C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
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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?
[32.1] What do I need to know when mixing C and C++ code?

Here are some high points (though some compiler-vendors might not require all these; check with your compiler-vendor's documentation):

  • You must use your C++ compiler when compiling main() (e.g., for static initialization)
  • Your C++ compiler should direct the linking process (e.g., so it can get its special libraries)
  • Your C and C++ compilers probably need to come from the same vendor and have compatible versions (e.g., so they have the same calling conventions)

In addition, you'll need to read the rest of this section to find out how to make your C functions callable by C++ and/or your C++ functions callable by C.

BTW there is another way to handle this whole thing: compile all your code (even your C-style code) using a C++ compiler. That pretty much eliminates the need to mix C and C++, plus it will cause you to be more careful (and possibly —hopefully!— discover some bugs) in your C-style code. The down-side is that you'll need to update your C-style code in certain ways, basically because the C++ compiler is more careful/picky than your C compiler. The point is that the effort required to clean up your C-style code may be less than the effort required to mix C and C++, and as a bonus you get cleaned up C-style code. Obviously you don't have much of a choice if you're not able to alter your C-style code (e.g., if it's from a third-party).