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Function Extension Mechanism
The Windows Sockets DLL, WS2_32.DLL, is no longer supplied by each individual
stack vendor. As a result, it is no longer possible for a stack vendor to offer
extended functionality by just adding entry points to the WS2_32.DLL. To
overcome this limitation, Windows Sockets 2 takes advantage of the new WSAIoctl function to accommodate service providers who want to offer provider-specific
functionality extensions. This mechanism assumes, of course, that an
application is aware of a particular extension and understands both the semantics and
syntax involved. Such information would typically be supplied by the service
provider vendor.
In order to invoke an extension function, the application must first ask for a
pointer to the desired function. This is done through the WSAIoctl function using the SIO_GET_EXTENSION_FUNCTION_POINTER command code. The input
buffer to the WSAIoctl function contains an identifier for the desired extension function while the
output buffer contains the function pointer itself. The application can then
invoke the extension function directly without passing through the WS2_32.DLL.
The identifiers assigned to extension functions are globally unique
identifiers (GUIDs) that are allocated by service provider vendors. Vendors who create
extension functions are urged to publish full details about the function
including the syntax of the function prototype. This makes it possible for common and
popular extension functions to be offered by more than one service provider
vendor. An application can obtain the function pointer and use the function without
needing to know anything about the particular service provider that implements
the function.
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