MDAC 2.5 SDK - ODBC Programmer's Reference
Appendix G: Driver Guidelines for Backward Compatibility

SQLAllocConnect Mapping

When an application calls SQLAllocConnect through an ODBC 3.x driver, the call to SQLAllocConnect(henv, phdbc) is mapped to SQLAllocHandle as follows:

1. The Driver Manager allocates a connection and returns it to the application.
   
   
2. When the application establishes a connection, the Driver Manager calls

   SQLAllocHandle(SQL_HANDLE_DBC, InputHandle, OutputHandlePtr)
   

   in the driver with InputHandle set to henv, and OutputHandlePtr set to phdbc.