Can't make SO_RCVLOWAT socket option working under LINUX?

jf.argentino · 09-03-2009, 08:14 AM

Hello,

I want to unlock the "recv" call on an UDP socket when at least 32kbytes of data are available, to do so I make:

Code:

int arg = 32768;
setsockopt (socket_id, SOL_SOCKET, SO_RCVLOWAT, &arg, sizeof (arg));

but, if I display the return value of

Code:

char buffer[65536];
bytes = recv (socket_id, buffer, 65536, MSG_WAITALL);
printf ("%d bytes received\n", bytes);

I can see that I'm always getting out of "recv" with 1476 bytes available, which is one datagram size (datagrams aren't fragmented and I can't increase their size on the sending side)!
How can I get several datagrams with only one "recv" call, if it's not with the SO_RCVLOWAT socket option? Any idea?

remark: my linux kernel version is 2.6.26.8-57.fc8

Thank you for reading.

orgcandman · 09-03-2009, 09:00 AM

Quote:

Originally Posted by jf.argentino

Hello,

I want to unlock the "recv" call on an UDP socket when at least 32kbytes of data are available, to do so I make:

Code:

int arg = 32768;
setsockopt (socket_id, SOL_SOCKET, SO_RCVLOWAT, &arg, sizeof (arg));

but, if I display the return value of

Code:

char buffer[65536];
bytes = recv (socket_id, buffer, 65536, MSG_WAITALL);
printf ("%d bytes received\n", bytes);

I don't think these options mean what you think they mean. MSG_WAITALL is certainly only useful on a stream socket, where datagram boundaries are not the same as message boundaries.

Quote:

I can see that I'm always getting out of "recv" with 1476 bytes available, which is one datagram size (datagrams aren't fragmented and I can't increase their size on the sending side)!

How can I get several datagrams with only one "recv" call, if it's not with the SO_RCVLOWAT socket option? Any idea?

In short: you can't. In datagram sockets, a packet is a message. You could possibly ip fragment, but that usually never works the way you want it to work in a live net.

You might look into packet_mmap. It's not exactly multiple datagrams in one recv, but I'm guessing that you're trying to avoid lots of context switching. packet_mmap is a shared buffer between user and kernel space for receiving packets which has no context copying, and minimal structure overhead.

jf.argentino · 09-03-2009, 09:51 AM

Quote:

You might look into packet_mmap. It's not exactly multiple datagrams in one recv, but I'm guessing that you're trying to avoid lots of context switching.

Yes my purpose is to avoid context switching, i'll take a look at packet_mmap asap. Thank you