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Subjectread returns EAGAIN on a _blocking_ socket, but should not?

The SuS documents EAGAIN for read(2) as:

The O_NONBLOCK flag is set for the file descriptor and the thread would
be delayed.

And the linux manpage for read(2) says something similar. Yet, I do get
EAGAIN when reading from a blocking socket. Here is a short strace:

socket(PF_FILE, SOCK_STREAM, 0) = 3
connect(3, {sa_family=AF_FILE, path="/serv"}, 110) = 0
write(3, "\0\3", 2) = 2
write(3, "NEW", 3) = 3
... many writes and reads omitted
sendmsg(3, {msg_name(0)=NULL, msg_iov(1)=[{"\0", 1}], msg_controllen=20, {cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, {3}}, msg_flags=MSG_OOB|MSG_DONTROUTE}, 0) = 1
read(3, "\0\3", 2) = 2
read(3, 0x51d030, 3) = -1 EAGAIN (Resource temporarily unavailable)
write(2, "protocol error: unexpected eof f"..., 44) = 44

(the sendmsg call in the source, incidentally, specifies "0" as flags
argument, not MSG_OOB|MSG_DONTROUTE, but this is likely irrelevant).

Here is the server side for the last exchange above:

recvmsg(9, {msg_name(0)=NULL, msg_iov(1)=[{"\0", 1}], msg_controllen=24, {cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, {10}}, msg_flags=0}, 0) = 1
... ioctls on other fds omitted
write(9, "\0\3", 2) = 2
write(9, "END", 3) = 3

Now, the first backtrace creates a unix domain socket, connects to an
existing one, passes a file descriptor and then read() returns with
EAGAIN. Nowhere does it set the socket to blocking.

The full backtraces can be found as (client
getting EAGAIN) and at (server writing).

I am looking for the obvious logical error I made, but I can only
come up with the conclusion that read returning EAGAIN on a blocking
socket must be a kernel bug. It is also interesting that this code and
the server/client exchanges worked reliably _until_ I started to pass
filehandles from client to server.

I am using linux 2.6.14 on x86_64.

Thanks for your time and any insights you can give!

The choice of a
-----==- _GNU_
----==-- _ generation Marc Lehmann
---==---(_)__ __ ____ __
--==---/ / _ \/ // /\ \/ /
-=====/_/_//_/\_,_/ /_/\_\ XX11-RIPE
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