Re: snprintf, overlapping destination and source

[Julia Lawall <julia.lawall@lip6.fr>]

On Sat, 5 Dec 2015, Rasmus Villemoes wrote:

> I did a search for code doing
>
>   s[n]printf(buf, "...", ..., buf, ...)
>
> and found a few instances. They all do it with the format string
> beginning with "%s" and buf being passed as the corresponding parameter
> (obviously to append to the existing string). That works (AFAICT), both
> with the current printf implementation and with the string()
> modification which is now in -mm. It would obviously go horribly wrong
> if anything, even non-specifiers, precede the "%s" in the format
> string.
>
> The question is, do we want to officially support this particular case of
> overlapping src and dst? Or should we close our eyes and hope it will
> continue to work [1] and that it won't cause a caffeine-deprived hacker
> to accidentally think one could also prepend to a buffer by doing
> sprintf(buf, "...%s", ..., buf)? I'm actually surprised gcc doesn't warn
> about this.
>
> [1] Not that I can immediately think of a sane way to implement snprintf
> where it won't work, but you never know...
>
> My coccinelle-fu isn't sufficient to find cases where one of the buf
> instances is a more complicated expressions involving buf as a
> subexpression, as in
>
>   s[n]printf(buf, "...", ..., buf + 4, ...)
>
> or
>
>   s[n]printf(&buf[len], "...", ..., buf, ...)
>
> which would presumably always be wrong. Julia?

If you just want an argument expression that contains buf somewhere, you
can write <+...buf...+>.

julia