gcc -MDD with -isystem<include_path> wont add dependencies for “system” paths

Question

So I have the following setup (simplified version):

application/app1.hpp
application/app1.cpp
application/utils/utils1.hpp
application/utils/utils1.cpp

So when I compile app1.cpp I do so like this (this is a cut-down version of the compile):

g++ -Wall -Wextra -Werror -I application -isystem application/utils -MMD -MP -MF dep.d -c application/app1.cpp -o obj.o

Where I use -MDD to auto generate dependency information. I use -isystem to inhibit warnings from files in utils folder.

Note: that utils is a sub-module (ie a seperate project that compiles on its own). Therefore I don't want compile warnings/errors from that project. Therefore I am using -isystem application/utils to include folders. When you use isystem you don't get gcc warnings - which is great :)

However I just discovered that this is also the reason I am not getting complete depenecy outputs. Files included in the isystem directories are not added as dependecies in the gcc generated dep.d file.

So it seems I can either ignore the warnings from utils but not get depenency generation for it OR I can get the dependency output but not ignore the warnings.

I really want both:

• No warnings from utils
• Dependencies from utils folder (via gcc's -MMD)

Is that possible to get both behaviours somehow?

Some ideas of mine:

• I was thinking of somehow running the dependency pre-processor part on its own first and then the compile... but I did not see a way to do that
• Force include folders in the MMD part. I found that I can include specific files with -include but not folders and I don't have a list of files :(

Answer 1

I just found something that could work, but I am not sure how efficient it is. I found the -E gcc option which replaces -c (compile) with preprocess only). So I could do:

1. Generate the dependency info:

   g++ -I application -I application/utils -E application/app1.cpp -o /dev/null -MMD -MP -MF dep.d

2. Compile the file with minimal warnings:

   g++ -Wall -Wextra -Werror -I application -isystem application/utils -c application/app1.cpp -o obj.o

This will mean that the preprocessor runs twice - not sure how much work that is... but it seems to run pretty quickly compared to the compile phase.

If there is further ideas I am still very much open to them... I won't mark this answer for a while incase someone has a better idea...