C(sockets): 'Invalid argument' error when using MSG_OOB

elluva · 06-05-2005, 03:17 PM

When I try to receive Out-of-band data by using the MSG_OOB flag in rcv(), I always get an error saying 'Invalid argument'.
The weird thing is that I can get the data through a rcv() using 0 as flags, which can't be right since the data was sent with send() using the MSG_OOB flag.

nukkel · 06-06-2005, 01:50 PM

From man 2 getsockopts:

With protocols that support out-of-band data, the SO_OOBINLINE
option requests that out-of-band data be placed in the
normal data input queue as received; it will then be
accessible with recv or read calls without the MSG_OOB
flag. Some protocols always behave as if this option is
set.

So it's possible that on the receiving end SO_OOBINLINE is set, which means that even with send(..., MSG_OOB), the data will arrive in-band nonetheless. Maybe use an extra setsockopts() after calling socket(), just to be sure?

Just a tought

elluva · 06-08-2005, 05:17 AM

|->ok, thanks! Now what would be the option to use to negate this setting?
|
ok, forget about this, stupid questions 'man 2 setsockopt' told me:
... int setsockopt(int s, int level, int optname, const void *optval, socklen_t optlen);...
and Most socket-level options utilize an int parameter for optval. For setsockopt, the parameter should be non-zero to enable a boolean option, or zero if the option is to be disabled.

Thanks!

nukkel · 06-08-2005, 06:31 AM

To make sure oob data will be received out-of-band, do something along the lines of

Code:

int false = 0;
retval = setsockopt(sock, SOL_SOCKET, SO_OOBINLINE, &false, sizeof (int));
if (retval) exit (1);

Don't forget to include sys/types.h and sys/socket.h .

Again, I'm not sure this is what causes your problem, but it certainly has the right "symptoms"... Anyway, it won't hurt to include this code.

If this doesn't help, I'd suggest capturing the traffic using e.g. the ethereal tool, to make sure if the data is indeed being sent as OOB.

Happy programming!

nukkel · 06-08-2005, 06:34 AM

OK, I saw your edit too late ...

elluva · 06-08-2005, 08:15 AM

no probs, ethereal is a great idea... how didn't I think of this before?!

elluva · 06-08-2005, 09:16 AM

ok, I set the option SO_OOBINLINE to false, but it still come through otherwise. I am using TCP, TCP should support this shouldn't it? Ethereal marks them as urgent, so it really should work if everything is set right at the receiver side...