bin/166660: [libc] [patch] New util/shlib to change per-fd default stdio buffering mode

John Baldwin jhb at
Sat Apr 14 22:20:04 UTC 2012

The following reply was made to PR bin/166660; it has been noted by GNATS.

From: John Baldwin <jhb at>
To: Jeremie Le Hen <jeremie at>
Cc: bug-followup at
Subject: Re: bin/166660: [libc] [patch] New util/shlib to change per-fd default
 stdio buffering mode
Date: Sat, 14 Apr 2012 18:16:47 -0400

 On 4/14/12 9:11 AM, Jeremie Le Hen wrote:
 > On Tue, Apr 10, 2012 at 09:43:16AM -0400, John Baldwin wrote:
 >> On Monday, April 09, 2012 5:21:03 pm Jeremie Le Hen wrote:
 >>> Hi John,
 >>> On Mon, Apr 09, 2012 at 11:30:08AM -0400, John Baldwin wrote:
 >>>> I think it would be fine to do this in libc directly rather than via
 >>>> LD_PRELOAD.  That would let it work for static binaries as well as
 >>>> dynamic libraries.  My understanding is that this is how stdbuf works on
 >>>> Linux (glibc honors the relevant magic environment variables).  To that
 >>>> end, I think it would be ok to move this into libc directly.
 >>> I thought it would be too expensive to check for three (actually up to
 >>> six, see below) in such a critical path.  Moreover, this would have
 >>> lowered a lot my chances to see this committed simply because very few
 >>> committers would have taken the responsibility for this and the time to
 >>> handle the debates that would have sprouted.
 >>> Your point for static binaries is very valid but aren't you afraid of
 >>> the performance impact?  I'll try to spare some time this week to move
 >>> libstdbuf code into libc and do some benchmarks.
 >> Hmm, I hadn't considered the performance impact, but to be honest, this
 >> is stdio. :)  If it only happens once when stdio is first used then I think
 >> this is fine to do in libc.
 > I looked in the stdio source to see how I could implement there
 > efficiently, but the problem is that there isn't a single entry point.
 > The best I can do I think is basically something like this:
 >     int stdbuf_done = 0;
 >     void
 >     _stdbuf()
 >     {
 > 	/* libstdbuf code */
 > 	stdbuf_done = 1;
 >     }
 >     #define STDBUF()    if (!stdbuf_done) _stdbuf()
 > And scatter STDBUF() all around.  What do you think of it?
 > (FWIW, I checked how Linux implemented this, they used an additional
 > shared library.)
 Oh, ok then.  For some reason I thought it was done in the base libc
 rather than in a different shared library.
 John Baldwin

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