[cmake-developers] [CMake 0015223]: Object OUTPUT_EXTENSIONs selection is hard - if ever possible - to manage with cross compilation

[Mantis Bug Tracker]

The following issue has been SUBMITTED. 
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http://public.kitware.com/Bug/view.php?id=15223 
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Reported By:                Emmanuel Blot
Assigned To:                
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Project:                    CMake
Issue ID:                   15223
Category:                   CMake
Reproducibility:            always
Severity:                   major
Priority:                   normal
Status:                     new
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Date Submitted:             2014-10-28 17:07 CET
Last Modified:              2014-10-28 17:07 CET
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Summary:                    Object OUTPUT_EXTENSIONs selection is hard - if ever
possible - to manage with cross compilation
Description: 
For some (historical?) reasons, CMake considers the default object extension tas
".obj", which is quite unusual but on Windows.

CMake "Modules" files define:
IF (UNIX)
 set(CMAKE_<LANG>_OUTPUT_EXTENSION .o)
ELSE ()
 set(CMAKE_<LANG>_OUTPUT_EXTENSION .obj)
ENDIF ()

which makes the Unix style the exception, and Windows style the default rule.

However, when cmake is used to cross compile, many (if not most) OSes use the
Unix style, even if they are not Unix-based. 

The trouble is that UNIX variable seems to be mostly defined from the host
environment, not from the target environment.

The net result is that ".obj" extension is enforced for non-Windows targets,
which in turn rapidly becomes a nightmare to manage.

Forcing UNIX to 1 from a CMakeLists.txt sometimes helps for C source files -
although it is definitely a hack - but does not with ASM files for example.

Setting CMAKE_<LANG>_OUTPUT_EXTENSION from a CMakeLists.txt does not seem to be
recommended - from previous CMake ML posts - and does not work anyway.

Although I would personally favour .o to be the default extension and Windows
the exception, I guess that for at least compatibility with previous CMake
version this cannot be changed.

However, being able to easily define the extension for a non Unix project, or
for a cross compiled one is definitely a real need.



Additional Information: 
I've failed - up to know - to find a way to choose the proper extension, which
forced me to write ugly workaround in CMakeLists.txt files.
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Issue History 
Date Modified    Username       Field                    Change               
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2014-10-28 17:07 Emmanuel Blot  New Issue                                    
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