INF: How to Differentiate Multiple SQLAllocEnv() Calls

• Microsoft Open Database Connectivity, version 1.0

SUMMARY

In version 1.0 of Open Database Connectivity (ODBC), every time an application calls SQLConnect(), the Driver Manager calls the Driver s SQLAllocEnv(), SQLAllocConnect(), and SQLConnect() functions in succession. This means that an ODBC driver can be called to allocate multiple environments for the same application.

Since a driver must allocate only one environment per application, it must be able to distinguish between the first call to SQLAllocEnv from an application and subsequent calls to SQLAllocEnv()

MORE INFORMATION

Since only one environment should be allocated for an ODBC application by the driver, even though multiple calls to SQLAllocEnv are made from the Driver Manager, a driver can use the Windows function GetCurrentTask() to determine whether an environment has already been allocated for the calling application (task).

Typically, a driver can build a list of tasks that currently have the environments allocated, and on every call to SQLAllocEnv(), the driver can use this list to determine if the current task has an allocated environment. If the task is found in this list, the driver can return the previously allocated environment associated with that task, without having to allocate a new environment.

Drivers written to the ODBC version 2.0 specification need not use the above mechanism because the 2.0 Driver Manager does not call SQLAllocEnv() in the Driver on every SQLConnect(). Thus, an ODBC 2.0 driver is assured that its SQLAllocEnv() will be called only once on the first SQLConnect() from the application. Subsequent SQLConnect() calls in the application to that driver will not result in SQLAllocEnv() calls into the driver.