Re: [boost] Re: 1.30.0->1.30.2: no more thread support for Linux?

From: John Maddock (boost.regex_at_[hidden])
Date: 2003-08-31 05:33:28

> > Threading support is on when BOOST_HAS_THREADS is defined, and off when
it's
> > not, or forced off by defining BOOST_DISABLE_THREADS, you'll find both
of
> > these mentioned in the configure generated user.hpp (and in the config
> > docs).
>
> So if I my program runs only on systems that I know support threads,
> and I want shared_ptr to use threading support, my program can guarantee
> this by defining BOOST_HAS_THREADS? No matter whether the user's Boost
> install is -
>
> 1) Simply extracted from the tarball
> 2) Configured without thread support
> 3) Configured with thread support
>
> And I don't need to define anything about the platform thread library
being
> used, like BOOST_HAS_PTHREADS?

OK lets start with Linux and gcc as a specific and special case - in this
case I think that will work - BOOST_HAS_PTHREADS will be defined anyway, but
remember that your std lib will not be thread safe unless you define
_REENTRANT, and if you do that then BOOST_HAS_THREADS will get defined
anyway by the config system (either the out-the-box version or the
configure'd one).  Of course the user could always manually configure boost
in some obscure way, or deliberately disable thread support with
BOOST_DISABLE_THREADS, but if they've done that then you should probably be
emitting a #error not trying to work around it.

In the general case though, we're back to the situation that your code will
not be thread safe unless you invoke your chosen compiler with some magic
special flag (or in the case of IBM Visual Age use a different compiler
front end altogether), and if you do that then Boost.config will detect it's
presence by whatever macros it sets (usually but not always _REENTRANT) and
turn on BOOST_HAS_THREADS.  You should always regard BOOST_HAS_THREADS as
*information*, not as something you set yourself, in 99% of cases if you
find it's not set, then it's because the compiler in it's current mode isn't
capable of producing thread safe code.

Note for example that Boost.config explicitly defines BOOST_DISABLE_THREADS
for gcc on some platforms because we know that gcc isn't capable of
producing thread safe code on those platforms yet (even though they do have
a perfectly good pthread lib).

Finally if you're configuring your program via autoconf then there are some
nice looking autoconf macros on the net, for example:
http://ac-archive.sourceforge.net/Installed_Packages/acx_pthread.html.

Sorry to make this complicated, but threads _are_ complicated....

John.