cmake: How to debug bad flags

Question

I am currently having a bear of a time trying to compile a moderate sized library with a brand new toolchain, Assimp on Xcode6 with the new iOS 8.0 SDK.

Bundled with the project are various scripts and Xcode projects that have configurations for building on iOS, but unfortunately none of them work out of the box.

So far the farthest I have gotten is by using a build script which uses the cmake "Unix Makefiles" method to assemble static libs. Other methods would include using cmake to generate Xcode projects to use to build. I tried that also to no avail, and neither did the Xcodeproject that comes with the project in the repository (which I later learned was marked deprecated in one of the readme files).

Okay, so with this "Unix Makefiles" cmake script I have been able to generate some of the static libs (after manually forcing static lib generation inside the main CMakeLists.txt), but when it went on to build for i386 and x86_64 architectures for iPhoneSimulator it kept pulling in the headers for iOS which caused a torrent of compiler errors.

Luckily I followed a hunch and found assimp/code/CMakeFiles/assimp.dir/flags.make which is one of the cmake-generated files, and lo and behold, the entire cflags was in here, and once I removed the rogue header include path, the make call finally succeeds and I have my iPhoneSimulator static lib!

Okay so the question that I have is basically where do I get started when debugging these frustrating cmake problems. My relationship with cmake has always been a strained one because none of cmake's complexity and design principles ever made sense to me, and very infrequent are the times when cmake builds work for me out of the box... it is always something that almost works but then I have to spend hours debugging with make VERBOSE=1 and then haphazardly poking at generated files, which are of course all marked with warnings to not edit them as they are generated files.

I realize that some of the variables here are perhaps relevant to my troubles. But it isn't clear to me how I can debug these variables. Where do I go to print out these variables so that I can find which variable contains erroneous values? For example, in this most recent situation I had a -I flag that was cropping up in the wrong place. Luckily I was able to find a file that contained it using various large-hammer methods that involve grep but I am not close to actually fixing the build configuration to make the process any less painful in the future.

Answer 1

For complex CMakeLists.txt files I have found the variable_watch command can sometimes be useful (documentation here). It doesn't make it easy, but gives you another level of information.