We're looking to use cmake on a project which has some dependencies which are
not properly picked up by cmake's dependency scanner. I'm not looking to fix
the scanner, because I feel the scanner is fundamentally broken:
In order to properly scan a file for dependencies, you need to either use the
compiler specific dependency scanner [1][2], or implement a fully featured c
preprocessor. You can't do a partial job like the current dependency scanner
does. I'd encourage you to read "Recursive Make Considered Harmful" [3] which
discusses the importance of creating correct dependency graphs (with neither
missing, nor erroneous dependencies). I'll provide a few examples of where
cmake's method is lacking:
1. #define ARCH x86
#define ASM_INCLUDE <ARCH/asm.h>
#include ASM_INCLUDE
cmake adds a dependency on ASM_INCLUDE instead of x86/asm.h
2. #ifdef DEBUG
#include <debug_config.h>
#else
#include <config.h>
#endif
cmake adds a dependency on both debug_config.h and config.h
In a worst-case scenario here (one which our project has), this could
generate circular dependencies which cause the build to fail.
3. #if 0
#include <nonexistantfile.h>
#endif
cmake adds a dependency on nonexistantfile.h when it shouldn't have
My suggestion for a solution to this problem is to use the compiler specific
dependency generation methods. This could be done cleanly by converting
dependency generation into a plugin based system with plugins for all the
supported compilers.
I would have (tried to) made these changes to cmake already, but since this is
both a non-trivial change and an architectural change, I felt it necessary to
get an ok from you before I changed anything. I'm not really willing to do the
work if it won't be accepted upstream (and I'm not willing to use cmake if this
doesn't get fixed).
Thanks for your time
Russell Harmon
[1]
http://gcc.gnu.org/onlinedocs/gcc-4.5.0/gcc/Preprocessor-Options.html#index-dependencies_002c-make-867
[2] http://msdn.microsoft.com/en-us/library/hdkef6tk.aspx
[3] http://aegis.sourceforge.net/auug97.pdf
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