Is there a way to substitute for header files in the same directory as a source file?

[RobertCurtin-eaton]

First of all, I acknowledge that this is a bit of an XY problem. I'm open to alternative ideas.

Our goal

We have a file structure looking something like this:

src/file1.c
src/file1.h
src/file2.c
src/file2.h

We want to write a test suite for file1.c that includes a mocked or faked version of file2.h while still compiling file1.c in place.

By default, header files in the current directory have priority over anything else included, so this initially seems impossible using gcc.

Constraints

• This approach will be used in a number of projects that, for regulatory reasons, need to be changed as little as possible. This means we can't use solutions such as moving the header file to another directory or rewriting code to use dependency injection.

Context

What have we tried?

Using -I- has a stated side effect:

-I- inhibits the use of the directory of the current file directory as the first search directory for #include "file".

This works… for CMAKE_CXX_STANDARD 11 and 14. Unfortunately, we have teams using later versions and would like to stay up to date.

Later versions (17+) throw an error:

cc1plus: note: obsolete option ‘-I-’ used, please use ‘-iquote’ instead
...
/usr/include/c++/11/pstl/glue_memory_defs.h:13:10: fatal error: execution_defs.h: No such file or directory
   13 | #include "execution_defs.h"
      |          ^~~~~~~~~~~~~~~~~~

I'm not 100% sure, but it seems to me like glue_memory_defs.h is expecting execution_defs.h to be visible because it's in the same directory. I assume -I- is blocking this from happening. I wasn't able to find anything in the gcc list of new language features that indicated why this would have changed between versions, though.

Alternative solutions

We have an approach that involves copying files to a temporary directory, but this has the side effect of debugging / warnings / errors all pointing to the temporary files rather than the actual files, which is annoying for developers.

Don't you need the actual file2.h when you test file2.c?

Currently we're generating each test suite as an individual executable. This allows us to compile the actual file2.c in when it's being tested, but use a mock version when we're testing file1.c. If there's another approach that can get us to this point without modifying source code, we're open to it.