Need some C++ help: Linker can't find a function in my source

March 13, 2005 @ 8:55 pm

I feel like a freaking idiot today as I’m working on some modifications to the mod-isapi Apache module for Web Connection. After screwing around with trying to find a solution to the Basic Authentication problem I’ve finally thrown in the towel and decided I’m going to have to fix this the hard way right at the ISAPI module level.

Apache apparently doesn’t support automatic Authentication for anything but file based security - it doesn't automatically validate auth requests made from code. And to make things worse throws out the Authentication in the case of Basic Authentication (this is actually hardcoded into server with a #Define switch - talk about heavy handed behavior!)

I’ve tried to work around this in some way by using script mappings to force authentication but Apache behaves terribly unpredictably when dealing with script routings. So the only way to get this to work is to hack the isapi module itself.

Ok, I figured out how to get the headers (thanks to Christof Wollenhaupt from Prolib) and re-write the server variables as needed. My thought here is basically to read the incoming header check if it’s basic auth and if the user is not already validated by Apache try to authenticate the user using Windows Authentication. The end result here would be the same behavior that you see in IIS ISAPI extensions. The code for this is not that tough and I got everything to compile just fine at the moment.

Unfortunately I needed to bring in some external code for Base64 decoding the Auth headers and I can not for the life of me to get the project to link now. I’m not a C++ guy as it is and project settings and nested include files give me one hell of a headache <g>, but usually I can figure things out. Not this time.

Here’s the scenario:

I have the original ISAPI mod code which sits in a separate project underneath the main Apache build. It has a single C source file. My Base64 code is just a separate C++ class which has two simple functions that expose decoding and encoding of Base64:

DWORD Base64StringEncode(char *lcInput, DWORD dwInputSize, char *lcOutput );

DWORD Base64StringDecode( char *lcInput,DWORD dwInputSize);

The ISAPI module imports the header just fine and compiles just fine as does my source file. However, the linker complains:

mod_isapi error LNK2019: unresolved external symbol _Base64StringDecode referenced in function _isapi_handler

It can’t find my function and thinks it’s external. But it’s obviously in the same project and compiling just fine, so why is it not finding it? It looks like it might be a calling convention problem but I tried changing the calling convention to my method but had no luck there either.

Anybody have any ideas at what to look at? This is driving me nuts…

Feedback for this Weblog Entry

re: Need some C++ help: Linker can't find a function in my source
by Brian Schkerke March 14, 2005 @ 6:31 am

Probably related to C++ mangling. Try changing your prototype to read similar to...

extern "C" int MyFunction();

This prevents the C++ compiler from mangling your function names (in order to achieve polymorphism) and allows the C code to see it.

Hope that's your solution!

re: Need some C++ help: Linker can't find a function in my source
by just looking March 14, 2005 @ 12:36 pm

some things to check (you may have already done them all):

the C file needs to #include the C++ header of your class

the C++ file needs to #include the library header files for any functions it uses from an external library

look for typo's in your function call

make sure the function call parameter types match the prototype signature exactly

if using VC++, right click on the offending call and "go to definition of Base64StringDecode". See if it takes you to your class.

try this: http://support.microsoft.com/default.aspx?scid=kb;EN-US;q191239

good luck

re: Need some C++ help: Linker can't find a function in my source
by Rick Strahl March 14, 2005 @ 12:56 pm

The solution turned out to be using Apache's built in Base64 encoding so I could avoid the whole issue altogether.

Decoded = ap_pbase64decode(r->pool, Encoded);

I did go back and add the class and function call back in an extern "C" did make this work. I didn't realize until much later that the Apache code was using plain C while my add in code was C++ hence the impedenance mismatch.

This code is doing all sorts of weird stuff though - maybe I haven't done plain C code in a really long time, but the compiler doesn't allow inline variable declarations. So doing:

char *Decoded = ap_pbase64decode()

doesn't work - Decoded had to be declared at the top of the function.

Also while debugging the compiler was not displaying the content of char pointers and skipping over whole sections of code that use them (point arithmetic). Never seen that either.

All I can say is, man am I glad I don't have to write C++ code for a living...

RE: http://www.west-wind.com/
by max.guernsey@harbingersoftware.com (Max Guernsey I March 19, 2005 @ 7:19 pm

> ...but the compiler doesn't allow inline variable declarations...
That tripped me up when I had to go back to C programming after a long C-free period.
> ...man am I glad I don't have to write C++ code for a living...
I second that.

encoding and decoding
by Chris May 18, 2006 @ 2:21 pm

hi..

i need urgent help with encoding and decoding with C++..
i havve no idea what code to use..
please help
chris

Help with programming the "F1, F2...." keys
by Tim August 01, 2006 @ 6:11 am

Hi i was just wondering if one could help me with incoorperating those keys. my scenario is that i need to make a program with options that need to use those keys but me being a beginner i am not sure. i was wondering is you could help. thanks