When an application calls SQLAllocConnect through an ODBC 3.x driver, the call to SQLAllocConnect(henv, phdbc) is mapped to SQLAllocHandle as follows:
The Driver Manager allocates a connection and returns it to the application.
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.