make dependencies: skip vendor and package headers with gcc?

Question

I'm starting a new project and thinking of using gcc 6.3.1 -MM to generate the dependencies into a file called Make.Dep, that I'll include from Makefile.

The -M option outputs all headers, including system headers. The -MM option doesn't output system headers, but I'm still buried in literally thousands of vendor and package headers such as Sybase and Boost, which I don't think will change (and if they do I'm happy to have to do a full rebuild manually).

Obviously I could wrap gcc -MM in a perl script or what have you that knows what directories I consider packages, but is there some more widely-accepted solution?

Note that one of my vendors' headers look for specific gcc-defined pre-processor symbols to configure their portability. I'd rather not curate a set of such symbols manually to allow dependency generation with some non-gcc method (e.g., makedepend).

Answer 1

Instead of -I, use -isystem to state directories that you don't wish to be output with -MM.

This is not mentioned currently at https://gcc.gnu.org/onlinedocs/gcc/Preprocessor-Options.html despite it seeming to be very closely tied to -M and -MM options.

Example: this creates correct dependencies of foo.cpp and bar.cpp, including Sybase headers:

gcc -MM -I/opt/nmr/sap/sybaseASE/sybclient-16.0.3-7/OCS-16_0/include foo.cpp bar.cpp

Example: this does the same, but not including Sybase headers:

gcc -MM -isystem /opt/nmr/sap/sybaseASE/sybclient-16.0.3-7/OCS-16_0/include foo.cpp bar.cpp

Here is a sample Makefile implementation for gmake. The patsubst function is a pattern substitution using % as a part that matches on the "before" that is then captured and used in the "after." isystem appears to need a space after it, but this is easy to generate with patsubst as the percent sign keeps the space from being truncated. The minus option on -include tells gmake not to complain if the file named doesn't exist. This allows you to use gmake to make depend and produce Make.Dep even before there is a Make.Dep. Finally, this assumes $(PkgIncDirs) hold package include directories none of which should be changing, while $(ProjIncDirs) would be include directories inside the project that you'd want dependencies to be generated for.

depend:
    gcc -MM $(CFlags) $(Defines) $(patsubst -I%, -isystem %, $(PkgIncDirs)) $(ProjIncDirs) $(Source) >Make.Dep

-include Make.Dep