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[Bug libc/4977] SEGV in strlen() of string argument of vsnprintf call on RHEL WS3/64-bit


------- Additional Comments From timp at pulsic dot com  2007-08-29 16:34 -------
(In reply to comment #3)
> Passing it once to some function is just fine.  What your testcase does
> wrong is passing the same va_list object to another function, while the standards
> say that if you pass it to some function and that function uses va_arg on it,
> then the object can be only passed to va_end, nothing else.
> On some architectures it can work even multiple times, 

yep - that's what I was alluding too - poor language on my part 

> that's mainly
> architectures which define va_list as a pointer (e.g. i386), but note
> that doing so is highly unportable.  

indeed - it's one thing that I hadn't fully appreciated until now

> On other architectures, va_list as is
> a one element array containing some structure (e.g. x86_64).  If you pass
> that to some function, you just pass its address to it, the object actually
> resides in the function which invoked va_start or va_copy, if their argument
> is an automatic va_list variable.  So, the first vsnprintf will modify that
> va_list object to point after all arguments it consumed and when you call
> vsnprintf again with the same va_list (== the same address of some automatic
> struct), it will start looking at garbage.

ah - I've just realised that I missed the requirement for the va_end after the
first vsnprintf() from my reading of the specification - thanks for the
clarification. As with many things, now that it's been pointed out to me, it
does seem  obvious :)

I sit enlightened.
Cheers Jakub

Tim


-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=4977

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