INF: Error Flushing by Driver Manager (110473)

SUMMARY

This article discusses how the Driver Manager calls a driver's SQLError to flush the errors in the driver's error buffer. Chapter 16 of the ODBC SDK "Programmer's Reference" documents the ODBC convention for error text, return codes, and rules for handling error conditions.

Under the Returning Error Messages section in chapter 16, it reads:

Each time the application calls SQLError, the driver returns the next error message in the buffer. When the application calls a different function, the driver discards the current contents of the error message buffer.

It is unclear from the last sentence whether the driver clears the current error buffer on entry into the next function, or whether the Driver Manager calls the driver to clear its error buffer before entry into the next function.

This article explains how an application calls ODBC function B() after a previous call to ODBC function A(), the Driver Manager actually calls the driver's SQLError() first and then calls the driver's function B().

MORE INFORMATION

The implication of the Driver Manager calling the Driver's SQLError() is that on entry to function B() in the Driver, the Driver has already cleared the error buffer. Developers need to note this when debugging their drivers.

Also, when the Driver Manager calls the Driver's SQLError(), it passes the appropriate ODBC handles, but for other arguments to SQLError() like szSQLState, pfNativeError, it passes a NULL. The SQLError() function for Drivers must be written, such that when passed NULL arguments for everything except ODBC handles in SQLError(), the errors in the Driver's error buffer are cleared.

The following is a diagram showing how this process is accomplished by the Driver Manager.

Application         Driver Manager                Driver
---------------------------------------------------------------------
A()         ======>    A()           ======>     A() = SQL_ERROR
= SQL_ERROR <======   <======        <======     Returns SQL_ERROR

//Application does not call SQLError()
B()         ======>    B()
                       SQLError()     ======>    SQLError()
                              Loop for clearing errors
                  <=======
                       DM calls Driver's B()
                       ======>        =======>    B() = SQL_SUCCESS
=SQL_SUCCESS <======   <======        <=======    Returns SQL_SUCCESS