Re: UB status of snprintf on invalid ptr+size combination?

[Vincent Lefevre <vincent@vinc17.net>]

On 2023-03-16 11:29:31 +0100, Stephan Bergmann wrote:
> On 15/03/2023 13:39, Vincent Lefevre wrote:
> > No, it is not obvious. If the C standard does not say that this is
> > the size of the array, then it does not have to be the size of the
> > array. The C standard just says:
> > 
> >    Otherwise, output characters beyond the n-1st are discarded rather
> >    than being written to the array, and a null character is written at
> >    the end of the characters actually written into the array.
> 
> But in 7.1.4 "Use of library functions" the standard also says
> 
> > If a function argument is described as being an array, the pointer
> > passed to the function shall have a value such that all address
> > computations and accesses to objects (that would be valid if the
> > pointer did point to the first element of such an array) are
> > valid.
> 
> which could be construed as meaning that the n-1st array element must always
> be accessible, even if a given invocation is known to always generate less
> then n output characters.

But the standard does not say that n is the size of the array.
The size of the array could be the maximum of n and the size
corresponding to the untruncated output string.

Similarly, for strncpy, I would not see n as the size of the arrays,
i.e. it is not allowed for the implementation to read characters
past a null character (possibly unless this does not have unwanted
effects), even though such characters would be among the first n
characters.

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