32-bit or 64-bit? Using C code

Question

Is there a way to find out if the machine is 32-bit or 64-bit by writing some code in C?

Answer 1

#include <limits.h>
/* ... */
printf("This machine's pointers to void, in a program "
       "compiled with the same options as this one and "
       "with the same compiler, use %d bits\n",
    (int)sizeof (void*) * CHAR_BIT);

Answer 2

If by "some code in C" you mean standard C code, then the answer is no, it is not possible. For a C program, the "world" it can see is created entirely by the implementation (by the compiler). The compiler can emulate absolutely any "world", completely unrelated to the underlying hardware.

For example, you can run a 32-bit compiler on a 64-bit machine and you will never be able to detect the fact that this is a 64-bit machine from your program.

The only way you can find out anything about the machine is to access some non-standard facilities, like some OS-specific API. But that goes far beyond the scope of C language.

Needless to add, the already suggested in other answers methods based on sizeof and other standard language facilities do not even come close detect the bit-ness of the machine. Instead they detect the bit-ness of the platform provided by the compiler implementation, which in general case is a totally different thing.