CMake Command Line Definitions Not Perpetuating To

Question

I have a cmake cross compiler toolchain file, abridged as:

set(CMAKE_SYSTEM_NAME Linux)
if( DEFINED TC_PATH )
    message( STATUS " TC_PATH IS defined. ${TC_PATH}" )
else()
    message( FATAL_ERROR " TC_PATH not defined." )
endif()

set(CMAKE_C_COMPILER ${TC_PATH}/usr/bin/i586-linux/i586-linux-gcc )
set(CMAKE_CXX_COMPILER ${TC_PATH}/usr/bin/i586-linux/i586-linux-g++ )
set(CMAKE_LINKER ${TC_PATH}/usr/bin/i586-linux/i586-linux-ld )

I call cmake, setting the TC_PATH as well as the toolchain file:

~/CMakeTest/output $ cmake -DTC_PATH:PATH=/opt/toolchain -DCMAKE_TOOLCHAIN_FILE=../toolchain.cmake ../

It appears cmake is invoking the toolchain file multiple times. On the first two time, the TC_PATH check succeeds, but later, after identifying the compilers, it throws an error:

--  TC_PATH IS defined. /opt/toolchain
--  TC_PATH IS defined. /opt/toolchain
-- The C compiler identification is GNU 4.9.1
-- The CXX compiler identification is GNU 4.9.1
-- Check for working C compiler: /opt/toolchain/usr/bin/i586-linux/i586-linux-gcc
CMake Error at /home/gnac/CMakeTest/toolchain.cmake:4 (message):
   TC_PATH not defined.
Call Stack (most recent call first):
  /home/gnac/CMakeTest/output/CMakeFiles/3.0.2/CMakeSystem.cmake:6 (include)
  CMakeLists.txt:2 (project)

So, outside of setting a permanent environment variable in the shell, how I can set the TC_PATH variable via the command line so that it will be remain in context while executing the cmake generate command?

Answer 1

Your toolchain needs to be self-sufficient. The step that fails is a try_compile() which is not getting your cached variables.

Your toolchain file does not look like you're cross-compiling (it does not have a CMAKE_SYSTEM_NAME) so you can do one of the following (besides setting the CC and CXX environment variables as you have mentioned):

1. It's sufficient to give the full path to your C and/or CXX compiler (depending on which languages you enabled), CMake will detect the rest of your GNU toolchain automatically

   cmake -DCMAKE_C_COMPILER:PATH=/opt/toolchain/usr/bin/i586-linux/i586-linux-gcc 
         -DCMAKE_CXX_COMPILER:PATH=/opt/toolchain/usr/bin/i586-linux/i586-linux-g++ ...
   

2. Add the following to your toolchain to skip the compiler tests (because not all options may be passed to it or because your compiler/linker of choice will not produce a valid executable)

   set(CMAKE_C_COMPILER_WORKS 1 CACHE INTERNAL "")
   set(CMAKE_CXX_COMPILER_WORKS 1 CACHE INTERNAL "")
   

   Or just use the CMakeForceCompiler macros, but the use is "Discouraged. Avoid using this module if possible."

3. Use configure_file() to put the path into your toolchain file (just make sure to do it before the project() call)

4. Prefer find_program() if the possible paths of your toolchain are known over setting it from the outside (see e.g. here)

References

• CMake FAQ: How do I use a different compiler?
• how to specify new gcc path for cmake
• CMake Cross Compiling
• cmake: problems specifying the compiler (2)
• cmake cross-compile with specific linker doesn't pass arguments to armlink
• CMake: In which Order are Files parsed (Cache, Toolchain, …)?