Re: sendmsg

[Kurt Van Dijck <kurt.van.dijck@eia.be>]

Hey Wolfgang,

On Tue, Feb 07, 2012 at 03:11:13PM +0000, Wolfgang wrote:
> Hi Kurt,
> 
> I am back working a little my gw.
> 
> As the defined in the j1939-21 the maximum dlc is 1785. 

Yes.
This stack also supports the Extended Transport Protocol as defined
in iso11783-6, which extends to several MegaBytes...

> 
> static struct {
> 	int pkt_len;
> } s = {
> 	.pkt_len = 1024,
> };
> 
> buf = malloc(s.pkt_len);
> 
> recvmsg(sockfd, &msg, 0);
> 
> Did I get that right, that a dlc more than 1024 bytes is not possible with
> recvmsg or sendmsg.
That is not a limitation of recvmsg or sendmsg. 1024 in jspy.c is just
a decent default. Via options a different (bigger) packet size can be selected.
For a complete explanation, man 2 recvmsg, look for MSG_TRUNC.
That will explain what happens when I supply a buffer to recvmsg that is smaller
than the packet I'm receiving. The received packet is then truncated.

> For my application I won't need so many data
Most application don't need such packets ....
> but for understanding the code!
Very well!
> 
> Kind regards,
> Wolfgang
> 
> 
> I forget to mention, that I set s.pkt_len = 1785 and it seemed to work with
> sendmsg and recvmsg. But UIO_MAXIOV is defined as 1024, so what do I get wrong?
> 
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