Will read() ever block after select()?

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Actually it should not block (that is what select() is for!), but in fact, it might, exceptionally. Normally, read() should return up to the maximum number of bytes that you’ve specified, which possibly includes zero bytes (this is actually a valid thing to happen!), but it should never block after previously having reported readiness.

Nevertheless, see the Linux select man page:

Under Linux, select() may report a
socket file descriptor as “ready for
reading”, while nevertheless a
subsequent read blocks. This could
for example happen when data has
arrived but upon examination has wrong
checksum and is discarded. There may
be other circumstances in which a file
descriptor is spuriously reported as
ready. Thus it may be safer to use
O_NONBLOCK on sockets that should not
block.

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