Detecting 64bit compile in C

Question

is there a C macro or some kind of way that i can check if my c program was compiled as 64bit or 32bit at compile time in C?

Compiler: GCC Operating systems that i n

Answer 1

The question is ambiguous because it doesn't specify whether the requirement is for 64-bit pointers or 64-bit native integer arithmetic, or both.

Some other answers have indicated how to detect 64-bit pointers. Even though the question literally stipulates "compiled as", note this does not guarantee a 64-bit address space is available.

For many systems, detecting 64-bit pointers is equivalent to detecting that 64-bit arithmetic is not emulated, but that is not guaranteed for all potential scenarios. For example, although Emscripten emulates memory using Javascript arrays which have a maximum size of 232-1, to provide compatibility for compiling C/C++ code targeting 64-bit, I believe Emscripten is agnostic about the limits (although I haven't tested this). Whereas, regardless of the limits stated by the compiler, Emscripten always uses 32-bit arithmetic. So it appears that Emscripten would take LLVM byte code that targeted 64-bit int and 64-bit pointers and emulate them to the best of Javascript's ability.

I had originally proposed detecting 64-bit "native" integers as follows, but as Patrick Schlüter pointed out, this only detects the rare case of ILP64:

#include 
#if UINT_MAX >= 0xffffffffffffffff
// 64-bit "native" integers
#endif

So the correct answer is that generally you shouldn't be making any assumptions about the address space or arithmetic efficiency of the nebulous "64-bit" classification based on the values of the limits the compiler reports. Your compiler may support non-portable preprocessor flags for a specific data model or microprocessor architecture, but given the question targets GCC and per the Emscripten scenario (wherein Clang emulates GCC) even these might be misleading (although I haven't tested it).

Generally speaking none of these scenarios can be relied upon to give any reliable indication of whether a 64-bit address space and non-emulated 64-bit arithmetic is available, thus they are basically useless (w.r.t. to said attributes) except in the context of a build system that is not agnostic. Thus for said attributes, it is preferred to set build macros that so the build system can select which variant is compiled.