[CMake] CMAKE_C_FLAGS and CMAKE_C_COMPILER cross compilation woe

[Andreas Mohr]

Hi,

On Tue, May 14, 2013 at 02:38:17PM -0400, cmake-request at cmake.org wrote:
> Date: Tue, 14 May 2013 10:02:56 -0700
> From: Thomas Taranowski <tom at baringforge.com>

> I am running into a situation when cross compiling where I cannot get
> cmake to emit the write CFLAGS for my cross-compilation toolchain
> correctly.
> 
> If I force the C compiler, via the CMakeForceCompiler mechanism, the
> CMAKE_C_FLAGS do not get passed to gcc during the build.
> 
> Example make VERBOSE=true snapshot.  Note there are no CFLAGS, only
> include paths:
> /usr/local/msp430/bin/msp430-gcc
> -I/home/tom/src/t3gtb.1/t3_i2c_mm/public
> -I/home/tom/src/t3gtb.1/t3_i2c_mm/src/msp430
> -I/home/tom/src/t3gtb.1/t3_i2c_mm/src    -o
> CMakeFiles/t3_i2c_mm_support.dir/src/msp430/t3g_bsp.c.obj   -c
> /home/tom/src/t3gtb.1/t3_i2c_mm/src/msp430/t3g_bsp.c
> 
> 
> Now, I can eliminate the need to force by passing in a -mmcu option
> (required for this processor to find the correct link map), which
> allows the TRY_COMPILE process to pass.  So I set:
> 
> set(CMAKE_C_COMPILER ${CROSS_COMPILE}gcc ${CMAKE_C_FLAGS})
> 
> This works to allow cmake to do it's thing and figure out what it
> needs to about the toolchain.  However, it seems as though the cflags
> is now fixed, because the include_directories directive has absolutely
> no effect.  It's as if cmake thinks that if I pass in some additional
> flags to the CMAKE_C_COMPILER, that it can no longer append anything
> to cflags.

This is the usual CMAKE_<lang>_FOO per-LANGUAGE mechanism
(for pre-defined "languages" and custom-setup ones), e.g. CMAKE_C_COMPILER,
..._LINKER etc. variables.


I believe you're missing the usual (and usually existing!) *extension* parts
within these expressions, which would be the <...> parts (<FLAGS> etc.).

For details, see all the Platform setup modules within CMake's
pre-installed modules, i.e. INSTALL_PREFIX/share/cmake-VER/Modules/Platform/ ,
since *this* is where the game gets played, i.e. where all the magic
happens, thus that is the authoritative place to start investigating about
custom platform setup questions.

HTH,

Andreas Mohr