10. Understanding ODBC Connectivity : ODBC Programming : How ODBC Applications Connect to a Database
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How ODBC Applications Connect to a Database
ODBC applications are never linked directly against the ODBC driver DLL or shared library. Instead, the applications are linked against the ODBC Driver Manager, which can be the Microsoft ODBC Driver Manager, the unixODBC Driver Manager, or the ODBC CLI.
The ODBC application has no knowledge of the driver until a connection is made. During a successful connection, the ODBC Driver is dynamically loaded into the program image. After loading, the functions called in the ODBC application are passed down to the driver implementation of the function. In this way, the same ODBC application can be used with different ODBC drivers, and thus can be used with a variety of different databases. This portability is a principal aim of ODBC.
The ODBC Driver is designed to work from a driver manager such as unixODBC or the ODBC CLI. Applications link with the driver manager. The ODBC driver is loaded into the driver manager when the first connection is made to a database. One can theoretically link directly with the ODBC driver, leaving out the driver manager, but this is not supported and may yield unpredictable results.
The ODBC Driver, the ODBC CLI, and the unixODBC Driver Manager are implemented as shared libraries on UNIX, but the names vary according to platform conventions. For example, on Linux, for 32-bit Ingres: the ODBC Driver is implemented as $II_SYSTEM/ingres/lib/libiiodbcdriver.1.so and libiiodbcdriverro.1.so, and the ODBC CLI is implemented as libiiodbc.1.so.