cc65 ( https://github.com/cc65/cc65 https://github.com/cc65/cc65 ) doesn't do C99 "and never will" according to https://cc65.github.io/doc/cc65.html https://cc6

cc65 ( https://github.com/cc65/cc65 https://github.com/cc65/cc65 ) doesn't do C99 "and never will" according to https://cc65.github.io/doc/cc65.html https://cc65.github.io/doc/cc65.html but it's probably the most developed C compiler for 6502-based systems.