the codes of functions is:

 51 #undef  __FD_SET
 52 static __inline__ void __FD_SET(unsigned long __fd, __kernel_fd_set *__fdsetp)
 53 {
 54         unsigned long __tmp = __fd / __NFDBITS;
 55         unsigned long __rem = __fd % __NFDBITS;
 56         __fdsetp->fds_bits[__tmp] |= (1UL<<__rem);
 57 }

I can't understand the usage of __rem,How to understand it? thank you.

PS:

 21 #undef __NFDBITS
 22 #define __NFDBITS       (8 * sizeof(unsigned long))