__int64 attribute
The keyword __int64 specifies a valid integer supported by the MIDL compiler.
__int64 identifier-name;
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identifier-name
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Specifies a valid MIDL identifier. Valid MIDL identifiers consist of up to 31 alphanumeric and/or underscore characters and must start with an alphabetic or underscore character.
For a discussion of how to use __int64, see hyper.