UB status of snprintf on invalid ptr+size combination?

[Stephan Bergmann]

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.