Variably Sized Data Structures

When variably sized data structures are used to transmit information between TAPI and the application, the application is responsible for allocating the necessary memory. The amount of memory allocated must be at least large enough for the fixed portion of the data structure, and is set by the application in the dwTotalSize member of the data structure. The dwUsedSize and dwNeededSize members are filled in by TAPI. If dwTotalSize is less than the size of the fixed portion, then LINEERR/ PHONEERR_STRUCTURETOOSMALL is returned. If a function returns success, then all the fields in the fixed portion have been filled in. The dwUsedSize and dwNeededSize members can be compared to determine if all variable parts have been filled in, and how much space would be required to fill them all in.

If dwNeededSize is equal to dwUsedSize, then all fixed and variable parts have been filled in. If dwNeededSize is larger than dwUsedSize, some variable parts may have been filled in, but exactly which variably sized fields have been filled in is undefined. No variable part is ever truncated, and variable parts that would have been truncated due to insufficient space are indicated by having both of their corresponding "Offset" and "Size" parts set to zero. If these are not both zero (and no error was returned), they indicate the offset and size of valid, nontruncated variable-part data.

An application can always guarantee that all variable parts are filled in by allocating and indicating dwNeededSize bytes for the structure and calling the "Get" function again until the function returns success and dwNeededSize equals dwUsedSize. This should happen on the second try except for race conditions that cause changes in the size of variable parts between calls, which should be a rare occurrence.