How to Dynamically Load and Call a Function from a DLL in C ?

Dynamically Loading a Function from a DLL

Question:

You have created a DLL with a function named funci() and are attempting to load and access this function from C . However, you encounter the error "'funci' was not declared in this scope." How can you use the DLL's pointer to access the function?

Answer:

Step 1: Load the DLL

HINSTANCE hGetProcIDDLL = LoadLibrary("C:\path\to\test.dll");

This loads the DLL into the process's memory.

Step 2: Resolve Function Address

You need to get the function's address using GetProcAddress.

typedef int (__stdcall *f_funci)(); // Define function pointer
f_funci funci = (f_funci)GetProcAddress(hGetProcIDDLL, "funci");

Step 3: Verify Function Address

Check if the function address was obtained successfully.

if (!funci) {
  std::cout << "Could not locate the function" << std::endl;
  return EXIT_FAILURE;
}

Step 4: Call the Function

Once you have the function pointer, you can call it.

std::cout << "funci() returned " << funci() << std::endl;

Step 5: Freeing the DLL (Optional)

Release the DLL handle using FreeLibrary() to unload the DLL from memory.

FreeLibrary(hGetProcIDDLL);

Additional Tips:

• The WinAPI function GetProcAddress requires the function name as a string, so make sure to specify the correct name.
• The calling convention of the exported function (e.g., __stdcall) must match the one used when defining the function pointer in your program.
• Properly exporting functions from the DLL is essential. Use the __declspec(dllexport) attribute to export the function.

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