INF: When and How an ODBC Driver DLL Gets Unloaded

SUMMARY

A driver gets unloaded by the ODBC Driver Manager when an application calls the ODBC SQLDisconnect() function by passing the handle to the database connection made by that driver. This article discusses how the driver is unloaded.

MORE INFORMATION

All ODBC calls by an application are intercepted and routed by the ODBC Driver Manager (DM). When an application calls the SQLDisconnect() function on a particular connection, the DM calls the driver's SQLDisconnect() function.

When the driver returns a status of SQL_SUCCESS or SQL_SUCCESS_WITH_INFO on this call, the DM then calls the driver's SQLFreeConnect() and SQLFreeEnv() in quick succession. Also, since the driver has made a successful disconnect on the call to SQLDisconnect(), the DM calls the FreeLibrary() function in the Windows API to reduce the reference count of the driver DLL by one. When the reference count of the Driver DLL gets to zero (0) after repeated calls to SQLDisconnect() by the application for all connections made by that driver, Windows will unload the driver DLL.

Driver developers may note minor implications arising out of the above. Note that every time an application calls SQLDisconnect(), the DM, besides calling the driver's SQLDisconnect(), also calls the Driver's SQLFreeConnect() and SQLFreeEnv() immediately. Thus SQLFreeEnv() is called on every SQLDisconnect(). When the application calls SQLFreeConnect() and SQLFreeEnv(), the DM does its own cleanup and does not make the same calls to the Driver because these calls would have been made at the time of SQLDisconnect().

If an ODBC application terminates without calling SQLDisconnect on all open connections, Windows will unload the Driver Manager if its reference count is reduced to zero. However, the Driver Manager cannot unload the driver DLL because the WEP of the Driver Manager (or any Windows DLL) cannot call the FreeLibrary function.