redirect stdout and stderr to different processes

Question

I am testing the answer here but when I use a C program instead of bash script, I couldn't get it to work properly.

This is the C program.

// tss.c

#include <stdio.h>

int main()
{
   while (1)
   {
      fprintf(stdout, "stdout: sias\n");
      fprintf(stderr, "stderr: cias\n");
      sleep(5);
   }

   return 0;
}

This is the test result, where I only see stderr not stdout.

$ $ { tss 2>&1 1>&3 3>&- | sed -u 's/^/err: /'; } 3>&1 1>&2 | sed -u 's/^/out: /'
err: stderr: cias
err: stderr: cias

I then notice that I need to do "fflush(stdout)" in the program. But one thing I don't understand why it is not needed for stderr, and is there a way to get it to work without inserting fflush?

Does ["C equivalent of autoflush (flush stdout after each write)?"](https://stackoverflow.com/questions/214271/c-equivalent-of-autoflush-flush-stdout-after-each-write) answer your question? — Gordon Davisson, Oct 24 '22 at 02:02
Re "*I only see stderr not stdout.*", That's only because you didn't wait long enough. When the buffer fills or the program exits, the buffer will be output. Note that the `unbuffer` tool can be used to fool STDOUT back into line-buffered behaviour. — ikegami, Oct 24 '22 at 03:49

score 1 · Accepted Answer · answered Oct 24 '22 at 02:20

I then notice that I need to do "fflush(stdout)" in the program. But one thing I don't understand why it is not needed for stderr,

stdout is buffered by default -- line buffered when it is connected to a terminal, but block buffered otherwise. Connecting the program's stdout to a pipe produces the "otherwise" case. fflush() is how you send the buffered output to the pipe before the buffer fills.

stderr is unbuffered by default. Unless you change that, output to stderr is sent to the output device immediately.

and is there a way to get it to work without inserting fflush?

Without fflush(), the output to stdout will be emitted in buffer-size chunks. If you want stdout to be unbuffered instead, like stderr, then you can achieve that with setvbuf().

redirect stdout and stderr to different processes

1 Answers1