Keep on getting LNK2019 error when mixing C and C++ - c++

Question

I'm trying to call C code from C++. I have 3 files: pubsub.cpp, pubsublib.c & pubsublib.h.
I know I have to use extern "C" in order to call a C function from C++.
I've literally tried everything, but I am still getting the following LNK2019 unresolved external symbol error:
LNK2019 unresolved external symbol _thisIsATestFunction referenced in
function _main
pubsublib.c
#include "pubsublib.h"
void thisIsATestFunction()
{
// do something
}
pubsublib.h
#ifndef pubsublib_H
#define pubsublib_H
void thisIsATestFunction();
#endif
pubsub.cpp
#include <string>
#include <iostream>
#ifdef __cplusplus
extern "C"{
#endif
#include "pubsublib.h"
#ifdef __cplusplus
}
#endif
using namespace std;
int main() {
thisIsATestFunction();
return 0;
}
I have also tried to just include the functions but I'm getting the same error. I'm really lost at the moment and have searched a lot... Any help would be greatly appreciated.

Answer 1

It works perfectly fine when you actually link with the necessary file.
When you compile pubsub.cpp you have to add pubsublib.obj to the command line. For example, this solved the problem for me using Visual Studio 2015:
cl -c pubsublib.c
cl pubsub.cpp pubsublib.obj
If you're building with the IDE, you simply need to make sure that pubsublib.c is a dependency and pubsublib.obj is specified as input to the link step.

Answer 2

You have to add a reference to the library in your application.
If you have both projects in one solution in Visual C++ 2015 you
right click your application project
select "Add..." from the drop down menu
select "Reference" from the menu
check you library in the list
If you have each project in its own solution you
right click you application project
select "Properties"
open "Linker" section "Input" page
add your .lib file to the "additional dependencies"
Your application should compile without LNK2019 after doing so.
There are also "build dependencies" to manage, but these do not affect the libraries linked to your project (only the build order).
(I am using a german edition of Visual Studio so please fix the translations if some of them are wrong)

Answer 3

Your declaration works fine, as proves the error message (_thisIsATestFunction is indeed a C-undecorated name).
Now two possibilities remain
the file pubsublib.c was not compiled as C, causing the compiled function to get decorated;
the file pubsublib.c was compiled as C, but didn't take part to the linking.
Only your solution/project(s) file can tell the truth. Can we see a screen copy of the Solution Explorer ?

Answer 4

This linkage error is produced when "pubsublib.c" file is compiled as C++ file (in g++). Please change your project options so that all .cpp files are compiled with g++, and all .c files are compiled with gcc.
After changing settings make sure that "pubsublib.c" file is really compiled with gcc. It can be found in compilation output like following:
g++.exe -c main.cpp -o main.o
gcc.exe -c pubsublib.c -o pubsublib.o
g++.exe main.o pubsublib.o -o Project1.exe