[Click] process two packets

[Beyers Cronje]

Just a note on a bug in Ian's code. The two error checks in the beginning to
check if p1 or p2 is NULL, you have to kill the packet before returning. You
always MUST do one of these steps in a push path:

1) Push the packet and return
2) Kill the packet and return
3) Store the packet for later use, upon which you have to do (1) or (2)
above when processing the stored packet.

Returning push without killing or pushing the packet results in a memory
leak.

Beyers

On Tue, May 3, 2011 at 3:17 PM, Ian Rose <ianrose at eecs.harvard.edu> wrote:

> Sure.  As a dumb example, You could create an element with 2 inputs that
> swaps the timestamps across pairs of packets as they are received before
> outputting them on 2 outputs.  The code might work something like this
> (warning: written totally from memory, not compiled, sure to contain bugs):
>
> // class variables:
> Packet *p1 = NULL, *p2 = NULL;
>
> // main logic in push() method
> void MyElement::push(int port, Packet *p)
> {
>   if (port == 0) {
>     if (p1 != NULL) {
>       // ERROR: 2 packets in a row received from port 0
>       return;
>     } else {
>       p1 = p;
>     }
>   } else if (port == 1) {
>     if (p2 != NULL) {
>       // ERROR: 2 packets in a row received from port 1
>       return;
>     } else {
>       p2 = p;
>     }
>   } else {
>     // ERROR: bad port - misconfigured?
>     return;
>   }
>
>   if ((p1 != NULL) && (p2 != NULL)) {
>     // swap timestamps
>     Timestamp t = p1->timestamp_anno();
>     p1->set_timestamp_anno(p2->timestamp_anno());
>     p2->set_timestamp_anno(t);
>
>     // output both (modified) packets
>     output(0).push(p1);
>     output(1).push(p2);
>
>     // clear variables -- ready to receive another pair
>     p1 = NULL;
>     p2 = NULL;
>   }
> }
>
> Note that the code is simple and assumes that packets arrive exactly as
> pairs (input0 and input1).  So this is ok:
>
> packet on input0   \ __ a pair
> packet on input1   /
> packet on input0   \ __ a pair
> packet on input1   /
> packet on input1   \ __ a pair
> packet on input0   /
> (etc)
>
> But this fails:
>
> packet on input0   \ __ NOT a pair!
> packet on input0   /
>
>
> Hope that makes sense,
> - Ian
>
>
> On 05/03/2011 05:45 AM, Jessica Shahper wrote:
> > Hi all,
> >
> > As far I have seen, the elements in click can work on a single packet at
> a time,
> > what if I want to create an element that can compare the contents in a
> header
> > and can even mix the contents of several packets together?
> > Is it possible to design a new element that can take several packets
> through
> > different inputs and can work on them at a time?
> >
> > Regards-
> > Jessica
> > _______________________________________________
> > click mailing list
> > click at amsterdam.lcs.mit.edu
> > https://amsterdam.lcs.mit.edu/mailman/listinfo/click
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