(In iOS/OS X SDK's) Where are __i386__ and __x86_64__ defined?

Question

In iOS and OS X SDK's, where are those architecture values defined?

The reason I am interested is because I used to conditionally compile code for the iOS Simulator as follows:

#ifdef __i386__
   // Simulator-only code
#endif

And just noticed that I haven't updated it for the new 64-bit iOS Simulator by also checking for __x86_64__, and I'm also curious about other defines as there doesn't seem to be an official Apple documentation file for this.

Answer 1

Those values are defined by CLANG at compilation time depending on the target you happen to be building for, but were formalized by the GNU C spec for use in GCC.