[CMake] CMAKE_C_FLAGS and CMAKE_C_COMPILER cross compilation woe

[Thomas Taranowski]

Thanks for the response Andreas.  For those poor souls who come after,
the solution is to do the following:

set(CMAKE_C_FLAGS "-mmcu=msp430fr5739 -D__MSP430FR5739__
-ffunction-sections -fdata-sections -DGCC_MSP430 -Wall -g -std=c99"
CACHE STRING "" FORCE)

Using the FORCE attribute forces the specified cflags to take effect.

Thomas Taranowski
Certified netburner consultant
baringforge.com


On Tue, May 14, 2013 at 11:55 AM, Andreas Mohr <andi at lisas.de> wrote:
> 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
> --
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