Linking error in simple C program using a C++ header

Question

I am following an intro tutorial http://www.linuxinsight.com/files/alp/alp-ch01-getting-started.pdf.

I have created the source files main.c, reciprocal.cpp and reciprocal.hpp. I have been able to successfully compile these files. The problem comes when I go to link them and I receive the following error message:

main.o: In function `main':
main.c:(.text+0x30): undefined reference to `reciprocal'
collect2: ld returned 1 exit status

I gather that this is somehow not using the header files correctly, however I do not really know what to try as I've followed the steps quite thoroughly.

If anyone has an idea why its throwing that error I would appreciate it very much.

Thanks

**UPDATE Here is the code for the three source files:

main.c

#include <stdio.h>
#include "reciprocal.hpp"

int main (int argc, char **argv) {
    int i;
    i = atoi(argv[1]);
    printf("The reciprocal of %d is %g\n", i, reciprocal(i));

    return 0;
}

reciprocal.cpp

#include <cassert>
#include "reciprocal.hpp"

double reciprocal(int i) {
    assert(i != 0);
    return 1.0/i;
}

reciprocal.hpp

#ifdef __cplusplus
extern "C" {
#endif

double reciprocal(int i);

#ifdef __cplusplus
}
#endif

I have actually modified reciprocal.hpp since originally posting. The only change I've made was to remove the extern keyword from the function signature. It previously read

extern double reciprocal(int i);

This change allowed me to link the program and I can now run it. I think what may have been occurring is that the second extern was overriding the first one. If anyone has any insight on this, I would be interested in knowing.

Thanks for your assistance everyone.

Answer 1

When you mix C and C++ code together, you can run into trouble because the two languages have different linkages. That is, if you were to crack open the object files for a compiled C and C++ file, you would find that the names inside of those object files were different.

In particular, C++ compilers tend to use a technique called "name mangling" in which function names are mixed with extra information about the types of their arguments. For example, a function

char foo(int);

might actually have the internal name

char@foo@int

inside of the generated object file. The problem is that C code does not do this, so if you compiled a C file and tried to make a reference to the name foo, the linker wouldn't find it, because the name of the function in the generated C++ file is char@foo@int instead of foo.

To fix this, C++ has a feature that allows you to explicitly tell the linker not to mangle the name and to make the generated code look like it was intended for a C program. To do this, you can declare C++ functions like this:

extern "C" char foo(int);

Now, the generated object file will contain the name foo without adornment in a way that is compatible with what the C code expects.

To fix your problem, try adding one of these extern "C" declarations to your C++ source file that defines reciprocal.

A note: if you have a project that mixes C and C++ code, you should always have the main function compiled as C++ code. C++ introduces extra initialization and cleanup code into its executables that are not present in C code. If you compile main as C code, this extra logic might not get added into the program, and so you may get inexplicable crashes at runtime.

Hope this helps!