On 02/27/2014 01:18 PM, Cody Rigney wrote:
>> On Fri, Feb 21, 2014 at 10:02 AM, Andrew Haley <aph@xxxxxxxxxx> wrote:
>>> It's hard for me to give a 100% answer to that one, but GCC has an
>>> idea what memory is reachable from every pointer. So, this won't
>>> clobber memory that's unreachable or has a different type from that
>>> pointer. It probably doesn't matter.
>>>
>
> Doing "=m" (*a) in the output operand worked! And so did "memory" to
> the clobber list. Thanks!
I have been informed that this is wrong, sorry.
The correct way to do it is like so:
---------------------------------------
If you know how large the accessed memory is, you can add it as input or
output but if this is not known, you should add `memory'. As an example,
if you access ten bytes of a string, you can use a memory input like:
{"m"( ({ struct { char x[10]; } *p = (void *)ptr ; *p; }) )}.
---------------------------------------
If you use a zero-length array in the struct (i.e. char x[0]) that will
effectively do the job if you don't know how large the array is, but there
seems to be some doubt whether this is guaranteed. A memory clobber really
is guaranteed, but seems a bit like overkill.
Andrew.
