The user_marshal Attribute

The [ user_marshal] attribute is an ACF-type attribute similar in syntax to [ represent_as]. As with the IDL attribute, [ wire_marshal], it offers a more efficient way to marshal data across a network. As an ACF attribute, [user_marshal] lets you marshal custom data types that are unknown to MIDL. Each application-specific type has a corresponding transmittable type that defines the wire representation.

Your application-specific type can be a simple, composite, or pointer type. The main restriction is that the type instance must have a fixed, well-defined memory size. If the size of your type instance needs to change, use a pointer field rather than a conformant array. Alternatively, you can define a pointer to the changeable type.

As with the [wire_marshal] attribute, you supply routines for the sizing, marshaling, unmarshaling, and freeing passes. The following table describes the four user-supplied routine names. The <type> is the userm-type specified in the [user_marshal] type definition.

Routine	Description
<type>_UserSize	Sizes the RPC data buffer before marshaling on the client or server side.
<type>_UserMarshal	Marshals the data on the client or server side.
<type>_UserUnmarshal	Unmarshals the data on the client or server side.
<type>_UserFree	Frees the data on the server side.

 

These user-supplied routines are provided by either the client or the server application, based on the directional attributes.

If the parameter is [ in] only, the client transmits to the server. The client needs the <type>_UserSize and <type>_UserMarshal functions. The server needs the <type>_UserUnmarshal and <type>_UserFree functions.

For an [ out]-only parameter, the server transmits to the client. The server needs the <type>_UserSize and <type>_UserMarshal functions, while the client needs the <type>_UserMarshal function.

Related topics

The wire_marshal Attribute

Marshaling Rules for user marshal and wire_marshal

user_marshal

wire_marshal

NdrGetUserMarshalInfo