Re: GIOChannel & Intercepting stdout

[Hrvoje Niksic <hrvoje niksic avl com>]

On 08/25/2010 08:26 PM, Chris Vine wrote:
> This won't work.  file descriptor number 1 (stdout) is a file descriptor
> you write to, not read from.
>
> If you connect stdout to the write file descriptor of a pipe with
> dup2(), and then connect your GIOChannel object to the read file
> descriptor of the pipe, it should work.  Whether it does or not will
> depend on the libraries which you say are writing to stdout.  If they
> expect a terminal on stdout you may need to resort to pseudo-ttys.

The good news is that libraries shouldn't require a terminal for stdout 
because then they wouldn't work with stdout being redirected to a file 
(highly unlikely).

The bad news is that this is harder to do correctly than it sounds.  If 
you connect stdout to a pipe, then a thread (or a subprocess) should 
exist to collect output from the pipe, or the program deadlocks whenever 
a library prints a chunk of output larger than the pipe buffer.

The other bad news is that the dup2/pipe solution is very Unix-specific. 
 It is not clear if this is a problem to Richard.

Here is an example code that creates a simple pipe.  Hooking the read 
end of the pipe to GIO, and reading it in a separate thread is left as 
an excercise to the reader.

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

static void
printout(void)
{
  printf("foo\n");
  printf("bar\n");
  fflush(stdout);
}

static char *
collect(int fd)
{
  static char buf[512];
  int size = read(fd, buf, sizeof(buf) - 1);
  if (size < 0)
    exit(1);
  buf[size] = '\0';
  return buf;
}

int main()
{
  // create a pipe
  int pipefd[2];
  if (pipe(pipefd) < 0)
    exit(1);
  // make fd 1 alias for the write end of the pipe
  if (dup2(pipefd[1], 1) < 0)
    exit(1);
  close(pipefd[1]);
  printout();
  fprintf(stderr, "recevied: %s", collect(pipefd[0]));
  return 0;
}