Hello, this is the link to this exact question on Reddit <https://www.reddit.com/r/gcc/comments/ftm75d/gcc_modules_branch_cmi_mapping/>(in the GCC subreddit). They told me to ask to this email, so I'll try: -------------------------------------------------------------------------------------------------------- Hello, I've built the module branch of GCC from source. I got the source via: git clone --branch devel/c++-modules git://gcc.gnu.org/git/gcc.git SRCDIR All seems to work, but I cannot use the feature that was the reason why I opted for this experimental version: C++20 modules. I created a very simple project, where I have a module interface (module1 in module1.cc) and a main (in Main.cpp). I'm able to compile the module alone with the line: g++ -fmodules-ts module1.cc -c. It creates a module1.o file and a gcm.cache folder, and inside it a module1.gcm file. When I try to compile the Main.cpp with g++ -fmodules-ts Main.cpp things stop working, and I get these errors: module: error: failed to read compiled module: No such file or directory module: note: compiled module file is 'gcm.cache/module.gcm' module: note: imports must be built before being imported module: fatal error: returning to the gate for a mechanical issue Now, the errors are quite explicit, but I cannot figure out what they're trying to tell me. The first line says that there isn't any compiled module file, but the second line tells me the compiled module file is in gcm.cache/module.gcm . And the third line tells me to build modules before importing them, but this is exaclty what I've done! I read about module mappers <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1184r1.pdf>, so I tried to write a file (mapper) working as a mapper. I wrote inside only one line: *module1 gcm.cache/module1.gcm* . But now, if I compile with: g++ -fmodules-ts -fmodule-mapper=mapper Main.cpp I get: module: error: failed to read compiled module: Unknown CMI mapping module: note: imports must be built before being imported module: fatal error: returning to the gate for a mechanical issue And again this means very little to me. Last I tried to compile the module and the main at the same time with: g++ -fmodules-ts -fmodule-mapper=mapper module1.cc Main.cpp and g++ -fmodules-ts module1.cc Main.cpp but I get the exact same errors I got before (both with and without the module mapper file). I really don't know what to do, I know it is an experimental feature, but in the GCC web page they say it is a usable feature <https://gcc.gnu.org/wiki/cxx-modules>, so I'm making some mistakes I cannot identify. If you need more information I'll provide it as soon as possible! --- *Lapo Falcone *
