Thread: Converting simple C++ example to C, to use existing DLL

I'm attempting to use an existing .dll file from within my C code. My coworker who created this .dll has a simple example written in C++ that uses this file, which example code I'm able to use with no problems.

However, I'm now attempting to "translate" his C++ code for use in my own C code, and so far have been getting long lists of compiler errors (understandably) because I'm attempting to use his C++ syntax within my C file. After searching online & on this forum, I haven't yet encountered the information I need, in order to convert his C++ code into a C syntax that will compile.

This is his example code:

Code:

int a = 5, b = 2;

typedef int (*sumPointer)(int a, int b); // Shorthand to make other lines shorter
HINSTANCE dll;           // Pointer to the .dll
sumPointer sum;         // Pointer to "sum" function within the .dll

dll = LoadLibrary("RC.dll");
sum = (sumPointer)GetProcAddress(dll, "sum");

sum(a,b);

Once I can get this simple example working within my C code, I'll be able to apply this same principle to the real .dll & function calls that I need to use. (I'm aware that the comment indicators above need to be changed to C format, for starters.)

I appreciate any suggestions you have that will allow me to properly implement the above code in C. Thanks!

Was the DLL written in C or in C++?
If it was written in C++; calling it from C may be impossible to very hard to do.

Tim S.

In case it's useful, I'll paste below the list of compiler errors I get, when I attempt to create a mex file using my current C code (with corrected comment indicators) and a related .lib file:

controller_CMEXsfcn.c(230) : error C2065: 'HINSTANCE' : undeclared identifier
controller_CMEXsfcn.c(230) : error C2146: syntax error : missing ';' before identifier 'dll'
controller_CMEXsfcn.c(230) : error C2065: 'dll' : undeclared identifier
controller_CMEXsfcn.c(231) : error C2275: 'sumPointer' : illegal use of this type as an expression
controller_CMEXsfcn.c(229) : see declaration of 'sumPointer'
controller_CMEXsfcn.c(231) : error C2146: syntax error : missing ';' before identifier 'sum'
controller_CMEXsfcn.c(231) : error C2065: 'sum' : undeclared identifier
controller_CMEXsfcn.c(233) : error C2065: 'dll' : undeclared identifier
controller_CMEXsfcn.c(233) : warning C4013: 'LoadLibrary' undefined; assuming extern returning int
controller_CMEXsfcn.c(234) : error C2065: 'sum' : undeclared identifier
controller_CMEXsfcn.c(234) : warning C4013: 'GetProcAddress' undefined; assuming extern returning int
controller_CMEXsfcn.c(234) : error C2065: 'dll' : undeclared identifier
controller_CMEXsfcn.c(234) : warning C4047: '=' : 'int' differs in levels of indirection from 'sumPointer'
controller_CMEXsfcn.c(236) : warning C4013: 'sum' undefined; assuming extern returning int

First you need to #include <windows.h> for the LoadLibrary() call.
Next you need to #include the header for the dll

C++ DLLs can usually be accessed by C programs, but using classes in C is a major pain in the backside since you have to define your own VTBL for class functions (C++ does this invisibly) and call them by function pointers... As a general rule if the DLL contains only functions you should be able to use it. But when it includes Classes, you need to be a well established masochist to get it working.

In the long run it would probably be easier to use his work as a model and then re-write the DLL...

Ok, I reviewed the source code specifically for my DLL, and it *does* start with 'extern "C" '. I should mention that both the example code/.dll as well as the real versions of those files, were specifically created for my use with my *C* code, so I believe that they all *should* be compatible with C.

That having been established, are there any suggestions about how the few lines of C++ code in my OP might be able to be translated into C? I appreciate all input.

Originally Posted by CodeKate

That having been established, are there any suggestions about how the few lines of C++ code in my OP might be able to be translated into C? I appreciate all input.

I see nothing in your code you posted that is NOT valid C code; but, you still need to include the needed header files!

Tim S.

Update: just by including the <windows.h> and .dll header call (duh), as Tater suggested, the code is now compiling successfully with no warnings. This isn't necessarily the end of my issues, but is a huge step forward. Thanks, Tater (and Tim)!

Originally Posted by CodeKate

Update: just by including the <windows.h> and .dll header call (duh), as Tater suggested, the code is now compiling successfully with no warnings. This isn't necessarily the end of my issues, but is a huge step forward. Thanks, Tater (and Tim)!

Well... now you know why we use header (.h) files....

Good luck with the rest of the project.

Well... now you know why we use header (.h) files....

yeah, thanks. It's not that I was unfamiliar with header files, but just that there were already a number of them included in my code, and I wasn't sure if others were missing...

Good luck with the rest of the project.

Thanks a lot!

Originally Posted by CodeKate

yeah, thanks. It's not that I was unfamiliar with header files, but just that there were already a number of them included in my code, and I wasn't sure if others were missing...
Thanks a lot!

"I use 10 of them, what do you want?" doesn't work here.