[Boost-users] pcap_loop callback with boost::function and boost::bind

[Clemens Gruber]

Hi,

I am writing a statistical protocol analyzer running under Windows with
WinPcap.

The problem is: The pcap_loop function for starting to capture the
packets wants a C style callback function, but I need that callback
function to have access to several class variables, so this callback
function should be a member function.
I know that it is not possible to use a member function directly,
because of the additional parameter, the pointer to the class.

I thought, maybe I could use boost::function and boost::bind, but I am
not sure if my solution is correct, so it would be very nice if some of
you guys could just look over it and tell me if this is a good way to do
this.

This is my code for starting the capture:

void MyClass::startCapturing()
{
boost::function < void (u_char *, const struct pcap_pkthdr *, const
u_char *) > f;
f = boost::bind(&MyClass::packetHandler, this, _1, _2, _3);
pcap_handler p = *(f.target<pcap_handler>());

pcap_loop(hnd, 0, p, NULL);
}

This the callback function I want pcap_loop to call:

void MyClass::packetHandler(u_char *param, const struct pcap_pkthdr
*header, const u_char *pkt_data)
{
/* Do something with the packet.. */
}

And this is the signature of the callback argument in the pcap_loop
function, as defined in pcap.h:
typedef void(*) pcap_handler(u_char *param, const struct pcap_pkthdr
*pkt_header, const u_char *pkt_data)

If you have an alternative, I would be glad to hear it! Maybe there's a
much simpler solution, I didn't think about.

Best regards,
Clemens
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[Marshall Clow]

There's a simple solution that doesn't involve bind - write a "trampoline function".

I see from the WinPcap documentation that the first parameter to the callback function is "user supplied", in other words, WinPcap just holds it and passes it to the callback.

Here's a sample callback, then.
/// Warning - code written in mail client - not compiled, not tested…..

void pcap_trampoline ( u_char *param, const struct pcap_pkthdr *pkt_header, const u_char *pkt_data ) {
MyClass *me = static_cast<MyClass *> ( param );
me->packetHandler ( header, pkt_data ); // no need to pass 'param' on.
}

and you set it up like this:

MyClass capture_obj;
pcap_t *handler;
….. // set up code
pcap_loop ( handler, 0, pcap_trampoline, static_cast<u_char *> ( &capture_obj ));

Mixing pointer-to-POD and pointer-to-object is not great, but when you've got a C interface, you do what you have to.

-- Marshall