winsock与unix posix中select参数的区别

winsock:
int select (
  int nfds,
  fd_set FAR * readfds, 
  fd_set FAR * writefds, 
  fd_set FAR * exceptfds, 
  const struct timeval FAR * timeout);

nfds
[in] This parameter is ignored; it is included only for compatibility with Berkeley sockets.

unix posix:
int select(int nfds,
        fd_set *readfds,
        fd_set *writefds,
        fd_set *errorfds,
        struct timeval *timeout);

The nfds argument specifies the range of file descriptors to be tested. The select() function tests file descriptors in the range of 0 to nfds-1.

winsock此参数忽略无效，unix posix中必须有实际的意义，代表文件描述符的范围。