[CMake] No <LANG>_FLAGS_<CONFIG> target property.

[Michael Hertling]

On 03/14/2011 11:55 PM, Óscar Fuentes wrote:
> Hendrik Sattler <post at hendrik-sattler.de>
> writes:
> 
>>> "The COMPILE_FLAGS property sets additional compiler flags used to build
>>> sources within the target."
>>>
>>> So it *adds* flags. I want to *set* flags, i.e. replace the existing
>>> ones.
>>
>> So you add global compiler flags which are not meant to be global? A usual 
>> person would say: "then do not set them globally!".
>> It's really that simple...
> 
> No, I want to replace those automatically setted by CMake.

AFAIK, CMake doesn't set compilation flags by itself, perhaps unless a
specific toolchain/platform file enforces this. Rather, the flags are
roughly set up as follows - for C compilations:

1) CMAKE_C_FLAGS for all configurations; explicitly set or initialized
   by the CFLAGS environment variable when CMake is run initially.
2) Addition of CMAKE_C_FLAGS_<CONFIG> for the configuration.
3) Addition of COMPILE_FLAGS source/target properties.

If you set neither CFLAGS/CMAKE_C_FLAGS nor CMAKE_C_FLAGS_<CONFIG> nor
COMPILE_FLAGS, your source files should be compiled without any flags,
so the art is to specify the latters such that each source file will
be compiled with the correct/desired set of them.

However, you're right insofar that there's no possibility to specify
flags which will be used solely for a certain target *and* a certain
configuration, but in my experience, this is almost never necessary.
Do you have a special problem in this regard, i.e. a flag that must
be activated for one target in one configuration only?

Furthermore, the user is free to choose the compiler for the project,
and the compilation flags are - basically, at least - specific to the
compiler, so one should be particularly conservative w.r.t. hardcoding
them in CMakeLists.txt files, IMO. Besides, the compilation flags are
also mentioned in the linker command line which is required sometimes
[1]; not doing so might result in subtle failures. The CMAKE_C_FLAGS
and CMAKE_C_FLAGS_<CONFIG> are passed to the linker, too, but the
COMPILE_FLAGS aren't, AFAICS.

Finally, if you really need to drop a flag from a compilation command
and can't avoid its addition by other means, you could use one of the
RULE_LAUNCH_COMPILE properties to intercept the compiler command and
adjust the command line to whatever is necessary [2]. Regrettably,
this is restricted to the Makefile generators. Alternatively, you
might modify the CMAKE_C_COMPILE_OBJECT rule variable within the
affected directories, especially w.r.t. the <FLAGS> placeholder,
see [3].

Regards,

Michael

[1] http://www.mail-archive.com/cmake@cmake.org/msg30015.html
[2] http://www.mail-archive.com/cmake@cmake.org/msg35215.html
[3] http://www.mail-archive.com/cmake@cmake.org/msg35282.html