C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 32:
32.1 What do I need to know when mixing C and C++ code?
32.2 How can I include a standard C header file in my C++ code?
32.3 How can I include a non-system C header file in my C++ code?
32.4 How can I modify my own C header files so it's easier to #include them in C++ code?
32.5 How can I call a non-system C function f(int,char,float) from my C++ code?
32.6 How can I create a C++ function f(int,char,float) that is callable by my C code?
32.7 Why is the linker giving errors for C/C++ functions being called from C++/C functions?
32.8 How can I pass an object of a C++ class to/from a C function?
32.9 Can my C function directly access data in an object of a C++ class?
32.10 Why do I feel like I'm "further from the machine" in C++ as opposed to C?
[32.7] Why is the linker giving errors for C/C++ functions being called from C++/C functions?

If you didn't get your extern "C" right, you'll sometimes get linker errors rather than compiler errors. This is due to the fact that C++ compilers usually "mangle" function names (e.g., to support function overloading) differently than C compilers.

See the previous two FAQs on how to use extern "C".