How to modify gcm.cache path when using gcc-11 to build c++20 modules?

Question

I am trying to let xmake support gcc-11 to build c++20 modules, but I have some problems.

gcc-11 will generate the gcm.cache directory in the current directory by default. How can I modify this default path to the specified other directory?

I know that clang has a -fmodules-cache-path= option to modify the cache path of modules, but I did not find a similar option for gcc.

Does anyone know? thanks

• alexpanter: I have not been able to find it. There is nothing in the man-page for g++, neither is it mentioned in their modules documentation. So I assume (at least for the moment) that it is not possible. Hopefully, they will add it one day.
• user826236: Read this page: gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Module-Mapper.html You'd have to create your own "module-mapper" to make this work. I have my own custom build environment, so I'm thinking of a super simple program that reads all of your source, and creates the module-map file, and supplies it as an environment variable.

Answer 1

This worked for me gcc-11 on ubuntu 22.04 (Makefile fragment):

MAPPER_DIR := /path/to/obj/dir
CXXFLAGS += -fmodules-ts '-fmodule-mapper=|@g++-mapper-server -r'$(MAPPER_DIR)

The caveat I ran into is that g++-mapper-server won't create the root directory, so make sure its created first. You'll still have to work out dependencies so that the gcm is created/updated before the importing code is compiled.

---

The explanation is | invokes a mapper process via pipe, @ resolves the process name from the gcc tools directory (cf gcc manual). The -r option sets the root directory. Take care protecting the spaces and the | from make and shell.

The default mapper server takes arguments - you can tweak and play with it - here's the options on my version:

$ /usr/lib/gcc/x86_64-linux-gnu/11/g++-mapper-server --help
Usage: g++-mapper-server [OPTION...] [CONNECTION] [MAPPINGS...] 
C++ Module Mapper.
 -a, --accept Netmask to accept from
 -f, --fallback Use fallback for missing mappings
 -h, --help Print this help, then exit
 -n, --noisy Print progress messages
 -1, --one One connection and then exit
 -r, --root DIR Root compiled module directory
 -s, --sequential Process connections sequentially
 -v, --version Print version number, then exit
Send SIGTERM(15) to terminate
For bug reporting instructions, please see:
<file:///usr/share/doc/gcc-11/README.Bugs>.

• ruki: Thanks, I have supported it using module mapper. github.com/xmake-io/xmake/blob/…