@mainframed767 @smallsco Apple’s default/standard compiler for A/UX is K&R only, so it will compile only old/specially prepared C sources.
Then, there’s the “extra” ANSI compiler from Apple. http://archive.3rz.de/Mac/a-ux/A-UX%20Developer%20Tools%201.1%20(Apple%20ANSI%20C%20compiler).iso.gz that I archived from original CDs. It’s more capable and was part of the SDK for A/UX 3.
Or you could use gcc.




