This is a simple and lightweight ringbuffer implementation I did for a hiring test. It was done with GCC on Linux/x86_64, and was tested with several other (embedded & similar) compilers like CC65 and VBCC.
To build it on Linux, use:
make
Then ./rd_test launches the simple test code.
To build the Commodore 64 binary of the test code (for fun), use:
make -f Makefile.C64
Make sure you have CC65 installed and in the path. Copy the resulting rb_test.c64 file to a medium your C64 can read using your favorite method, and LOAD/RUN. :)
If you don't have a C64 or emulator at hand, observe the C64 version in it's full glory here.