How do I get a verbose output for CMake? - cmake

Question

I would like to investigate why I have this error:
$ cmake ..
-- The C compiler identification is unknown
-- The CXX compiler identification is GNU 5.4.0
-- Check for working C compiler: /cygdrive/c/Users/Ycr/Home/bin/arm-none-eabi-gcc
-- Check for working C compiler: /cygdrive/c/Users/Ycr/Home/bin/arm-none-eabi-gcc -- broken
CMake Error at /usr/share/cmake-3.6.2/Modules/CMakeTestCCompiler.cmake:61 (message):
The C compiler "/cygdrive/c/Users/Ycr/Home/bin/arm-none-eabi-gcc" is not
able to compile a simple test program.
Unfortunately after the error:
I have no idea of what CMake did. I don't have a verbose log of the command it executed.
The CMakeFiles/cmTC_e4aa4.dir was cleaned after the error, so I have no possibility to explore the issue myself.
How should I investigate such an error?
I tried to use the --debug-trycompile option. This time CMake creates a CMakeTmp folder which makes perfectly without errors. However, I still have this CMakeFiles/cmTC_e4aa4.dir that generates errors and even with the option CMake unlinks the folder.

Answer 1

Getting a Verbose Log
The try_compile() calls that CMake does in the beginning to test the compiler, gives a detailed error output on the console and writes it to
[your binary output directory]/CMakeFiles/CMakeError.log
I've checked the source code again and there is no CMake option that would give more a more detailed output for CMake's internal try_compile() calls.
You could just force the output to standard output by adding some variable_watch() calls to your main CMakeLists.txt before your project() call like:
variable_watch(__CMAKE_C_COMPILER_OUTPUT)
variable_watch(__CMAKE_CXX_COMPILER_OUTPUT)
Keeping the Temporary Files
To keep the temporary file of try_compile, add --debug-trycompile to the cmake command line.
But be aware that the multiple compiler tests at the beginning overwrite the artifacts of previous ones:
It may however change the results of the try-compiles as old junk from a previous try-compile may cause a different test to either pass or fail incorrectly. This option is best used for one try-compile at a time, and only when debugging.
References
How to keep generated temporary files?
CMake error at CMakeLists.txt:30 (project): No CMAKE_C_COMPILER could be found

Answer 2

For me, none of the log files in my output directory contained useful information from try_compile(), even when using --debug-trycompile.
I ended up using the OUTPUT_VARIABLE option to capture and then print the output like this:
try_compile(<options> OUTPUT_VARIABLE TRY_COMPILE_OUTPUT)
message(WARNING ${TRY_COMPILE_OUTPUT})