Re: [Boost-build] Liking to pthreads (Was: CVS: boost/tools/build/v2/tools python.jam, 1.3 0, 1.31)

From: Vladimir Prus (ghost_at_[hidden])
Date: 2006-09-18 04:12:49

On Thursday 07 September 2006 09:28, Vladimir Prus wrote:

> > > In what way is errno different?
> >
> > James wasn't allowed to post to this list, and I quite impolitely
> > forwarded his comments here, for which I have apologized.  But he has
> > a point.  Suffice it to say that he convinced me <threading>multi is
> > required in general, even if on many systems it can probably be left
> > out for many programs.  Basically -mt has to be assumed to change the
> > ABI used by the standard library.
>
> The errno point is interesting.
>
> > So I think <threading>multi should be the default way to link with
> > pthreads, and the user should have to supply an explicit override if
> > she wants to live dangerously.
>
> I'll check with an expert, and get back ;-)

Finally done. I've talked with Daniel Jacobowitz who's close enough to libc 
and binutils and gcc to just know things we tried to guess. I was told that:

1. For glibc and libstdc++, -pthread does not change any interface in a 
link-incompatible way. On errno, specifically:

> errno always expands to (*__errno_location ()).  Up until 2.2 or so,
> you could also say "int errno" and get the main thread's errno, without
> including <errno.h> - but it was a bug to do that.

2. -pthread basically just sets _REENTRANT and add pthread to linker command 
line. It does not cause any link-incompatibile changes itself.

3. Any library can make link-incompatible changes in its interface when 
_REENTRANT is defined, but that's up to each individual library.

So, assuming we don't know about specific problems with Python, we should 
assume it's safe to link MT libpython into single-threaded library.

I hope that's sufficient.

- Volodya

-- 
Vladimir Prus
http://vladimir_prus.blogspot.com
Boost.Build V2: http://boost.org/boost-build2