MDAC 2.5 SDK - ODBC Programmer's Reference
Appendix G: Driver Guidelines for Backward Compatibility
When an application calls SQLAllocConnect through an ODBC 3.x driver, the call to SQLAllocConnect(henv, phdbc) is mapped to SQLAllocHandle as follows:
SQLAllocHandle(SQL_HANDLE_DBC, InputHandle, OutputHandlePtr)
in the driver with InputHandle set to henv, and OutputHandlePtr set to phdbc.