Re: Modify buffering of standard streams via environment variables (not LD_PRELOAD)?

[Zachary Santer]

> On Sun, 21 Apr 2024, wrotycz wrote:
> >
> > It seems that it's 'interleaved' when buffer is written to a file or
> > pipe, and because stdout is buffered it waits until buffer is full or
> > flushed, while stderr is not and it doesn't wait and write immediately.
>
> Right; my point was just that stdout and stderr are still separate streams
> (with distinct buffers & buffering modes), even if fd 1 & 2 refer to the
> same pipe.

As I guess I should've expected, the behavior differs between a bash
script and a compiled program.

$ cat ./abc123
#!/bin/bash

printf '%s' 'a' >&2
printf '%s' '1'
printf '%s' 'b' >&2
printf '%s' '2'
printf '%s' 'c' >&2
printf '%s' '3'
printf '\n' >&2
printf '\n'

exit 0;
$ ./abc123
a1b2c3

$ ./abc123 2>&1 | cat
a1b2c3

$ cat ./abc123.c
#include <stdio.h>

int main()
{
  putc('a', stderr);
  putc('1', stdout);
  putc('b', stderr);
  putc('2', stdout);
  putc('c', stderr);
  putc('3', stdout);
  putc('\n', stderr);
  putc('\n', stdout);

  return 0;
}
$ gcc -o abc123.exe abc123.c
$ ./abc123.exe
a1b2c3

$ ./abc123.exe 2>&1 | cat
123
abc
$ stdbuf --output=0 --error=0 -- ./abc123.exe 2>&1 | cat
123
abc
$

I probably shouldn't go around assuming that things are smart. I'll
accept that adding logic to glibc to test if any given set of file
descriptors are pointing to the same file or pipe and ensuring that
anything written to any one of those file descriptors is always
actually written to the stream for the first one, for instance, would
probably be overkill.