[CMake] Updated FindThreads.cmake in tracker

[Marcel Loose]

On Sun, 2009-09-20 at 19:32 -0400, Philip Lowman wrote:
> On Sun, Sep 20, 2009 at 3:02 PM, Hendrik Sattler
> <post at hendrik-sattler.de> wrote:
>         Am Sonntag 20 September 2009 17:16:19 schrieb Philip Lowman:
>         
>         > Hello,
>         >
>         > I've merged (optional) Pthreads-win32 support into a
>         FindThreads.cmake
>         > attached in the tracker and added some documentation on how
>         to use it.
>         > Since FindThreads isn't my module I wanted to throw this up
>         on the mailing
>         > list for feedback.
>         >
>         > http://www.cmake.org/Bug/view.php?id=6399
>         >
>         >
>         > Also, in regard to a previous mailing list thread about
>         FindThreads...
>         >
>         > I'm not sure which platform the block "Check if compiler
>         accepts -pthread"
>         > is executed on.  The documentation I attached to the code
>         advises calling
>         > "target_link_libraries(target ${CMAKE_THREAD_LIBS_INIT})".
>          After grokking
>         > the code a bit further I'm now guessing this "-pthread"
>         argument is
>         > technically accepted by the linker and not needed by the
>         compiler, but it
>         > would be nice to know this for sure to ensure the
>         documentation is correct.
>         
>         
>         gcc says:
>         -pthread
>                   Adds support for multithreading with the pthreads
>         library.  This
>                   option sets flags for both the preprocessor and
>         linker.
>         
>         So it may work to only use it during linking but this may
>         cause subtle failure
>         on some platforms.
>         
>         When writing FindThreads.cmake, it would be better to really
>         rewrite it and
>         use the common naming standards for cmake modules.
> 
> 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?
> 
> 
> -- 
> Philip Lowman

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.

Marcel Loose