hi all How to understand some codes in the function of __FD_SET?

Dave Hylands dhylands at gmail.com
Sun Nov 3 13:07:28 EST 2013


On Sat, Nov 2, 2013 at 11:35 PM, lx <lxlenovostar at gmail.com> wrote:

> hi all:
>        the codes of functions is:
>  51 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L51> #undef  __FD_SET <http://lxr.oss.org.cn/ident?v=2.6.16;i=__FD_SET>
>  52 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L52> static __inline__ void __FD_SET <http://lxr.oss.org.cn/ident?v=2.6.16;i=__FD_SET>(unsigned long __fd, __kernel_fd_set <http://lxr.oss.org.cn/ident?v=2.6.16;i=__kernel_fd_set> *__fdsetp)
>  53 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L53> {
>  54 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L54>         unsigned long __tmp = __fd / __NFDBITS <http://lxr.oss.org.cn/ident?v=2.6.16;i=__NFDBITS>;
>  55 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L55>         unsigned long __rem = __fd % __NFDBITS <http://lxr.oss.org.cn/ident?v=2.6.16;i=__NFDBITS>;
>  56 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L56>         __fdsetp->fds_bits[__tmp] |= (1UL<<__rem);
>  57 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L57> }
> I can't understand the usage of __rem,How to understand it? thank you.

fds_bits is an array of entries which are each __NFDBITS long (in your
example, each entry is 32-bits). __tmp gives you the index into the array.
__rem is the remainder of the division, and gets  you the bit number within
the 32-bit entry.

Dave Hylands
Shuswap, BC, Canada
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