Asynchronous Functions

An operation that completes asynchronously performs part of its processing in the function call made by the application and the remainder of it in an independent execution thread after TAPI has returned from the function call. To ensure completion of the call's processing, the service provider vectors the request to another active entity in the systemsuch as a LAN server, add-in hardware, a switch, or a networkand then returns to the application. At this time, either a negative error result or a positive request ID is returned to the application.

At the time of asynchronous completion (the service provider has received an interrupt from the hardware, meaning that a message must be delivered), the service provider calls TAPISRV.EXE and reports that "Event X has just taken place. Deliver an appropriate message to all concerned applications." When TAPISRV.EXE receives this message, it forwards the message the TAPI dynamic-link library, in the application's process, which in turn posts a window message, signals an event handle, or posts to an IO completion port, according to the message notification scheme selected by the application in lineInitializeEx or phoneInitializeEx.

When the asynchronous portion of the operation completes, a LINE_REPLY (or PHONE_REPLY) message is sent to the application. This message contains, as one of its parameters, the request ID returned by the function call. This request ID allows the application to determine which original request has completed. (Applications should remember the request IDs of all their requests in progress so that reply messages can be properly handled.) A second parameter to the LINE_REPLY (or PHONE_REPLY) message is the asynchronous return value. This is either a negative value (for an error) or zero if the operation completed successfully. For asynchronous operations, any of the return values may be returned as part of the function return or as the dwParam2 parameter in the _REPLY message. The value 0, which indicates success, will only be returned in the LINE_REPLY message, and never as the function's return value.

The initialize functions (lineInitializeEx and phoneInitializeEx) tell TAPI how to send these messages to the application.

Note In some cases, if a multithreaded application calls an asynchronous function from a thread other than the thread from which the application initialized the line or phone device, the application may receive the LINE_REPLY or PHONE_REPLY message before the asynchronous function has returned. In such cases, the application should save the message parameters and wait until the asynchronous function returns and the request ID is known before processing the message.

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