[Winpcap-users] Problems with Tutorial "Filtering the traffic"

[Gianluca Varenni]

----- Original Message ----- 
From: "Guy Harris" <guy at alum.mit.edu>
To: <winpcap-users at winpcap.org>
Sent: Wednesday, March 15, 2006 12:55 PM
Subject: Re: [Winpcap-users] Problems with Tutorial "Filtering the traffic"


> Gianluca Varenni wrote:
>> Uhm, the error tells me you are writing a managed c++ application (BTW, a 
>> managed C++ MFC application, quite interesting).
>>  I suggest you to read this FAQ
>>  http://www.winpcap.org/misc/faq.htm#Q-29
>
> ...
>
>> The problem is due to the fact that the standard winpcap include file
>> "pcap.h" contains only a forward declaration of "struct pcap", but not
> > the actual definition of it. As a consequence, the Managed C++
> > compiler does not emit any metadata for that type, since there's no
> > definition for it.
>>
>> There are two solutions to the problem:
>>
> >
>>     1. Include "pcap-int.h" instead of "pcap.h". This include the
> >        actual definition for the type "struct pcap"
> >
>>     2. Add a fake definition of "struct pcap". The simplest one is
> >        "struct pcap{};".
>
> Managed C++ requires extra work for opaque structures?  Sigh....

Well, managed C++ is not exactly "C++"... it has its own extensions to 
supported the managed types of the CLR (and its limits).


>
> Is there some sort of #ifdef so that pcap.h could do "struct pcap{};" in 
> Managed C++ and "struct pcap;" in C and "unmanaged" C++?  (Does C++ 
> support opaque structures in that fashion?  If so, why doesn't Managed C++ 
> do so?  "We don't care, we don't have to - we're Microsoft"?)

Well, WinPcap/libpcap was not born to be used in Managed C++, I'm not too 
much inclined at the moment in adding such an #ifdef to the main libpcap 
include file (pcap.h) for this issue. Regarding "why didn't Microsoft do 
that in MC++?" I don't know at all, but I'm pretty sure there is a reason 
for this behavior. Just to give you one: if you *don't* have the "struct 
pcap{}" declaration, you cannot declare a variable of that type (i.e. 
"struct pcap foo;" is not allowed), you can only declare pointers to that 
structure. If you add the fake declaration, you can declare a variable (i.e. 
"struct pcap foo;" is compiled), *but* the structure is completely *wrong* 
(since the size of foo is not the actual size of "struct pcap" as defined in 
pcap_int.h). I'm not trying to defend Microsoft, just observing that maybe 
that was a good (or not so good) reason for this.

Have a nice day
GV


>
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