What is the range of illegle memory addresses?

Peng Yu pengyu.ut@gmail.com
Tue May 18 21:35:55 GMT 2021

So, to write a portable C program, no values other than NULL should be
used as an invalid memory address?

On 5/18/21, Peter Bergner <bergner@linux.ibm.com> wrote:
> On 5/18/21 10:55 AM, Peng Yu via Gcc-help wrote:
>> 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?
> There is no such list that defines which virtual addresses are valid and
> which
> are not.  It depends on your OS, target, system config, etc.  I've heard of
> some OS systems where 0 was ok to access and NULL was defined to be some
> other
> non-zero value.
> Also, -1 is sometimes valid too. If you're running on a ppc64 (ie,
> big-endian)
> Linux kernel and execute a 32-bit application and your system has enough
> memory,
> the kernel will give you the full 4G virtual address space to your
> application.
> Peter


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