### Export Functions from a DLL

Each function that you want to export from the DLL must be an external function, either by default or by being qualified with the extern keyword. Otherwise, the linker will not find your function references and will generate errors.

There are several ways to export the functions in a DLL so that they are available to other programs:

Using the _Export keyword in the source files
  With this method, you can only export the functions by name.You cannot control which ordinal is assigned to a particular function. This is the easiest method for exporting functions, but it can cause problems if other programs that use the DLL depend on a particular set of ordinals. If the DLL has to be updated, the compiler may assign different ordinals to the exported functions.
Using _declspec(dllexport) in the source files
  _declspec(dllexport) is used in the same way as the _Export keyword. As with _Export, you can only export functions by name, and you cannot control which ordinal is assigned to a function.
Using #pragma export in the source files
  With this method, you can only export the functions by name, but you have the option of choosing the ordinal for a function yourself or letting the compiler choose it for you. With this method, you can specify the assignment of ordinals to exported functions. When you update a DLL, you can keep these assignments. This means that programs that use functions from this DLL will not have to be updated when the DLL is updated.

> Note that even if you use #pragma export or _Export to export your function, there are some situations where you may still need to create a module definition file.

Using a Module Definition file.
  This method offers the most flexibility: functions can be exported with or without a name. You have the option of choosing the ordinal for a function yourself or allowing the compiler to choose it.
  The advantage of using a .DEF file to export functions is that changing the DLL will not affect other programs that use functions in the DLL. However, it can be difficult to write and maintain C++ .DEF files because you must use the mangled names of the functions that you want to export.
  With a module definition file, the load time can be greater for the code that uses the DLL if you have chosen to keep names resident in memory.

Create a Module Definition File

Examples Using _Export, #pragma export and _declspec