Re: What is meant by the callback function

[Guy Harris <guy () alum mit edu>]

On Sep 23, 2007, at 9:08 PM, Varuna De Silva wrote:

> On 9/23/07, Guy Harris <guy () alum mit edu> wrote:
>
> > If you're modifying libpcap to support a new type of capture, you  
> > don't
> > write the callback function,
>
> I am doing this for my device to be supported by wireshark. As I  
> understand
> I dont need to write the callback function???

You don't need to write the callback function.  Wireshark already has  
a callback function, capture_pcap_cb(), in it.

> > you write code in the read_op routine to call the callback function;
>
> Yes, I did so as following, ( I am guilty, of copying it directly from
> pcap-septel.c)
>
> int muwis_read(pcap_t *p, int cnt, pcap_handler callback, u_char  
> *user)
> {
> callback(user, &pcap_header, dp);
> :
> :
> }
>
> As I understand this need to be called for each packet captured.

Yes.

> > a pointer to the callback function is passed to the read_op routine.
>
> This is the "user" in the above code.

No.  That's the "callback" in the above code.

"user" is a "u_char *" that's passed to your routine, and that you  
pass directly through to the callback; you don't need to do anything  
with it - and, as you don't know what it is (it's something that the  
callback routine uses), you *shouldn't* do anything with it other than  
pass it to the callback routine.

> Now my question is what will
> be this "user" function in my case. That is when I call xxxx_read()
> pointer to what function will I have to pass to it as " *user "

You don't call xxxx_read() - libpcap does that, in pcap_dispatch(),  
pcap_loop(), pcap_next(), and pcap_next_ex().

Your read routine will just take the "user" argument passed to it, and  
pass it to the callback function.
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