[CMake] Updated FindThreads.cmake in tracker

[Marcel Loose]

On Mon, 2009-09-21 at 23:25 -0400, Philip Lowman wrote:
> On Mon, Sep 21, 2009 at 6:07 AM, Marcel Loose <loose at astron.nl> wrote:
>         
>         On Sun, 2009-09-20 at 19:32 -0400, Philip Lowman wrote:
>         > On Sun, Sep 20, 2009 at 3:02 PM, Hendrik Sattler
>         >
>         > I'm hesitant to rewrite anything that works, especially if
>         it works on
>         > platforms I don't have access too. :)
>         >
>         > Include & library variables probably could be added if it
>         can be done
>         > in a safe way with the compile checks already in place.
>         >
>         > Not sure on -pthread, we've never added it to our gcc
>         command lines
>         > before.  Looking at the man page, it's only a compile flag
>         under
>         > IA-64, RS-6000, PPC, and SPARC.  Would recommending people
>         add it to
>         > their compiler flags and only rely on the output of
>         > ${CMAKE_THREAD_LIBS_INIT} for linking be the right thing to
>         do?
>         
>         
>         Hmm, don't know if the documentation is correct.
>         
>         I tested this on Linux x86_64 (gcc 4.3) and on an old i686
>         (gcc 3.2). On
>         both systems, when I diff the output of 'gcc -E -dM' and 'gcc
>         -E -dM
>         -pthread' I get '#define _REENTRANT 1'.
>         
>         So, -pthread clearly defines an extra preprocessor variable.
> 
> We've always defined _REENTRANT manually and specified -lpthread but
> looking into this further I'm guessing we're just getting lucky since
> we've never built on platforms where this doesn't work.
> 
> http://stackoverflow.com/questions/875789/gcc-do-i-need-dreentrant-with-pthreads
> 
> Also regarding the lack of a global -pthread in the docs, this post
> was kind of illuminating.
> 
> http://lists.freebsd.org/pipermail/freebsd-threads/2003-September/001202.html
> 
> So the gist of it is, if your only Unix compiler is gcc and if you add
> "-pthread" to your CMAKE_C_FLAGS/CMAKE_CXX_FLAGS you really have no
> need for FindThreads at all, as far as I can tell.
> 
> 
> -- 
> Philip Lowman

Well, yes, unless you're considering Mac OSX a Unix system as well,
which IMHO it is, nowadays. GCC gives an 'unsupported option' warning
when specifying '-pthread'. So clearly, '-pthread' is not supported on
Mac OSX.

Marcel Loose