On all Linux architectures, "unsigned long" and "long" are always register-sized. For 32-bit architectures, "long" is 32 bits (the same as "int" which is always 32 bits), and for 64-bit architectures, "long" is 64 bits (the same as "long long" which is always 64 bits). This is different from Windows, in which "long" is always 32 bits, even on 64-bit architectures (and you have to use "long long" if you want 64 bits).

Therefore, Linux-only code (including the Linux kernel itself) commonly uses "long" or "unsigned long" to mean "register-sized" and "large enough to fit a pointer"; portable code normally uses "size_t" or "uintptr_t" for that.

The minimum size for unsigned long is 4 bytes