Re: [boost] boost/config/suffix.hpp: spurious BOOST_HAS_THREADS with g++ 3.4

From: Jonathan Wakely (cow_at_[hidden])
Date: 2004-05-10 06:17:57

On Mon, May 10, 2004 at 12:14:59PM +0200, Christoph Ludwig wrote:

> The problem turns out to be that gcc 3.4 defines the preprocessor
> variable _REENTRANT if certain standard library headers are included
> even if the compiler is *not* called with the '-pthread' command line
> option.

I believe this is only when GCC is configured with posix threads
(either explicitly with --enable-threads=posix, or implicitly on POSIX
systems where the default threading model is pthreads).

If you explicitly configure with --enable-threads=single (or the
equivalent --disable-threads) then _REENTRANT won't be defined, and
boost/config.hpp correctly detects that threads are disabled.
This doesn't help solve your problem, of course.

>         The following test exhibits this behavior and shows that it
> causes BOOST_HAS_THREADS to be erroneously defined in
> boost/config/suffix.hpp if, e.g., the standard header string is
> included:
[snip]
> I have no idea what the gcc developers recommend in order to detect
> multi-threading support or if this should be reported to the gcc
> developers as a bug. But it will certainly break many programs using
> Boost.

I've been meaning to get around to asking that on the gcc list.
Unfortunately gcc.gnu.org seems to be down today.

GCC provides the inline __gthread_active_p() to tell if if the program
is linked to the pthread library, but I don't know how to give the same
info to the preprocessor at compile time.

> A user can work around this problem by either including
> boost/config.hpp before any other headers (even Boost headers since
> boost/lexical_cast.hpp, for example, includes string before
> boost/config.hpp). Or the user builds everything with MT-support, even
> if the program itself is single threaded. Neither workaround is
> satisfying.

Or (as I've been doing) explicitly defining BOOST_DISABLE_THREADS when
you know your program is single threaded.

> PS: FWIW, I could trace back the problem to a change in
>     include/c++/i686-pc-linux-gnu/bits/gthr-default.h. This change was
>     discussed in
>     http://gcc.gnu.org/ml/gcc-patches/2003-07/msg01607.html.

FWIW the google cache version of that is here: http://tinyurl.com/2xksr

jon

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