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

[Vincent Lefevre <vincent@vinc17.net>]

On 2023-03-20 13:31:33 -0400, Siddhesh Poyarekar wrote:
> In glibc we tend to skip a test as UNSUPPORTED when resources to run a test
> are considered uncommon (from the perspective of a test system) and cannot
> be expected to be met in all environments.

In MPFR, we have some tests that need much memory, which are enabled
only if the MPFR_CHECK_LARGEMEM environment variable is set. But this
means that some issues would not be detected on particular platforms
by end users (MPFR_CHECK_LARGEMEM is mainly for developers).

> It looks like mpfr_snprintf and glibc snprintf are incompatible in this
> context and the latter should not be used to implement the former if the n >
> __bos(dest) use case is important to it.
> 
> Then again, I wish mpfr_snprintf also similarly tightened its requirements,
> but that's a discussion for another day.

Well, if the C standard is changed, I suppose that we will align to
it. We can still recommend not to use n > buffer size if this is not
supported by snprintf (even though the MPFR code is currently
guaranteed to work in this case).

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