30:48

标签: linux pipe parent-child

我正在尝试使用C实现此linux命令。 ls -l | cut -b 1

我试图这样做的方式是

  

在父进程中调用ls -l   将ls -l的输出放在一个文件中(写入文件)

     

在子进程中调用cut   读取文件(在父进程中写入的文件)   将剪切应用于文件   打印输出

这是迄今为止我所做的

/* 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;
   }
}

这段代码完美无缺。并在展台上打印输出测试声明。以及ls -l输出。但文件是空的。我究竟做错了什么。 我也试过freopen如下..仍然是空文件。 :/

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

1 个答案:

答案 0 :(得分:0)

你没有在孩子中调用剪切,文件描述符也搞乱了。

要执行任务,您必须关闭父级的标准输出,并在execvp 之前使父级中的写入结束标准输出。在孩子中,您必须关闭孩子的stdin ,并在execvp 之前将读取结束作为stdin给您的孩子。以这种方式your parent's stdout is stdin of your child(创建管道b / w两个)。

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;
   }
}