使用select接口写高精确延时。
intselect(intmaxfdp,fd_set*readset,fd_set*writeset,fd_set*exceptset,structtimeval*timeout);
void usleep(unsigned long usec)
{
struct timeval tv;
tv.tv_sec = usec / 1000000;
tv.tv_usec = usec % 1000000;
int err;
do {
err = select(0, NULL, NULL, NULL, &tv);
} while(err < 0 && errno == EINTR);
}