[PATCH] Cygwin: mmap: allow remapping part of an existing anonymous, mapping

[Ken Brown]

On 1/8/2025 10:24 AM, Corinna Vinschen wrote:
> On Jan  7 21:27, Ken Brown wrote:
>> On 1/7/2025 3:18 PM, Corinna Vinschen wrote:
>>> - mmap_record::prot flag, should be an array of protection bits per page
>>>     (POSIX page i e., 64K, not Windows page).
>>
>> Question: Since it only takes 3 bits to store all possible protections, do
>> you think it's worth the trouble to pack the protections, so that each byte
>> stores the protection bits for 2 pages?  Or should I just use an array of
>> unsigned char, with 1 byte for each page?  Or did you have something else in
>> mind?
> 
> I hadn't thought deeply about this.  I had a vague notion of a ULONG
> array to match windows protection bits, but, as you note above, we
> really only need 3 bits.
> 
> I don't think we have to define this as a bit field array, given this
> isn't readily available in C and you would have to add bitfield
> arithmetic by yourself.  So, yeah, a char or maybe better uint8_t
> might be the best matching type here.
Another question: Adding this array to mmap_record, we have two flexible 
arrays in the class: one for page_map and one for the protection array. 
My understanding is that a class or struct can have only one flexible 
array member, and it has to be at the end.  What's the best way to deal 
with that?  The only thing I can think of is to use a pointer instead of 
an array for the protections, and then allocate memory for it separately 
when an mmap_record is created.  Or is there a better way?

Thanks.

Ken