Behavioral Changes and ODBC 3.x Drivers
The environment attribute SQL_ATTR_ODBC_VERSION indicates to the driver whether it needs to exhibit ODBC 2.x behavior or ODBC 3.x behavior. How the SQL_ATTR_ODBC_VERSION environment attribute is set depends on the application. ODBC 3.x applications must call SQLSetEnvAttr to set this attribute after they call SQLAllocHandle to allocate an environment handle and before they call SQLAllocHandle to allocate a connection handle. If they fail to do this, the Driver Manager returns SQLSTATE HY010 (Function sequence error) on the latter call to SQLAllocHandle.
For more information on behavioral changes and how an application acts, see Behavioral Changes.
ODBC 2.x applications and ODBC 2.x applications recompiled with the ODBC 3.x header files do not call SQLSetEnvAttr. However, they call SQLAllocEnv instead of SQLAllocHandle to allocate an environment handle. Therefore, when the application calls SQLAllocEnv in the Driver Manager, the Driver Manager calls SQLAllocHandle and SQLSetEnvAttr in the driver. Thus, ODBC 3.x drivers can always count on this attribute being set.
If a standards-compliant application compiled with the ODBC_STD compile flag calls SQLAllocEnv (which may occur because SQLAllocEnv is not deprecated in ISO), the call is mapped to SQLAllocHandleStd at compile time. At run time, the application calls SQLAllocHandleStd. The Driver Manager sets the SQL_ATTR_ODBC_VERSION environment attribute to SQL_OV_ODBC3. A call to SQLAllocHandleStd is equivalent to a call to SQLAllocHandle with a HandleType of SQL_HANDLE_ENV and a call to SQLSetEnvAttr to set SQL_ATTR_ODBC_VERSION to SQL_OV_ODBC3.
In certain driver architectures, there is a need for the driver to appear as either an ODBC 2.x driver or an ODBC 3.x driver, depending on the connection. The driver in this case might not actually be a driver but a layer that resides between the Driver Manager and another driver. For example, it might mimic a driver, like ODBC Spy. In another example, it might act as a gateway, like EDA/SQL. To appear as an ODBC 3.x driver, such a driver must be able to export SQLAllocHandle, and to appear as an ODBC 2.x driver, must be able to export SQLAllocConnect, SQLAllocEnv, and SQLAllocStmt. When an environment, connection, or statement is to be allocated, the Driver Manager checks to see if this driver exports SQLAllocHandle. Since the driver does, the Driver Manager calls SQLAllocHandle in the driver. If the driver is working with an ODBC 2.x driver, the driver must map the call to SQLAllocHandle to SQLAllocConnect, SQLAllocEnv, or SQLAllocStmt, as appropriate. It must also do nothing with the SQLSetEnvAttr call when behaving as an ODBC 2.x driver.
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