Merging some convenient (but old) code I found laying around into a new project, I came across this build error:
Undefined symbols for architecture i386: "_GetCurrentKeyModifiers", referenced from: -[Class mouseDown:] in Class.o ld: symbol(s) not found for architecture i386 clang: error: linker command failed with exit code 1 (use -v to see invocation)
Scrolling through the source for this class, I found calls to GetCurrentKeyModifiers() scattered about. This function is from Carbon, the AppKit way to get the same thing is:
[[NSApp currentEvent] modifierFlags]
Swapping out GetCurrentKeyModifiers() calls for this cleared those build errors right up.