Messages in this thread | | | From | Daniel Kabs <> | Subject | unix(7) and MSG_TRUNC semantics | Date | Thu, 11 Jan 2007 15:41:03 +0100 |
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Hello!
For IPC, I use unix domain datagram sockets. I receive messages by calling recv(). The man page recv(2) tells me about the flags argument to a recv call, namely: MSG_TRUNC Return the real length of the packet, even when it was longer than the passed buffer. Only valid for packet sockets. Thus I used recv() with flags MSG_TRUNC|MSG_PEEK in order to detect message truncation due to insufficient buffer size.
Strangely enough, MSG_TRUNC seems to get ignored by the kernel: If the message received is larger than the receive buffer I supplied, the function returns the size of the buffer. I reckon, the function should return the real message size instead.
To work around this problem, I use the ioctl FIONREAD instead.
On the other hand, in this mailing list, I found an old bug report describing the same problem using UDP sockets:
http://groups.google.com/group/fa.linux.kernel/browse_frm/thread/fb6acbb527507e26/ad0b2ba33b6b66fa
UDP sockets seem to have been patched by now. From linux/net/ipv4/udp.c: udp_recvmsg() ... err = copied; if (flags & MSG_TRUNC) err = skb->len - sizeof(struct udphdr); ...
Why doesn't unix_dgram_recvmsg() in linux/net/unix/af_unix.c contain code to this effect? Is this a feature or a bug?
Cheers Daniel Kabs - 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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