Is adding additional source directory possible?

[maxgerhardt]

@godofplatform

What is the -L for, not for CPP files but only actual libraries?

Yes, libraries in the sense of a precompiled libsomething.a file. Not meant for a source file.

@godofplatform

If so, is there any way to add a directory for common CPP files that is outside of the “src” directory?

They must be in src/ folder or in a library for PlatformIO to trigger its compilation. You can create a folder in which you create a library folder in which you put the common C/C++ files. Two projects can reference this extra library folder via lib_extra_dirs without duplication. This has been asked before, e.g. at Sharing source files between 2 pio projects.