GetProcAddress function in C++

Question

Hello guys: I've loaded my DLL in my project but whenever I use the GetProcAddress function. it returns NULL! what should I do? I use this function ( double GetNumber(double x) ) in "MYDLL.dll"

Here is a code which I used:

typedef double (*LPGETNUMBER)(double Nbr);
HINSTANCE hDLL = NULL;
LPGETNUMBER lpGetNumber;
hDLL = LoadLibrary(L"MYDLL.DLL");
lpGetNumber = (LPGETNUMBER)GetProcAddress((HMODULE)hDLL, "GetNumber");

yes, I wrote that DLL and make .def file and compile it with lib.exe — Alireza
Try dumpbin /exports mydll.dll in a command prompt to see what the names of the exports are in your dll. You get dumpbin with your VS, or for free from MS as part of the platform SDK. — Fozi
@Alireza: If GetProcAddress() returns null, then it means that Windows can't locate that entry point in the DLL. There is also no reason to cast the hDLL variable, LoadLibrary() returns a HMODULE which is the first input parameter to GetProcAddress(). — Lucky Luke
@Lucky Luke: To rephrase what you said: On non-stone-age systems HINSTANCE and HMODULE are one and the same. — Boaz Yaniv

John Dibling John Dibling · Accepted Answer · 2011-05-17T13:34:56

Checking return codes and calling GetLastError() will set you free. You should be checking return codes twice here. You are actually checking return codes zero times.

hDLL = LoadLibrary(L"MYDLL.DLL");

Check hDLL. Is it NULL? If so, call GetLastError() to find out why. It may be as simple as "File Not Found".

lpGetNumber = (LPGETNUMBER)GetProcAddress((HMODULE)hDLL, "GetNumber");

If lpGetNumber is NULL, call GetLastError(). It will tell you why the proc address could not be found. There are a few likely scenarios:

There is no exported function named GetNumber
There is an exported function named GetNumber, but it is not marked extern "c", resulting in name mangling.
hDLL isn't a valid library handle.

If it turns out to be #1 above, you need to export the functions by decorating the declaration with __declspec(dllexport) like this:

MyFile.h

__declspec(dllexport) int GetNumber();

If it turns out to be #2 above, you need to do this:

extern "C"
{
  __declspec(dllexport) int GetNumber();
};

GetProcAddress function in C++

3 Answers

MyFile.h