Linking error when Calling a C header file - c++

Question

I am calling a C function from a header file I wrote in Qt written in Cpp. I keep getting a linking error when I try to compile my Qt Application.
here is the header file:
#ifndef GROUND_SERVER_H
#define GROUND_SERVER_H
#ifdef __cplusplus
extern "C" {
#endif
struct system_info{
char id[33];
};
/* Support function for the below function */
void Generate_Key(char*,char*,char*);
/* Runs the actual key generation as well as
moves the file to the respectful card or
USB drive inserted in the system that will
act as the user system key */
void run_key_generation(struct system_info*,char*,char*);
/* Function to run the server on a selected
port at which the medium that the server
is to listen on will be connected. */
void run_server(unsigned short);
void generate_id();
#ifdef __cplusplus
};
#endif
#endif

Answer 1

#include "foo.h" is only the textual inclusion of the contents of foo.h into the current compilation unit. If you implement a function in a different file, then you need to compile that file, too, and link the resulting object files to form an executable (or library).
This does also apply when mixing C and C++ (or most other compiled) code: You compile the source code files with the compiler suitable for the language they're written in, and finally link everything together.
So:
foo.h
#ifndef FOO_H
#define FOO_H 1
#ifdef __cplusplus
extern "C" {
#endif
int answer(void);
#ifdef __cplusplus
}
#endif
#endif
foo.c
int answer(void) {
return 42;
}
bar.cc
#include <iostream>
#include "foo.h"
int main(int argc, char ** argv) {
std::cout << "The answer is " << answer() << std::endl;
return 0;
}
To create an executable from these files, you need to:
gcc -c foo.c # Compile C file
g++ -c bar.cc # Compile C++ file
g++ -o foobar foo.o bar.o # Link