C++20 module mapper

[Alexander Christensen via Gcc-help <gcc-help@xxxxxxxxxxx>]

Hi, I hope I write to the right place.

I would like to know how to use the module mapper, documented here https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Module-Mapper.html,
to change the location of the expected gcm.cache directory. Given an example project layout:

src/
    Makefile
    project_1/
        // .cpp source files
        Makefile
    project_2/
        // .cpp source files
        Makefile

Compiling from inside project_1 and project_2 folders does not work with the default module mapper, as gcc expects the module cache
to be located in current working directory, which is a problem if project_2 references modules built in project_1.
However, I can specify a module-mapping file. But I can't get it to work.

What would be the correct syntax for such a file, located in project_2, to override the default module cache to read from modules in
src/gcm.cache/ and NOT src/project_2/gcm.cache/ ? This does not work:

// module_mapping.txt
'$root' ../gcm.cache/

And I do not want to prepend "cd .. && " to all build commands in src/project_2/Makefile.


Sincerely,
Alexander