Re: [Qemu-devel] [PATCH 5/9] qapi_sized_buffer

[Stefan Berger <stefanb@linux.vnet.ibm.com>]

On 03/13/2013 07:18 PM, mdroth wrote:
> On Wed, Mar 13, 2013 at 06:00:24PM -0400, Stefan Berger wrote:
>> On 03/13/2013 04:52 PM, mdroth wrote:
>>
> Visitors don't have any knowledge of the data structures they're visiting
> outside of what we tell them via the visit_*() API.
>
> [...]
>
> For example, a visitor for a 16-element array of:
>
> typedef struct ComplexType {
>      int32_t foo;
>      char *bar;
> } ComplexType;
>
> would look something like:
>
> visit_start_carray(v, ...); // instruct visitor how to calculate offsets
> for (i = 0; i < 16; i++) {
>      visit_type_ComplexType(v, ...) // instruct visitor how to handle elem
>      visit_next_carray(v, ...); // instruct visitor to move to next offset
> }
> visit_end_carray(v, ...); // instruct visitor to finalize array

Given this example above, I think we will need the sized buffer. The 
sized buffer targets  binary arrays and their encoding. If I was to 
encode an 'unsigned char[n]' (e.g., n=200) using n, or n/2 or n/4 loops 
like above breaking it apart in u8, u16 or u32 respectively I think this 
would 'not bed good' also considering the 2 bytes for tag and length 
being added by ASN.1 for every such datatype (u8,u16,u32). The sized 
buffer allows you to for example take a memory page and write it out in 
one chunk adding a few bytes of ASN.1 'decoration' around the actual data.

    Stefan