linux-kernel - Does Linux select() violate POSIX?

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Message-ID: <BANLkTik9HzR_oJ7FocQ=o+zNoK22tUTjBg@mail.gmail.com>
Date:	Sat, 18 Jun 2011 10:06:18 -0700
From:	Nemo Publius <nemo@...f-evident.org>
To:	linux-kernel@...r.kernel.org
Subject: Does Linux select() violate POSIX?

Suppose I  have a file descriptor referencing a TCP/IP socket in blocking mode.

Suppose select() reports that the descriptor is ready for reading.

If I then call recv() on that descriptor, can it _ever_ block?

According to the Linux select man page
(http://linux.die.net/man/2/select), the answer is yes:

"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."

According to the POSIX specification for select
(http://pubs.opengroup.org/onlinepubs/9699919799/functions/select.html),
the answer is no:

"A descriptor shall be considered ready for reading when a call to an
input function with O_NONBLOCK clear would not block, whether or not
the function would transfer data successfully. (The function might
return data, an end-of-file indication, or an error other than one
indicating that it is blocked, and in each of these cases the
descriptor shall be considered ready for reading.)"

There are only three possibilities:

1) I am mis-reading the POSIX spec.
2) The Linux select() man page is wrong.
3) Linux select() violates the POSIX spec.

So, which is it?

Thanks!
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