C restore stdout to terminal

Question

I am working with a multi-thread program.

First I redirect my stdout to a certain file. No problem there (I used dup2(fd, 1) where fd is the file descriptor for the file).

Afterwards, I need to redirect my stdout to the terminal again.

My first approach:

      /*Declaration*/
      fpost_t  stream_sdout;
      /*code*/
      if ( fgetpos( stdout, &stream_sdout) == -1 )
          perror(Error:);

It says illegal seek.
No idea why this is happening.
But if I get this to work, then I only need to use fsetpos(stdout, &stream_stdout) and it should work.

My second idea, was to to copy the stdout using dup2(stdout, 4) to the file descriptor table, at position 4. But that ain't working either.

How can I switch the standard output back to its original destination (terminal, pipe, file, whatever)?

Answer 1

#include <unistd.h>

...

int saved_stdout;

...

/* Save current stdout for use later */
saved_stdout = dup(1);
dup2(my_temporary_stdout_fd, 1);

... do some work on your new stdout ...

/* Restore stdout */
dup2(saved_stdout, 1);
close(saved_stdout);

Answer 2

Before you do the dup2(fd, STDOUT_FILENO), you should save the current open file descriptor for standard output by doing int saved_stdout = dup(STDOUT_FILENO); (letting dup() choose an available file descriptor number for you). Then, after you've finished with the output redirected to a file, you can do dup2(saved_stdout, STDOUT_FILENO) to restore standard output to where it was before you started all this (and you should close saved_stdout too).

You do need to worry about flushing standard I/O streams (fflush(stdout)) at appropriate times as you mess around with this. That means 'before you switch stdout over'.

Answer 3

If the program runs on a Linux environment, you can freopen ("/dev/stdout", "a", stdout).

But if you know that stdout was the terminal, freopen ("/dev/tty", "a", stdout) or the equivalent for other OSs—even Windows.