Create dynamic library using MSVC command line

Question

I've read a lot of posts and yet I don't understand how to create a simple dynamic library on windows using MSVC in command line. What I'm doing is:

1º) Code the DLL

dynamic.h

#pragma once

__declspec(dllexport) void HelloWorld();

dynamic.c

#include "dynamic.h"
#include <stdio.h>

void HelloWorld(){
    printf("Hello World");
}

2º) Compile it

cl /LD dynamic.c

(it compiles correctly and without errors generating dynamic.dll and dynamic.lib)

3º) Try to test it

main.c

#include<stdio.h>
#include"dynamic.h"
int main(){

    HelloWorld();

    return 0;
}

cl main.c dynamic.lib

ERROR (by cl.exe x64)

main.cpp
Microsoft (R) Incremental Linker Version 14.16.27034.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:main.exe
main.obj
.\dynamic.lib
main.obj : error LNK2019: unresolved external symbol "void __cdecl HelloWorld(void)" (?HelloWorld@@YAXXZ) referred in main
main.exe : fatal error LNK1120: unresolved externals

Please teach me how dynamic libraries compilation really work because I can't understand

Answer 1

Try this way in dynamic.h :

#ifdef __cplusplus  
extern "C" {
#endif 
    __declspec(dllexport) void HelloWorld();
#ifdef __cplusplus 
}
#endif 

Use 'dumpbin.exe /exports dynamic.dll' to show exported symbols

Answer 2

In main.c it needs to see the function declaration like this:

__declspec(dllimport) void HelloWorld();

So you cannot use the same dynamic.h as you currently have, for both building the DLL and building main.c .

Normally people will use a preprocessor setup so the same header file has a different declspec depending who is including it, for example:

// dynamic.h
#ifndef DLL_FUNCTION 
#define DLL_FUNCTION __declspec(dllimport)
#endif

DLL_FUNCTION void HelloWorld();

dynamic.c (in the DLL):

#define DLL_FUNCTION __declspec(dllexport)
#include "dynamic.h"