Calling functions in a DLL from C

Question

I have a solution in VS 2008 with 2 projects in it  One is a DLL written in C   and the other is a simple C   console application created from a blank project  I would like know how to call the functions in the DLL from the application   Assume I am starting with a blank C   project and that I want to call a function called int IsolatedFunction int someParam   How do I call it

User · Accepted Answer

There are many ways to do this but I think one of the easiest options is to link the application to the DLL at link time and then use a definition file to define the symbols to be exported from the DLL.

CAVEAT: The definition file approach works bests for undecorated symbol names. If you want to export decorated symbols then it is probably better to NOT USE the definition file approach.

Here is an simple example on how this is done.

Step 1: Define the function in the export.h file.

int WINAPI IsolatedFunction(const char *title, const char *test);

Step 2: Define the function in the export.cpp file.

#include <windows.h>

int WINAPI IsolatedFunction(const char *title, const char *test)
{
    MessageBox(0, title, test, MB_OK);
    return 1;
}

Step 3: Define the function as an export in the export.def defintion file.

EXPORTS    IsolatedFunction          @1

Step 4: Create a DLL project and add the export.cpp and export.def files to this project. Building this project will create an export.dll and an export.lib file.

The following two steps link to the DLL at link time. If you don't want to define the entry points at link time, ignore the next two steps and use the LoadLibrary and GetProcAddress to load the function entry point at runtime.

Step 5: Create a Test application project to use the dll by adding the export.lib file to the project. Copy the export.dll file to ths same location as the Test console executable.

Step 6: Call the IsolatedFunction function from within the Test application as shown below.

#include "stdafx.h"

// get the function prototype of the imported function
#include "../export/export.h"

int APIENTRY WinMain(HINSTANCE hInstance,
                     HINSTANCE hPrevInstance,
                     LPSTR     lpCmdLine,
                     int       nCmdShow)
{
    // call the imported function found in the dll
    int result = IsolatedFunction("hello", "world");

    return 0;
}

User · Answer

Presuming you re talking about dynamic runtime loading of DLLs  you re looking for LoadLibrary and GetProAddress  There s an example on MSDN

User · Answer

You can either go the LoadLibrary GetProcAddress route  as Harper mentioned in his answer  here s link to the run-time dynamic linking MSDN sample  again  or you can link your console application to the  lib produced from the DLL project and include the hea h file with the declaration of your function  as described in the load-time dynamic linking MSDN sample   In both cases  you need to make sure your DLL exports the function you want to call properly  The easiest way to do it is by using   declspec dllexport  on the function declaration  as shown in the creating a simple dynamic-link library MSDN sample   though you can do it also through the corresponding  def file in your DLL project   For more information on the topic of DLLs  you should browse through the MSDN About Dynamic-Link Libraries topic

User · Answer

The following are the 5 steps required    declare the function pointer Load the library Get the procedure address assign it to function pointer call the function using function pointer   You can find the step by step VC   IDE screen shot at http   www softwareandfinance com Visual CPP DLLDynamicBinding html  Here is the code snippet    int main            declspec dllimport  bool GetWelcomeMessage char  buf  int len      used for static binding           typedef bool   GW  char  buf  int len        HMODULE hModule   LoadLibrary TEXT  TestServer DLL         GW GetWelcomeMessage    GW  GetProcAddress hModule   GetWelcomeMessage         char buf 128       if GetWelcomeMessage buf  128     true          std  cout  lt  lt  buf          return 0

User · Answer

Can also export functions from dll and import from the exe  it is more tricky at first but in the end is much easier than calling LoadLibrary GetProcAddress  See MSDN    When creating the project with the VS wizard there s a check box in the dll that let you export functions   Then  in the exe application you only have to  include a header from the dll with the proper definitions  and add the dll project as a dependency to the exe application   Check this other question if you want to investigate this point further Exporting functions from a DLL with dllexport

User · Answer

Might be useful  https   www codeproject com Articles 6299 Step-by-Step-Calling-C-DLLs-from-VC-and-VB-Part-4 For the example above with  quot GetWelcomeMessage quot  you might need to specify  quot   stdcall quot  in the typedef field before the function name if getting error after calling imported function

User · Answer

When the DLL was created an import lib is usually automatically created and you should use that linked in to your program along with header files to call it but if not then you can manually call windows functions like LoadLibrary and GetProcAddress to get it working

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