why would a socket read() set errno to EWOULDBLOCK but still read?

[Dan Bernstein]

In article <1991Jun27.220701.21108 at athena.mit.edu> mlevin at jade.tufts.edu writes:
>    I am doing a read() on a connected TCP socket (BSD 4.3) marked as
> non-blocking. For some reason, the read returns the proper number of
> characters read (or sometimes 0), but sets errno to EWOULDBLOCK.

The Golden Errno Rule:

	He who inspects errno before a system call returns failure
	shall be buried before he is dead.

---Dan