[PATCH] open_memstream() and open_wmemstream()

Jilles Tjoelker jilles at stack.nl
Thu Feb 7 21:12:25 UTC 2013

On Tue, Feb 05, 2013 at 03:46:43PM -0500, John Baldwin wrote:
> I've written an implementation of open_memstream() and
> open_wmemstream() along with a set of regression tests.  I'm pretty
> sure open_memstream() is correct, and I believe open_wmemstream() is
> correct for expected usage.  The latter might even do the right thing
> if you split a multi-byte character across multiple writes.  One
> question I have is if my choice to discard any pending multi-byte
> state in the stream anytime a seek changes the effective position in
> the output stream.  I think this is correct as stdio will flush any
> pending data before doing a seek, so if there is a partially parsed
> character we aren't going to get the rest of it.

I don't think partially parsed characters can happen with a correct
application. As per C99, an application must not call byte output
functions on a wide-oriented stream, and vice versa.

Discarding the shift state on fseek()/fseeko() is permitted (but should
be documented as this is implementation-defined behaviour).
State-dependent encodings (where this is relevant) are rarely used

The conversion to bytes and back probably makes open_wmemstream() quite
slow but I don't think that is very important.

> http://www.FreeBSD.org/~jhb/patches/open_memstream.patch

The seek functions should check for overflow in the addition (for
SEEK_CUR and SEEK_END) and the conversion to size_t.

Jilles Tjoelker

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