Information Returned by Functions

An application receives two kinds of information as a result of a function call: the function's return value, and values written to data locations specified by the function's arguments.

If the function's return value is zero, the application knows that the function has completed synchronously. In this case, any values written as a result of the function call are reliable and can be used immediately. However, if the return value is positive, the function has not yet completed but it will complete asynchronously, at which time TAPI notifies the application by sending it an asynchronous reply message for the function. Once the application receives this message (and the message indicates success), any values returned by the function are considered to be reliable. However, before the message is received, the application should consider these values suspect and should not use them. Also, because asynchronous reply messages can take varying lengths of time to be sent, the application may not receive them in the same order in which it called their functions. This is why an application must retain the request IDs of its requests in progress so that it can identify and correctly respond to incoming asynchronous reply messages.

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