POSIX: capture child process output

Question

I am trying to fork and exec, with the parent process processing anything on stdout/stderr. Unfortuantely the parent process does not get any output. That is, the calls to read always return a non-positive integer.

Here's the code. Whats wrong?

#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/wait.h>

#include <iostream>
#include <string>

int main()
{               
    int out[2],err[2],in[2];
    pipe(out);
    pipe(err);
    pipe(in);

    int fork;
    if(!(fork=::fork()))
    {
        dup2(in[0],STDIN_FILENO);
        dup2(out[1],STDOUT_FILENO);
        dup2(err[1],STDERR_FILENO);

        execlp("/bin/ls","/bin/ls","-la","/");
    }

    fcntl(out[0], F_SETFL, fcntl(out[0], F_GETFL) | O_NONBLOCK);
    fcntl(err[0], F_SETFL, fcntl(err[0], F_GETFL) | O_NONBLOCK);
    for(;;)
    {
        int result;
        waitpid(fork,&result,WNOHANG);

        char ac[256];
        int cnt;
        while((cnt=read(out[0], ac, 256))>0)
    {
            std::cout << std::string(ac,ac+cnt) << std::flush;
        }
        while((cnt=read(err[0], ac, 256))>0)
        {
            std::cout << std::string(ac,ac+cnt) << std::flush;
        }
        if(WIFEXITED(result))
    {
            return 0;
        }
    }
}

When removing these two lines:

fcntl(out[0], F_SETFL, fcntl(out[0], F_GETFL) | O_NONBLOCK);
fcntl(err[0], F_SETFL, fcntl(err[0], F_GETFL) | O_NONBLOCK);

everything works as expected. I want non-blocking IO, though...

Thanks for reading my post.

Looks like this question: http://stackoverflow.com/questions/7292642/grabbing-output-from-exec/7292659 — Aif, Oct 09 '11 at 12:36

score -1 · Answer 1 · answered Oct 09 '11 at 13:33

-1

Your code does not do almost any error validation.

I for one would check the return value of errno, when you get -1 from read, there might be an issue there.

answered Oct 09 '11 at 13:33

Paulo Pinto

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POSIX: capture child process output

1 Answers1