Undefined reference to "function" - c

Question

So I have this very simple program but I can't seem to get rid of a simple error.
I have a Header file with this
#ifndef FUNCTIONLOOKUP_H_INCLUDED
#define FUNCTIONLOOKUP_H_INCLUDED
enum functions
{
foo,
bar
};
//predefined function list
int lookUpFunction(enum functions);
#endif // FUNCTIONLOOKUP_H_INCLUDED
And in the src file i have the definition of lookUpFunction
Now when I call the lookUpFunction() from my main where I included the header file it gives me a undefined reference to it. The other awnsered questions where of no help.
#include <stdio.h>
#include <stdlib.h>
#include "FunctionLookUp.h"
int main()
{
lookUpFunction(foo); <---
return 0;
}
Function implementation
#include <stdio.h>
#include "FunctionLookUp.h"
typedef void (*FunctionCallback)(int);
FunctionCallback functionList[] = {&foo, &bar};
void foo(int i)
{
printf("foo: %d", i);
}
void bar(int i)
{
printf("bar: %d", i);
}
int lookUpFunction(enum functions)
{
int test = 2;
//check if function ID is valid
if( functions >= sizeof(functionList))
{
printf("Invalid function id"); // error handling
return 0;
}
//call function
functionList[functions](test);
return 1;
}
I can't seem to figure out where this error comes from.

Answer 1

You must have some file similar to:
/* FunctionLookUp.c */
#include "FunctionLookUp.h"
int lookUpFunction(enum functions)
{
/* code ... */
return x;
}
somewhere in order to solve your problem

Answer 2

You never show code that implements the function.
So it's most likely that what you're seeing is a linker error, the call itself is fine but the linker cannot find the code to call, so it throws an error.
Just declaring a function can't magically make it appear from somewhere, you must write the actual function too.