select() 如何返回除 -1、0 或 1 之外的任何值？

Question

This is definitely just a dumb misunderstanding on my part, but the man page for select() states:

The timeout argument specifies the interval that select() should block waiting for a file descriptor to become ready. The call will block until either:

*a file descriptor becomes ready;

*the call is interrupted by a signal handler; or

*the timeout expires.

And furthermore, that

On success, select() and pselect() return the number of file descriptors contained in the three returned descriptor sets (that is, the total number of bits that are set in readfds, writefds, exceptfds) which may be zero if the timeout expires before anything interesting happens. On error, -1 is returned, and errno is set to indicate the error; the file descriptor sets are unmodified, and timeout becomes undefined.

So my question is -- if it stops blocking as soon as a file descriptor is ready, would it not immediately return 1? And if no fds become ready, it returns 0, otherwise error and returns -1.

Obviously in practice it returns more than 1: the whole point is that you should be able to read/write multiple fds, right?

Answer 1

由于现代抢占式多任务的工作方式，多个描述符可能会在您的进程被唤醒之前准备就绪，并且select调用对描述符进行计数。