[cesarb]

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.

[caf]

It's not even just Linux - way back in the mid 90s there was an agreement among UNIX vendors to use the "LP64" model for 64 bit architectures, and ILP32 was already the de facto standard for 32 bit UNIX-likes, so "long can hold a pointer" is somewhat of a tradition in the POSIXy world.

[dataflow]

Thanks, yeah. Any idea why they still do this instead of switching to `uintptr_t`?

[dxuh]

Afaik the kernel is built as C89 and uintptr_t was introduced in C99.

[dataflow]

Damn. I'd have they they could just typedef it manually, but interesting, thanks.

[caf]

It's not even about how the kernel is built. The userspace-side system call declarations need to be #include-able in C89 user code, so they can't use C99 types.