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docs: correct description of file-position

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Explain more when a file's size is changed by `file-position`.
This commit is contained in:
Matthew Flatt 2019-06-24 15:28:22 -06:00
parent 0d3d0d7d86
commit 87673902dc
1 changed files with 10 additions and 5 deletions
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15
pkgs/racket-doc/scribblings/reference/port-buffers.scrbl
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@ -99,11 +99,16 @@ called with a position argument on such a @tech{file-stream port}, the
@exnraise[exn:fail:filesystem].
When @racket[file-position] sets the position @racket[pos] beyond the
current size of an output file or (byte) string, the file/string is enlarged
to size @racket[pos] and the new region is filled with @racket[0]
bytes. If @racket[pos] is beyond the end of an input file or (byte) string,
then reading thereafter returns @racket[eof] without changing the
port's position.
current size of an output file or (byte) string, the file/string is
enlarged to size @racket[pos] and the new region is filled with
@racket[0] bytes; in the case of a file. In the case of a file output
port, the file might not be enlarged until more data is written to the
file; in that case, beware that writing to a file opened in
@racket['append] mode on Unix and Mac OS will reset the file pointer
to the end of a file @emph{before} each write, which defeats file
enlargement via @racket[file-position]. If @racket[pos] is beyond the
end of an input file or (byte) string, then reading thereafter returns
@racket[eof] without changing the port's position.
When changing the file position for an output port, the port is first
flushed if its buffer is not empty. Similarly, setting the position