Undefined Reference to a function

Question

I\'m using Linux and I have the following files:

main.c, main.h
fileA.c, fileA.h
fileB.cpp, fileB.h

The function F1() is decla

Answer 1

To be able to call a c++ function from c source code you neeed to give the appropriate linkage specification.

The format for specifying linkage specification is

extern "type_of_Linkage" <function_name>

So in your case, you should be using:

extern "C" void F1();

Answer 2

perhaps, use

extern "C" void F1();

Answer 3

If you're really compiling fileA.c as C, not C++, then you need to make sure that the function has the proper, C-compatible linkage.

You can do this with a special case of the extern keyword. Both at declaration and definition:

extern "C" void F1();
extern "C" void F1() {}

Otherwise the C linker will be looking for a function that only really exists with some mangled C++ name, and an unsupported calling convention. :)

Unfortunately, whilst this is what you have to do in C++, the syntax isn't valid in C. You must make the extern visible only to the C++ code.

So, with some preprocessor magic:

#ifdef __cplusplus
extern "C"
#endif
void F1();

Not entirely pretty, but it's the price you pay for sharing a header between code of two languages.

Answer 4

fileA.c needs to also include fileA.h I believe.

Answer 5

fileA.c can't include fileB.h (via fileA.h) because the C compiler doesn't know what extern "C" means, so it complains that it sees an identifier before a string. don't try to include fileB.h in fileA.c or fileA.h. its not needed