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[C] pipe redirection of child's stdin and stdout doesnt work - Printable Version

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+---- Thread: [C] pipe redirection of child's stdin and stdout doesnt work (/showthread.php?tid=4740300)

pipe redirection of child's stdin and stdout doesnt work - mr.cracker - Mar 21, 2015

I was trying to redirect childs STDIN and STDOUT to pipes using dup2().
child has : execl("/bin/sh")

I want to give commands from Parent.
Child executes it. Write o/p to stdout, which is redirected to pipe.
Then parent read from pipe, o/p to stdout of parent.

my code is,

Code
#include<stdio.h>
#include<wait.h>
#include<unistd.h>
#include<stdlib.h>
#include<string.h>

int main(){
    pid_t pid;
    int n, len;
    int pipe1[2], pipe2[2];
    char buff[1024], data[4096];

    pipe(pipe1);
    pipe(pipe2);

    if( (pid = fork()) < 0){
      return 1;
    }else if(pid == 0){ //child
      close(pipe1[1]);      //child closes write end of pipe 1 ( fd1[1]), and
      close(pipe2[0]);      //child closes read end of pipe 2 (fd2[0]).

      dup2(pipe1[0], STDIN_FILENO);   // pipe1[0] read pipe, which is opened in child. And it's companion
            // pipe1[1] write pipe is opened in parent.
            // whatever  write into pipe1[1]  will be recived by stdin of child.
      dup2(pipe2[1], STDOUT_FILENO);
      execl("/bin/sh", "/bin/sh", (char *)0);

      exit(0);
    }
    //parent

    printf("parent pid: %d\n", (int)getpid());
    close(pipe1[0]);      //parent closes read end of pipe 1 ( fd1[0])
    close(pipe2[1]);      //parent closes write end of pipe 2 (fd2[1])

while(1){
printf("\nenter command: ");

  //read pathname
    if(fgets(buff, 1024, stdin) == NULL){
      printf("error in fgets");
      exit(1);
    }

    len = strlen(buff);  // fgets() guarantees null byte at end
    //if(buff[len - 1] == '\n')
      //  len--;      //delete newline from fgets

    //write pathname to ipc channel
    if(write(pipe1[1], buff, len) != len){
      printf("write error in client\n");
    }
    printf("command written to child: %s\n", buff);

    /* read from IPC, write to standard output */
    n = read(pipe2[0], buff, 4096);
      printf("am reading\n");
      if(write(STDOUT_FILENO, buff, n) != n ){
      printf("write error inside while in client\n");
      }
}
}

it got stuck on here,
Code
while((n = read(pipe2[0], buff, 4096)) > 0){
I dont get any output.

Why child does not write into pipe2[1] ?
I think thats why system waits for some data in read() function.

Where i got mistake?
How can i make it work?

* - g4143 - Mar 21, 2015

I'm pretty sure you can't redirect a terminal's stdin and stdout that way.

* - mr.cracker - Mar 21, 2015

(Mar 21, 2015 02:00 AM)g4143 Wrote:  I'm pretty sure you can't redirect a terminal's stdin and stdout that way.

sorry, now its working, the problem was here.
Code
if(buff[len - 1] == '\n')
      len--;      //delete newline from fgets

Since file stream is line buffered in terminal , i need to keep that '\n' to get output.

* - g4143 - Mar 21, 2015

(Mar 21, 2015 05:05 AM)mr.cracker Wrote:  sorry, now its working, the problem was here.
Code
if(buff[len - 1] == '\n')
      len--;      //delete newline from fgets

Since file stream is line buffered in terminal , i need to keep that '\n' to get output.

Since file stream is line buffered in the shell.

For some reason I thought you were attempting to do this.
Code
execl("/usr/bin/xterm", "/usr/bin/xterm", (char *)0);/*open a terminal as a child*/

BTW, your program should have a return 0 at the end of main and it should #include <sys/types.h>.

* - mr.cracker - Mar 21, 2015

(Mar 21, 2015 08:56 AM)g4143 Wrote:  For some reason I thought you were attempting to do this.
Code
execl("/usr/bin/xterm", "/usr/bin/xterm", (char *)0);/*open a terminal as a child*/

i tried above code. It doesn't redirect stdin and stdout.!!!
Can you please tell me the reason ?

I have updated my code with working code above in my question.
now when i get a prompt i entered
"LLL" : that is a wrong command.
then i got,
Code
/bin/sh: 1: LLL: not found

After that it asks for input continuously. I think the child ask for input. Can you just check my code once more. Just run it. It is a working code. and try a invalid input.
I could not find the reason.

* - g4143 - Mar 21, 2015

(Mar 21, 2015 09:11 AM)mr.cracker Wrote:  i tried above code. It doesn't redirect stdin and stdout.!!!
Can you please tell me the reason ?

I have updated my code with working code above in my question.
now when i get a prompt i entered
"LLL" : that is a wrong command.
then i got,
Code
/bin/sh: 1: LLL: not found

After that it asks for input continuously. I think the child ask for input. Can you just check my code once more. Just run it. It is a working code. and try a invalid input.
I could not find the reason.

Well is LLL a valid bash command or program on your system? Try opening a terminal and typing LLL at the prompt.

* - mr.cracker - Mar 21, 2015

Can you please tell me, why below code doesn't redirect stdin and stdout ?

Code
execl("/usr/bin/xterm", "/usr/bin/xterm", (char *)0);/*open a terminal as a child*/