[Bug tree-optimization/104969] Likely a false positive of -D_FORTIFY_SOURCE=3

[siddhesh at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104969

--- Comment #2 from Siddhesh Poyarekar <siddhesh at gcc dot gnu.org> ---
(In reply to Martin Liška from comment #0)
> The original code is defective a bit as it wrongly assumes that
> (char*)str + (2 * i) is at maximum 'len' big. It's actually len - (2 * i)
> big. But it should be still valid code, am I right?

It doesn't overflow in this case, but specifying a length larger than the
actual buffer size is a standard violation.

"""
The snprintf() function shall be equivalent to sprintf(), with the addition of
the n argument which states the size of the buffer referred to by s. If n is
zero, nothing shall be written and s may be a null pointer. Otherwise, output
bytes beyond the n-1st shall be discarded instead of being written to the
array, and a null byte is written at the end of the bytes actually written into
the array.
"""

https://pubs.opengroup.org/onlinepubs/9699919799/functions/snprintf.html