error LNK2001: unresolved external symbol - c++

Question

I have two VC++ projects inside a sln file in visual studio 2010. I want to use a_flag in the file of another project, is this possible what i am doing below ?
Project 1:
**sample_header.h**
#ifndef SAMPLE_HEADER_API
#define SAMPLE_HEADER_API __declspec(dllimport)
#endif
extern SAMPLE_HEADER_API int a_flg;
**file1.cpp**
#define SAMPLE_HEADER_API __declspec(dllexport)
#include "sample_header.h"
// Intialization of external
int a_flag = 15;
void m_func()
{
int i = 0;
}
Project 2:
**file2.h**
#include <stdio.h>
**file2.cpp**
#include "file1.h"
#include "sample_header.h"
// provided path of "sample_header.h" in additional include directory as well
void main()
{
if(a_flag > 0)
{
std::cout << "FLAG" ;
}
}
I set project1 as a DLL, project2 as an EXE project.
In linking, I am getting this error :
error LNK2001: `unresolved external symbol "__declspec(dllimport) int a_flg" (__imp_?a_flg##3HA)` in file2.cpp
I have read Microsoft page here about the DLL creation and linking but no idea how to resolve this external symbol error.
Thanks !

Answer 1

You need to set the project that creates your .dll to also generate the .lib file (an import library).
A quick description of the linkage should be something like this :
DLL Dependency Project -> dependecy.dll + dependency.lib
Main Project -> depends at runtime to depedency.dll , depends at link time to dependency.lib.
In other words, your .dll is just another binary that exposes some function signatures.
At runtime you can choose for either c linkage which involves querying the dll for the exposed functors/variables via name (the hard way, but useful when you don't have the .dll source code at hand) or use a more elegant way where you link the generated static library with your main.
When using the 1st approach, you will need to treat inside your code if you cannot find a certain .dll .
When using the 2nd approach, your binary will know that it depends on a certain .dll when you try to run it .
Here's an answer you will find very useful :
How do I build an import library (.lib) AND a DLL in Visual C++?