MDAC 2.5 SDK - ODBC Programmer's Reference
Chapter 6: Connecting to a Data Source or Driver
The first task for any ODBC application is to load the Driver Manager; how this is done is operating-system dependent. For example, on a computer running Microsoft® Windows NT® Server/Windows 2000 Server, Windows NT Workstation/Windows 2000 Professional, or Microsoft Windows® 95/98, the application either links to the Driver Manager library or calls LoadLibrary to load the Driver Manager DLL.
The next task, which must be done before an application can call any other ODBC function, is to initialize the ODBC environment and allocate an environment handle, as follows:
SQLHENV henv1;
SQLAllocHandle(SQL_HANDLE_ENV, SQL_NULL_HANDLE, &henv1);
The Driver Manager does not call SQLAllocHandle in the driver at this time because it does not know which driver to call. It delays calling SQLAllocHandle in the driver until the application calls a function to connect to a data source. For more information, see "Driver Manager's Role in the Connection Process," later in this chapter.
When the application has finished using ODBC, it frees the environment handle with SQLFreeHandle. After freeing the environment, it is an application programming error to use the environment's handle in a call to an ODBC function; doing so has undefined but probably fatal consequences.
When SQLFreeHandle is called, the driver releases the structure used to store information about the environment. Note that SQLFreeHandle cannot be called for an environment handle until after all connection handles on that environment handle have been freed.
For more information about the environment handle, see "Environment Handles" in Chapter 4, "ODBC Fundamentals."