Messages in this thread | | | Date | Sun, 25 Dec 2005 16:16:11 +0100 | From | Marc Lehmann <> | Subject | read returns EAGAIN on a _blocking_ socket, but should not? |
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Hi!
The SuS documents EAGAIN for read(2) as:
[EAGAIN] 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 http://data.plan9.de/x.20407 (client getting EAGAIN) and at http://data.plan9.de/x.20406 (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 ---==---(_)__ __ ____ __ pcg@goof.com --==---/ / _ \/ // /\ \/ / http://schmorp.de/ -=====/_/_//_/\_,_/ /_/\_\ XX11-RIPE - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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