Executing another program, redirecting its output to a file, and returning control.

Okay, so basically I'm trying to write a program that I can use to convert video files and transfer them to my mp3 player.

The basic part worked fine initially, using system(), I would call ffmpeg, have it convert the file, then use system again to call mtp-sendtr. The problem is, I need to specify the length of the video in seconds to mtp-sendtr. I can check the length myself and input it manually, and that works, but since ffmpeg displays the length as it converts, it seems I could use this data somehow.

First, I tried simply piping the output from ffmpeg to a text file like "ffmpeg blah blah blah > output.txt" but that doesn't work, the file ends up being blank.

So I did this (I've replaced ffmpeg with ls just to simplify things):

Code:

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>


int main(int argc, char *argv)
{
int fd; /*file descriptor to the file we will redirect ls's output*/

if((fd = open("dirlist.txt", O_RDWR | O_CREAT))==-1){ /*open the file */
  perror("open");
  return 1;
}


dup2(fd,STDOUT_FILENO); /*copy the file descriptor fd into standard output*/

dup2(fd,STDERR_FILENO); /* same, for the standard error */


close(fd); /* close the file descriptor as we don't need it more  */

/*execl ls */
 execl("/bin/ls", "ls", (char *) 0);

printf("All done.\n");

return 0;
}

That works, of course, but then the program just ends. I don't know much about forks and dup2 and all that, and I'm trying to learn, but I can't find anything that seems to answer my questions.

I have a feeling I'm going about this in a convoluted way, and I'd LOVE to have someone explain to me what I need to do to make this work.

Read up on popen(), it will be easier for what you are trying to do, vis tapping the output from ffmpeg.

Originally Posted by IanKoro

I have a feeling I'm going about this in a convoluted way, and I'd LOVE to have someone explain to me what I need to do to make this work.

Yes, no matter how you slice it using C to do this is convoluted. It would be better done in a shell script, but if you are more comfortable programming in C than for bash you might as well try.

You need to fork somewhere.

execl() replaces the process image, so it will never return.

You need to first create pipes (using pipe()) for I/O, and then fork. In the child process, dup2() stdin and stdout to 2 ends of the pipes, close the unneeded ends, and call exec(). In the parent, close the unneeded ends (should be the opposite ends to the ones closed by the child), and then you can read/write from/to the pipes.

I have recently written a C++ wrapper for this. You may want to have a look.
Code review for a class that spawns a new process and allows IPC
(final code on the last post)
The AppInstance::start() function in particular.

OR, if you only need to read OR write to the child process (as opposed to both read AND write), you can just use popen().

As for shell redirection not working, maybe it writes to stderr? (People do that because stderr is usually not buffered, whereas stdout usually is)

Originally Posted by cyberfish

You need to fork somewhere.

execl() replaces the process image, so it will never return.

You need to first create pipes (using pipe()) for I/O, and then fork. In the child process, dup2() stdin and stdout to 2 ends of the pipes, close the unneeded ends, and call exec().

This is what popen() does and it does it properly. Just use popen().

Life would be so good if popen supports "rw" mode...

But yes, for this application, popen() is easier, since you presumably don't need to write to ffmpeg.

> Life would be so good if popen supports "rw" mode...
IIRC, some versions do (or at least I recall someone saying they do)

Ahh... well, I figured out how to fork it, thank you very much... I found an example that cleared things up tremendously.

I know C is a convoluted way of doing this, but I'm mostly trying to teach myself stuff.