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

[Vincent Lefevre <vincent@vinc17.net>]

About

  snprintf(buf, INT_MAX, "%s", "Hello world");

On 2023-03-14 14:39:42 -0700, Paul Eggert wrote:
> On 3/14/23 12:47, Simon Chopin via Libc-alpha wrote:
> > When the issue was first discovered[1], I didn't raise the issue because I
> > dismissed it as UB, but it reappeared in an unrelated context, and
> > colleagues pointed out that the wording in the standard doesn't actually
> > say that the `n` argument is the size of the array.
> 
> That sounds like a misreading of the C standard. Even though the standard
> often does not explicitly say that a size argument is the size of an array,
> it's obvious from context that this is intended. So Florian is correct here
> that the call with INT_MAX is not portable C code.

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.

In practice, it is possible that the check of the buffer size has
already been done somewhere else, so that using a larger value for n
is fine.

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Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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