bin/166660: [libc] [patch] New util/shlib to change per-fd
default stdio buffering mode
jhb at FreeBSD.org
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 FreeBSD.org>
To: Jeremie Le Hen <jeremie at le-hen.org>
Cc: bug-followup at freebsd.org
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;
> /* 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.
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