Urgent: DLL

I am told to write a DLL. I am told the client will give me the .h file like this, where they define the function:

dll.h

Code:

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extern "C"
{

        // Sets 'modelList' to a newline-separated ('\n') list of available propagation models.
        // The last name is followed by a '\n' separator too.
        __declspec(dllexport) void GetAvailablePropagationModels(BSTR* modelList);

        // Returns a pointer to a new instance of the specified propagation model. Needs to initialize the new model using initData.
        // Returns nullptr in case of error and then sets errorMessage.
        // Returns nullptr if the given initData.version is not supported.
        __declspec(dllexport) velox::propagation_api::IPropagationModel* GetPropagationModel(const wchar_t* name,
        velox::propagation_api::StaticData* initData, BSTR* errorMessage);
}

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I am told to implement these in my new DLL. It is confusing. i am new to windows programming. Usually if i write the DLL, i need to give the a header to the client with the above info.
Here i am told that they define the function, i need to implement it.
Please help me understand. It is quite urgent

Quote:

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Please help me understand

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Understand what? You have previously written dll's so you know their source structure etc. Here you have to write 2 function definitions given the above function declarations. Assuming you are familiar with velox etc, then the functions just need to do what the comments describe.

Thanks a lot kaud .
Usually, from my basic understanding, it is the person who writes the dll, gives the .h. Here i was told to use the one given by the client and implement the functions in it.
I just needed some input from the c++ windows experts that what i understand is correct

Why does it matter who writes the header files? Just implement the function bodies in a DLL.

Using C++? It may make a difference if the dll is written using a different compiler than the version used to compile the client.