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请问FD_SET,FD_ISSET这些函数是什么意思
typedef int32_t __fd_mask;
#define _NFDBITS (sizeof(__fd_mask) * /* 8 bits per byte */
#define __howmany(x,y) (((x)+((y)-1))/(y))
#ifndef _FD_SET
# define _FD_SET
typedef struct __fd_set {
long fds_bits[__howmany(FD_SETSIZE, (sizeof(long) * )];
} fd_set;
# ifndef _KERNEL
# ifdef __cplusplus
extern "C" {
# endif /* __cplusplus */
#ifdef _INCLUDE_HPUX_SOURCE
# define FD_SET(n,p) (((__fd_mask *)((p)->;fds_bits))[(n)/_NFDBITS] |= (1 <<
((n) % _NFDBITS)))
# define FD_CLR(n,p) (((__fd_mask *)((p)->;fds_bits))[(n)/_NFDBITS] &= ~(1 <<
((n) % _NFDBITS)))
# define FD_ISSET(n,p) (((__fd_mask *)((p)->;fds_bits))[(n)/_NFDBITS] & (1 <<
((n) % _NFDBITS)))
# define FD_ZERO(p) memset((void *)(p), (int) 0, sizeof(*(p)))
#else
# define FD_SET(n,p) (__fd_set1(n, p))
# define FD_CLR(n,p) (__fd_clr(n, p))
# define FD_ISSET(n,p) (__fd_isset(n, p))
# define FD_ZERO(p) memset((void *)(p), (int) 0, sizeof(fd_set)) |
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