How to tell if Address Sanitizer is active?

There's no macro, AFAIK. I usually add one as a part of the build type.

However, if it's just so that you can prevent a function being instrumented, you can tag them with __attribute__((no_sanitize_address)). E.g:

__attribute__((no_sanitize_address))
void myfunc()
{
   // Sketchy or hot-path code.
}

You might be looking for __has_feature(address_sanitizer):

http://clang.llvm.org/docs/AddressSanitizer.html#conditional-compilation-with-has-feature-address-sanitizer

There is __has_feature(address_sanitizer), an example how to use it is at http://clang.llvm.org/docs/AddressSanitizer.html :

#if defined(__has_feature)
# if __has_feature(address_sanitizer)
// code that builds only under AddressSanitizer
# endif
#endif

Perfect. Thank you.