Why stderr should be unbuffered

[Steve Summit]

In article <1990Dec6.050817.9741 at athena.mit.edu>, I wrote:
>In article <WNW7ZR7 at xds13.ferranti.com> peter at ficc.ferranti.com (Peter da Silva) writes:
>>In every case where I've got the source to a program and have figured out
>>where something like that came from it's been one of the two following
>>cases:
>>	if(!(fp = fopen(...))) {
>>		fprintf(stderr, "%s: ", argv[0]);
>>		perror(...);
>>	}
>>(where the fprintf stomped on errno when deciding what buffering to do) or:
>[other case deleted]
>
>This is a highly unfortunate case.  I believe that stderr should
>never be buffered; I'll discuss the tradeoffs in another article.

Once upon a time, errno was always unbuffered, and stdout was
always buffered.  Enough new users, who didn't know about (or
were too lazy to use) fflush, complained about missing output
that line buffering was invented, and enabled if stdout wasatty.
This may already have been a step in the wrong direction; line
buffering merely coddles people into thinking they don't need to
worry about fflush, and indeed code _usually_ works without it,
yet the implicit buffering decisions are occasionally incorrect
(e.g. when stdout is a pipe), so fflush is still a good idea, but
people are even less likely to use it.

Then someone had the brilliant idea to start buffering stderr.
(Presumably it was important to speed the delivery of voluminous
error messages.)  This solved a non-problem at considerable risk:
a user may now miss an important error message (or have it
delayed) because of an inappropriate stderr buffering decision.
(To be sure, ANSI X3.159 says that stderr must be at most line
buffered, so messages may be delayed only if they do not end in
newlines.)

If inefficient voluminous stderr output was truly a problem, a
better solution would have been to fix the offending voluminous
error message generators.  A program which can be expected to
generate voluminous output to stderr on most runs (examples I
know of are rcs, tar -v, cc, and lint) should be rewritten to
place those messages on stdout.  (It might seem inappropriate to
place error messages on stdout, since the whole point of stderr
is to keep error messages from disappearing down a stdout
pipeline, but note that each of the programs I mentioned
generates no output to stdout except in exceptional cases.  For
cc and lint, it can be argued that the error messages are simple
output, and _should_ be placed on stdout, to make piping to
filters or line printers easier, especially for unfortunate csh
users.  Indeed, many cc's probably do this already, and I'm
fairly sure that lint does.)

Since there may always be programs generating voluminous output
to stderr, and since even moderate amounts of unbuffered output
can be debilitating on heavily-loaded timesharing systems with
expensive system calls, it may still be desired to reduce
character-at-a-time error output.  This can be done
transparently, without buffering stderr and without requiring any
effort by programmers to ensure correct output, with only a
little work on the stdio implementor's part.

A quick-and-dirty solution is to special-case stderr inside, say,
fprintf, and temporarily buffer it.  This causes (more) reentrancy
problems, and required kludgey special handling for longjmp in
one system under which I saw it implemented.

A better solution is to sprinkle checks throughout the stdio code
wherever multiple characters might be available at once, and for
which putc's which translate to character-at-a-time write(2)'s
would be pointlessly, unnecessarily expensive.  I know of six
such places: puts, fputs, fwrite, and three places within the
printf common code (non-% format text, strings pulled in with %s,
and all other formatted strings, which are typically constructed
in a common buffer and printed by common code).

To be sure, special cases sprinkled throughout code (not even
nicely centralized) are generally undesirable, but stdio is
widely-used and important enough that uglifications for
performance reasons (can the eternal efficiency hack denouncer
really be saying this? :-) ) are justifiable.  I implemented such
speedups for unbuffered I/O in my own stdio package and it was
even less work than I thought it would be.

                                                 Steve Summit
                                                 scs at adam.mit.edu