[naner]

Is having large amounts of code in headers common in real world C code (aside from stb)? Seems like it would be a nightmare to chase down issues, though maybe compiler error messages have gotten smarter since I've last dealt with C.

[markrages]

Header files are not special in C. It is a common convention to put interface in .h and implementation in .c files. But the preprocessor expands all the #includes together into one big file before the compiler proper attempts to compile it.

Modern compilers have error messages that show you the whole chain of #includes, and on top is the filename and line number, whether it ends with ".c" or ".h" or anything else.