Hi developers,
Just wanted to share an experience that happened to me while using
libwireshark from an application outside of the wireshark source tree:
So I have this application I'm working on that uses libwireshark for all
packet dissection tasks. My application also crafts packets.
For a long time I was seeing that libwireshark was telling me that
received packets had incorrect IP, TCP and UDP checksums. I couldn't
figure out why because everything looked fine after dissection (correct
field values everywhere) and the same packets dissected outside of my
application showed correct checksums.
It turns out that the problem was that libwireshark was calling the
incorrect checksumming function due to a name collision with a function
provided by my application. The function in question is in_cksum().
libwireshark's in_cksum() has the following declaration:
int in_cksum(const vec_t *vec, int veclen);
and my application's in_cksum() has:
uint16_t in_cksum(uint8_t *data, size_t len);
I am linking against libwireshark but the in_cksum() that gets used by
both my application and libwireshark is my application's in_cksum().
Coincidentally both functions take similar arguments (a pointer and a
length) so the end result was incorrect checksums instead of a crash.
This was a bit difficult to track down because there are no compiler
or link warnings, and running the application through a debugger will
tell you that in_cksum() is getting called but the return value is
misteriously incorrect.
Anyway, my problem is now resolved - I just renamed my in_cksum() to
avoid the name collision. However, I wonder if in order to prevent
this from happening to others in the future you guys would be amicable
to renaming epan's in_cksum() to something less likely to cause name
collisions, like epan_cksum() or something like that. After all
in_cksum() is a very generic and widely used name for an Internet
Protocol checksumming function (it's in kernels, books, other projects,
etc.)
Another solution would be to make libwireshark's in_cksum() a private
function, but this is not possible, is it? I mean, once you have a
function defined in a separate source code file and you call it from
other source code files the function becomes a public function, at least
in the case of an application or shared library, or not necessarily?
Cheers,
Eloy Paris.-
