SQLSetEnvAttr
The ODBC Programmer's Reference defines how ODBC drivers should interpret the SQLSetEnvAttr attribute specifications from applications written to either the ODBC 2.x or ODBC 3.x API. The SQL Native Client ODBC driver complies with those rules.
One of the attributes controlled by SQLSetEnvAttr is whether connection pooling is to be used. If connection pooling is used with the SQL Native Client ODBC driver, the DriverCompletion parameter must be set to SQL_DRIVER_NOPROMPT when connecting with either SQLDriverConnect or SQLConnect.
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Concepts
ODBC API Implementation Details