On Thu, Jul 31, 2025 at 12:03 PM zxuiji via Std-Proposals
<std-proposals_at_[hidden]> wrote:
>
> thiago I used NULL there to indicate what would be passed if something like the following occurred:
>
> int leng = 0, index = 0;
> char *text = foo(&leng,&index);
> // oops didn't check foo succeeded:
> snprintf( buff, max, ".*s", leng, text + index );

That's interesting, but it's still UB the moment `text+index` is
executed. Which happens *before* `snprintf` gets called.

> On Thu, 31 Jul 2025 at 16:59, Thiago Macieira via Std-Proposals <std-proposals_at_[hidden]> wrote:
>>
>> On Thursday, 31 July 2025 08:56:26 Pacific Daylight Time zxuiji via Std-
>> Proposals wrote:
>> > snprintf( buff, max, "%.*s", length, NULL + index. ) could catch the
>> > invalid pointer (so long as index is small) before a segfault can occur and
>> > if stdlib is not in debug mode then allow the segafault to occur, otherwise
>> > return -1 and set errno to some relevant value
>>
>> Do note that the program is UB before snprintf() was called.
>>
>> Could such a check function ever be used *before* UB happened?
>>
>> --
>> Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
>> Principal Engineer - Intel Platform & System Engineering
>>
>>
>>
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