fd_set记录

#define __FD_SETSIZE 1024
#define __NFDBITS (8 * sizeof(unsigned long))
#define __FDSET_LONGS (__FD_SETSIZE/__NFDBITS)
typedef struct {
	unsigned long fds_bits [__FDSET_LONGS];
} __kernel_fd_set;
typedef __kernel_fd_set   fd_set;

1.将对应内存全部置为0

#define FD_ZERO(fdsetp) __FD_ZERO(fdsetp)
#define __FD_ZERO(fdsetp)   (memset (fdsetp, 0, sizeof (*(fd_set *)(fdsetp))))

2.映射fd到数组的某一个位上面

#define FD_SET(fd,fdsetp) __FD_SET(fd,fdsetp)
#define __FD_SET(fd, fdsetp)   (((fd_set *)(fdsetp))->fds_bits[(fd) >> 5] |= (1<<((fd) & 31)))

3.通过句柄fd检测某一位是否已经置了值

#define FD_ISSET(fd,fdsetp) __FD_ISSET(fd,fdsetp)
#define __FD_ISSET(fd, fdsetp)   ((((fd_set *)(fdsetp))->fds_bits[(fd) >> 5] & (1<<((fd) & 31))) != 0)

4.根据fd将此前SET过的值清除掉

#define FD_CLR(fd,fdsetp) __FD_CLR(fd,fdsetp)
#define __FD_CLR(fd, fdsetp)   (((fd_set *)(fdsetp))->fds_bits[(fd) >> 5] &= ~(1<<((fd) & 31)))