IO redirection problem [SOLVED]

Lexion · 2010-07-28 13:28:27

My goal is to write a program which redirects the io streams of a child program to a variety of files. I have gotten it to work, mostly.

#define _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <fcntl.h>

int shell_pid;

int redirect_io()
{
    // io stream file names
    char *out = "acs_stdout";
    char *in = "acs_stdin";

    // actual redirection
    freopen(out, "w", stdout);
    freopen(out, "w", stderr);
    freopen(in, "r", stdin);

    return 0;
}

int fork_shell()
{
    // fork another thread to run shell
    shell_pid = fork();
    if (shell_pid == 0)
    {
        char *cmd[] = {"/bin/bash", NULL};
        execv(cmd[0], cmd);
        _exit(0);
    }

    return 0;
}

int shell_write(char *s)
{
    // write command to stdin file
    fprintf(stdin, "%s\n", s);
    return 0;
}

int main()
{
    redirect_io();
    shell_write("ls"); // i have tried this command after fork_shell(). there is no difference
    fork_shell();
    waitpid(shell_pid, 0, 0); // wait until the shell is done

    return 0;
}

My problem is that after that first command (ls) is run, bash just quits. It completes the ls command and the correct output is in acs_stdout (my stdout file). Do any of you have any ideas on what might be wrong?

Last edited by Lexion (2010-07-28 16:22:29)

diegonc · 2010-07-28 14:21:49

Lexion wrote:

[...]
int redirect_io()
{
    [...]
    freopen(in, "r", stdin);

    return 0;
}
[...]
int shell_write(char *s)
{
    // write command to stdin file
    fprintf(stdin, "%s\n", s);
    return 0;
}
[...]

I don't think its related, but you are writting to a file opened for reading. I believe bash is exiting because after reading the ls command there's nothing left in the file and it gets an EOF.

You should be using pipes here:

int fork_shell()
{
    /* call pipe(), it will give you two file descriptors,
     * one for each end of the pipe.
     */
    // fork another thread to run shell
    shell_pid = fork();
    if (shell_pid == 0)
    {
        /* close the write end */
        /* reassing stdin to the read end, using dup() i think */
        /* reopen stdout to a file (or the write end of another pipe
         * if you want the output to go back to the parent process)
         */
        char *cmd[] = {"/bin/bash", NULL};
        execv(cmd[0], cmd);
        _exit(0);
    }

    /* you may reassing stdout to the write end of the pipe or use it
     * explicitly when sending commands to bash
     */

    return 0;
}

Last edited by diegonc (2010-07-28 14:24:02)

Lexion · 2010-07-28 14:46:02

Thanks for the quick reply. Bash no longer quits now, but when I run the program, it does this:

$ program
ls

And doesn't quit until I press Ctl-C. It appears to start bash, and then print "ls" to stdout. Here is the code that I changed:

int shell_pipe[2];
#define SHELL_PIPE_READ shell_pipe[0]
#define SHELL_PIPE_WRITE shell_pipe[1]

int fork_shell()
{
    pipe(shell_pipe);
    shell_pid = fork();
    if (shell_pid == 0)
    {
        close(SHELL_WRITE);
        dup2(SHELL_READ, STDIN_FILENO);
        char *cmd[] = {"/bin/bash", NULL};
        execv(cmd[0], cmd);
        _exit(0);
    }

    return 0;
}

diegonc · 2010-07-28 16:09:02

Looks like shell_write was not updated

Lexion · 2010-07-28 16:22:08

Thanks alot. It works. yay!

Arch Linux

#1 2010-07-28 13:28:27

IO redirection problem [SOLVED]

#2 2010-07-28 14:21:49

Re: IO redirection problem [SOLVED]

#3 2010-07-28 14:46:02

Re: IO redirection problem [SOLVED]

#4 2010-07-28 16:09:02

Re: IO redirection problem [SOLVED]

#5 2010-07-28 16:22:08

Re: IO redirection problem [SOLVED]

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