Unicode function calls to ODBC driver managers

CLI applications that are ODBC-compliant can access a Db2® database through the CLI/ODBC driver in one of two ways: linking to the CLI/ODBC driver library or linking to the ODBC driver manager library.

CLI applications can access a database by:

Direct access - An application links to the CLI/ODBC driver library and makes calls to exported CLI/ODBC functions. Unicode applications accessing the CLI/ODBC driver directly should access and perform transactions against the database using the CLI Unicode functions, and use SQLWCHAR buffers with the understanding that all Unicode data is UCS-2. To identify itself as a Unicode application, the application must connect to the database using either SQLConnectW() or SQLDriverConnectW().
Indirect access - An application links to an ODBC driver manager library and makes calls to standard ODBC functions. The ODBC driver manager then loads the CLI/ODBC driver and calls exported ODBC functions on behalf of the application. The data passed to the CLI/ODBC driver from the application might be converted by the ODBC driver manager. An application identifies itself to an ODBC driver manager as a Unicode application by calling SQLConnectW() or SQLDriverConnectW().

When connecting to a data source, the ODBC driver manager checks to see if the requested driver exports the SQLConnectW() function. If the function is supported, the ODBC driver is considered a Unicode driver, and all subsequent calls in the application to ODBC functions are routed to the functions' Unicode equivalents (identified by the 'W' suffix; for example, SQLConnectW()) by the ODBC driver manager. If the application calls Unicode functions, no string conversion is necessary, and the ODBC driver manager calls the Unicode functions directly. If the application calls ANSI functions, the ODBC driver manager converts all ANSI strings to Unicode strings before calling the equivalent Unicode function.

If an application calls Unicode functions, but the driver does not export SQLConnectW(), then the ODBC driver manager routes any Unicode function calls to their ANSI equivalents. All Unicode strings are converted by the ODBC driver manager to ANSI strings in the application's code page before calling the equivalent ANSI function. This might result in data loss if the application uses Unicode characters which cannot be converted to the application's code page.

Various ODBC driver managers use different encoding schemes for Unicode strings, depending on the operating system:

Table 1. Unicode string encoding schemes by operating system
Driver manager	Operating system
Driver manager	Microsoft Windows	Linux and UNIX
Microsoft ODBC Driver Manager	UTF-16*	not applicable
unixODBC Driver Manager	UCS-2	UCS-2
DataDirect Connect for ODBC Driver Manager	UTF-16*	UTF-8
* UTF-16 is a superset of UCS-2 and therefore is compatible