Path: utzoo!attcan!uunet!munnari!murtoa.cs.mu.oz.au!cs.mu.oz.au!kre
From: kre@cs.mu.oz.au (Robert Elz)
Newsgroups: comp.bugs.4bsd
Subject: Re: ungetc will put characters back into an _IOSTRG input stream
Keywords: writable strings, shared strings (text segment), sscanf, ungetc
Message-ID: <1231@murtoa.cs.mu.oz.au>
Date: 13 Feb 89 17:59:46 GMT
References: <20282@lll-winken.LLNL.GOV>
Sender: news@cs.mu.oz.au
Lines: 20

In article <20282@lll-winken.LLNL.GOV>, casey@lll-crg.llnl.gov (Casey Leedom) writes:
> Description:
> 	Ungetc will put characters back into an _IOSTRG input stream. 
> Fix:
> 	Debatable.

Maybe better to have ungetc() look at the character that's in the
buffer - if its the same as the one being pushed back (which it
always will be in sscanf()) then don't do the write.  ungetc() is
rare enough that the extra mem read & test aren't going to matter.

That way you don't get a segmentaion violation with write only strings
if you're just doing sscanf(), but you will if you try random other
ungetc's on a string (somehow).

You may also win doing this with normal writeable buffers - if the buffer
is currently mapped shared read, copy on write, you may be able to avoid the
copy (yes, I know, that will happen once a century...)

kre