The Synchronous/Asynchronous Model

Telephony operations complete either synchronously or asynchronously. These two models differ as follows.

• Asynchronous operations have a parameter named dwRequestID of the defined type DRV_REQUESTID as their first parameter. Such an operation performs part of its processing in the function call and the remainder of its processing in an independent execution thread after the function call has returned. When the function returns, it returns either a negative error result or the (positive) dwRequestID that was passed in the function call. The negative error result indicates that the operation is complete (and failed). The positive dwRequestID returned indicates that the operation continues in the independent thread. In such a case, the service provider eventually calls the ASYNC_COMPLETION procedure, passing the dwRequestID and a result code to indicate the outcome of the operation. The result code is zero for success or one of the same set of (negative) error results. In any case, the service provider must never return zero or any positive value other than dwRequestID from a function designated as asynchronous because doing so can produce unexpected results. Service providers should always copy input data from application memory into service provider memory before returning from an asynchronous function.
• Synchronous operations do not have dwRequestID as their first parameter. Such an operation performs all of its processing in the caller's execution thread, returning the final result when it returns. The result is zero for success or a negative error code for failure. The service provider does not call ASYNC_COMPLETION for synchronous operations.