Call a C function from C++ code

Question

I have a C function that I would like to call from C++. I couldn't use "extern "C" void foo()" kind of approach because the C function failed to be compiled using g++. But it compiles fine using gcc. Any ideas how to call the function from C++?

Answer 1

Compile the C code like this:

gcc -c -o somecode.o somecode.c 

Then the C++ code like this:

g++ -c -o othercode.o othercode.cpp 

Then link them together, with the C++ linker:

g++ -o yourprogram somecode.o othercode.o 

You also have to tell the C++ compiler a C header is coming when you include the declaration for the C function. So othercode.cpp begins with:

extern "C" { #include "somecode.h" } 

somecode.h should contain something like:

 #ifndef SOMECODE_H_  #define SOMECODE_H_   void foo();   #endif 

---

(I used gcc in this example, but the principle is the same for any compiler. Build separately as C and C++, respectively, then link it together.)

Answer 2

Let me gather the bits and pieces from the other answers and comments, to give you an example with cleanly separated C and C++ code:

The C Part:

foo.h:

#ifndef FOO_H #define FOO_H  void foo(void);  #endif  

foo.c

#include "foo.h"  void foo(void) {     /* ... */ } 

Compile this with gcc -c -o foo.o foo.c.

The C++ Part:

bar.cpp

extern "C" {   #include "foo.h" //a C header, so wrap it in extern "C"  }  void bar() {   foo(); } 

Compile this with g++ -c -o bar.o bar.cpp

And then link it all together:

g++ -o myfoobar foo.o bar.o 

Rationale: The C code should be plain C code, no #ifdefs for "maybe someday I'll call this from another language". If some C++ programmer calls your C functions, it's their problem how to do that, not yours. And if you are the C++ programmer, then the C header might not be yours and you should not change it, so the handling of unmangled function names (i.e. the extern "C") belongs in your C++ code.

You might, of course, write yourself a convenience C++ header that does nothing except wrapping the C header into an extern "C" declaration.