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manual: Explain our implementation-defined memstream semantics 

Posix Issue 8 adds an implementation-defined item we don't already
cover, about seeking backwards.  This defines our implentation.

https://issues.redhat.com/browse/RHEL-3008
https://pubs.opengroup.org/onlinepubs/9799919799/functions/open_memstream.html

Reviewed-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by Collin Funk <collin.funk1@gmail.com>
This commit is contained in:
DJ Delorie 2025-09-18 19:13:32 -04:00
parent 85c5b504aa
commit 8aa99c592b
1 changed files with 13 additions and 5 deletions
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18
manual/stdio.texi
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@ -5135,13 +5135,21 @@ remain valid only as long as no further output on the stream takes
place. If you do more output, you must flush the stream again to store
new values before you use them again.
A null character is written at the end of the buffer. This null character
is @emph{not} included in the size value stored at @var{sizeloc}.
A null character is written at the end of the buffer when the stream
is flushed, and at the current location when closed (these locations
may be different if @code{fseek} is used). This null character is
@emph{not} included in the size value stored at @var{sizeloc}.
You can move the stream's file position with @code{fseek} or
@code{fseeko} (@pxref{File Positioning}). Moving the file position past
the end of the data already written fills the intervening space with
zeroes.
@code{fseeko} (@pxref{File Positioning}). Moving the file position
past the end of the data already written fills the intervening space
with zeroes. Note that seeking backwards into existing written data
will change the effective ``end of file'' used by @code{fflush} and
@code{SEEK_END} (and where the trailing null character is written,
when closed). Thus, if you wish to do ``random-access'' I/O in a
memstream, it's important to use @code{fseek} to move the file
position to the desired data end (using @code{SEEK_POS}) before
closing it.
@end deftypefun
Here is an example of using @code{open_memstream}: