Standard Error and popen(): how to do it?

I want to open a process from C code, and be able to read its standard output and standard error, while being able to write to its standard input.

The closest I get to achieving this is using popen(), but this does not allow you to read the standard error stream. You could add "2>&1" to the command, but this will not make it possible to distinguish between the standard output and error data. Being able 开发者_如何学JAVAto seperate both streams is required for my application.

Python has popen3 and Ruby has Open3 to do these kind of things, but I can't seem to find a way to do it in C. Any help?

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

...


int pipe_err[2], pipe_out[2], pipe_in[2];

if (pipe(pipe_err) || pipe(pipe_out) || pipe(pipe_in)) { // abbreviated error detection
     perror("pipe");
     scream_and_run_around_frantically();
     exit(BAD);
}
pid_t pid = fork();
if (!pid) { // in child
    dup2(pipe_err[1], 2);
    dup2(pipe_out[1], 1);
    dup2(pipe_in[0], 0);
    close(pipe_err[0]);
    close(pipe_err[1]);
    close(pipe_out[0]);
    close(pipe_out[1]);
    close(pipe_in[0]);
    close(pipe_in[1]);

    // close any other files that you don't want the new program
    // to get access to here unless you know that they have the
    // O_CLOEXE bit set

    execl(program_path, program_name, arg1, arg2, arg3);
    /* only gets here if there is an error executing the program */
 } else { // in the parent
     if (pid < 0) {
           perror("fork");
           error_death_plague_stubbed_toe();
           exit(BAD);
     }
     child_err = pipe_err[0];
     close(pipe_err[1]);
     child_out = pipe_out[0];
     close(pipe_out[1]);
     child_in = pipe_in[1];
     close(pipe_in[0]);

     ...

You will probably want to have a look at

man 3 exec

This has lots of functions that turn the current program into a new program. They all have different interfaces, but use execve under the hood.

man 2 execve

Also:

man 2 fork

and

man 2 pipe

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You may want to consider using execl(). This is what is used internally by popen().

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If you're forking the child process, you can duplicate the handle of stderr and use it in the child.