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

John Baldwin jhb at
Tue Apr 10 14:00:17 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: Tue, 10 Apr 2012 09:43:16 -0400

 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.
 > > One more question, do you use the same environment variable as glibc for
 > > this, or do you use a different scheme?
 > I didn't like the GNU variable names (_STDBUF_I, _STDBUF_O and
 > _STDBUF_E) so I used STDBUF_0, STDBUF_1 and STDBUF_2 instead.  But the
 > former are supported for obvious compatibility reasons.  To be honest I
 > don't really care about the names, we can use the GNU ones if you think
 > it's better to avoid doing to much strcmp(3), especially if we but the
 > code in the libc startup path.
 If the variable values have the same semantics, then I think it is best to 
 simply use the same names as glibc.
 John Baldwin

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