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From: John Maddock (john_at_[hidden])
Date: 2008-04-11 13:30:23

Peter Dimov wrote:
> John Maddock
>>> My point however was that this is no longer true, starting from g++
>>> 4.1. _REENTRANT is now being set properly (and ignored by
>>> Boost.Config).
>> Is it?
> Yes. See the link I posted previously.
> This is Linux, _REENTRANT is not set, BOOST_HAS_THREADS is set - as
> far as I can see.

Ah, I think I see the issue here, and I'm not sure what the right thing to
do is - if libstdc++ is built thread safe and it has threading support, then
BOOST_HAS_THREADS gets turned on.

We did this for two reasons:

1) Prior to gcc-4.1.2 there's nothing in the compiler defines that let you
determine whether it's in multithread mode or not - we opened a gcc bug
report on this ages ago, and it claims to be fixed for 4.1.2:

2) In a sense gcc is either "always thread safe" or "never thread safe"
depending on how it and libstdc++ were built. For example on Linux (and
most other platforms I believe) -pthread does nothing except pass -lpthread
to the linker.

So... assuming (1) really has been fixed in gcc-4.1, what's the right thing
to do about (2)? There's a school of thought which says that if gcc was
built multithreaded then maybe all installed libraries should be as well
"just in case". The basic issue I guess is that it may be common to build
without -pthread and just add -lpthread to the linker instead?
Or to try and mix code built with and without -pthread?

Any thoughts...? John.

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