Re: Libpcap - pcapfindalldevs

[Johan Mazel <johan.mazel () gmail com>]

Ok.
Thanks a lot for the explanation.
Regards.
Johan Mazel

2009/9/4 Guy Harris <guy () alum mit edu>

> On Sep 4, 2009, at 8:45 AM, Johan Mazel wrote:
>
>  I wrote a short piece of code in C to show the problem.
> > My code is the following one:
> > *#include <stdio.h>
> > #include <pcap.h>
> >
> > int main(){
> >   struct pcap_if * found_devices;
> >   int result;
> >   char * errbuf;
> >   printf("Scanning\n");
> >   result = pcap_findalldevs(&found_devices, errbuf);
>
> That's not valid.
>
> Routines such as pcap_open_live() and pcap_findalldevs() take a "char
> *errbuf" argument.
>
> That does *not* mean that you can pass it a random uninitialized "char *"
> variable to it and expect it to work; said random uninitialized variable
> could well point outside the address space of the process - it could, for
> example, have all its bits zero, and most OSes leave the first page (and
> perhaps more pages) out of the address space, so that attempts to
> dereference null pointers will probably cause a crash rather than causing
> random unpredictable behavior (most C implementations represent a null
> pointer as a pointer with all its bits zero - and, no, the C standard
> doesn't require that).
>
> Furthermore, it also doesn't mean that you need to pass it a "char *"
> variable; what you need to pass it is an expression whose type is "char *".
>
> If you did
>
>        char errbuf[PCAP_ERRBUF_SIZE];
>        printf("Scanning\n");
>        result = pcap_findalldevs(&found_devices, errbuf);
>
> that should work - in most contexts in C, a reference to an array of type T
> is converted to a value of "pointer to type T" that points to the first
> member of the array (no, pointers and arrays are not the same thing in C).
>
>  If this is a bug in Libpcap,
> >
>
> It's not - it's a bug in your program.  Change
>
>        char * errbuf;
>
> to
>
>        char errbuf[PCAP_ERRBUF_SIZE];
>
> in your program.
>
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