deciding threading support at compile time

[Christoph Ludwig <cludwig at cdc dot informatik dot tu-darmstadt dot de>]

Hi,

with gcc 3.4.0, how can I decide at compile time (e.g., by a
preprocessor constant) whether gcc was called with the '-pthread'
command line option? 

Background: My compiler is configured as follows:

  cludwig@lap200:~/C++/gcc3.4/LaRed2> LANG=C ; g++ -v
  Reading specs from /opt/gcc/gcc-3.4.0/lib/gcc/i686-pc-linux-gnu/3.4.0/specs
  Configured with: ../gcc-3.4.0/configure --prefix=/opt/gcc/gcc-3.4.0
  --enable-threads=posix --enable-version-specific-runtime-libs
  --enable-languages=c,c++ --enable-__cxa_atexit --enable-c-mbchar
  --enable-concept-checks --enable-libstdcxx-debug --enable-c99
  --enable-libstdcxx-pch
  Thread model: posix
  gcc version 3.4.0

When testing gcc 3.4.0 with a program using Boost libraries, I found
that sometimes the preprocessor variable BOOST_HAS_THREADS is
spuriously set if, e.g., the C++ standard string header is included.
This makes the boost::smart_ptr library refer to pthread symbols. In
other translation units that do not include the C++ string header
BOOST_HAS_THREADS is not set whence boost::smart_ptr refers to a dummy
mutex. This is not only an obvious violation of the ODR, but also
causes linker errors. (I gave more details in my posting to the Boost
mailing
list. http://thread.gmane.org/gmane.comp.lib.boost.devel/102817) 

The string header eventually includes the header
c++/i686-pc-linux-gnu/bits/gthr-default.h. Since gcc 3.4, this header
unconditionally defines the preprocessor variable _REENTRANT. (A
rational for this change is given in
http://gcc.gnu.org/ml/gcc-patches/2003-07/msg01607.html.)
But the Boost configuration headers define BOOST_HAS_THREADS if
_REENTRANT is defined.

Hence my questions:
 - Is it correct that _REENTRANT is defined under linux even if the
   compiler was called without '-pthread'? 

 - How can I decide at compile time whether the command line option
   '-pthread' was present? Or whether any other MT-support is active?
   (I was told about __gthread_active_p(), but that does not help at
   *compile* time.)

 - Where can I find comprehensive documentation of the effects of the
   '-pthread' option under Linux on x86? The gcc 3.4.0 manual mentions
   this option as an IBM RS/6000 and PowerPC submodel option
   only. (E.g., does the compiler generate different code for the
   initialization of local static variables if '-pthread' is in
   effect?)

Thanks

Christoph


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