I am working with TFileStream in Delphi 2006. When I invoke TFileStream.Seek with an offset that is out of bounds I am getting different return values. When I seek to a position below the beginning of the stream, the function returns -1 and if I seek to a beyond the stream size, the function returns what would have been the position in the stream if the stream was that large. Is there a way to check whether a seek operation on the stream was successful? Why does TFileStream.Seek not fail when the seek offsets are out of bounds of the current stream size?
Thanks in advance.
Yes, you can seek beyond file size - where is no error here, the seek is successful. More than that, you can lock file region (see LockFile) beyond file size - that is also OK and is used by some RDBMS to implement table/record locking.
Also from MSDN:
It is not an error to set a file
pointer to a position beyond the end
of the file. The size of the file does
not increase until you call the
SetEndOfFile, WriteFile, or
WriteFileEx function. A write
operation increases the size of the
file to the file pointer position plus
the size of the buffer written, which
results in the intervening bytes
uninitialized.
So by setting a file pointer beyond the file size you can subsequently increase the file size (ex by the SetEndOfFile).
It calls a windows function and the result you get is from the windows function.
I would be inclined to check in your code if the Seek value is valid. If you need to do this alot then maybe create a descendant of TFileStream, something like TRangeCheckingFileStream which includes range checks in it's seek method and returns a value you can expect.
