Why is the bit size different between a syscall and its wrapper?

Hi.

I think I am missing something, but
is there any particular reason to
use a different bit size between
a syscall and its userspace wrapper?



For example, for the unshare syscall,

unshare(2) says the parameter is int.


SYNOPSIS
       #define _GNU_SOURCE
       #include <sched.h>

       int unshare(int flags);




In the kernel, it is unsigned long.


SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
{
        return ksys_unshare(unshare_flags);
}




I guess the upper 32-bit will be
zeroed out in the c library when
sizeof(int) != sizeof(unsigned long)
(i.e. 64-bit system), but I'd like to know
why we do it this way.


-- 
Best Regards
Masahiro Yamada