On Tue, 2021-05-18 at 10:55 -0500, Peng Yu via Gcc-help wrote:
> Hi,
>
> NULL (0) is a commonly used value for an invalid memory address. I
> also see -1 is used for an illegal memory address.
>
> Besides those values, I'd suspect that other values that are close to
> 0 (both negatives and positives) won't appear in real programs.
>
> Is there a limit on the safe range that can be assumed to not be used
> by user level code as valid addresses?
Using a value from dereferencing NULL is always undefined behavior.
Otherwise, it depends on platform, OS, and system configuration.
For example, on x86_64 Linux with 4-level page table [1] the userspace
address space is 47-bit. So in non-kernel code any pointers >= 2^{47}
can be considered invalid.
For another example, on 32-bit x86 Linux with KPTI enabled, a valid
userspace pointer can be very close to 0xFFFFFFFF. I'm not sure how
close, but I've observed some problem [2] caused by this.
Again, this question is off-topic. I can't see how this is related to
GCC anyway.
[1]: https://www.kernel.org/doc/html/latest/x86/x86_64/mm.html
[2]: https://github.com/python/cpython/pull/13205
--
Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University
