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Re: [Qemu-devel] [PATCH 01/23] memory: introduce memory_region_find()

From: Avi Kivity
Subject: Re: [Qemu-devel] [PATCH 01/23] memory: introduce memory_region_find()
Date: Mon, 19 Dec 2011 17:17:37 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:8.0) Gecko/20111115 Thunderbird/8.0

On 12/19/2011 05:10 PM, Anthony Liguori wrote:
> On 12/19/2011 09:04 AM, Avi Kivity wrote:
>> On 12/19/2011 04:50 PM, Anthony Liguori wrote:
>>>> +static int cmp_flatrange_addr(const void *_addr, const void *_fr)
>>>> +{
>>>> +    const AddrRange *addr = _addr;
>>>> +    const FlatRange *fr = _fr;
>>> Please don't prefix with an underscore.
>> Why not?  It's legal according to the standards, if that's your concern
>> (only _[_A-Z]+ are reserved).
> http://www.gnu.org/s/hello/manual/libc/Reserved-Names.html
> "In addition to the names documented in this manual, reserved names
> include all external identifiers (global functions and variables) that
> begin with an underscore (‘_’)"
> Now that might just mean that you're shadowing a global name, so maybe
> that's okay, but I think it's easier to just enforce a consistent rule
> of, "don't start identifiers with an underscore".

I rather like the pattern

   void callback(void *_foo)
         Foo *foo = _foo;


If the consensus is that we don't want it, I'll change it, but I do
think it's useful.

>>>> +MemoryRegionSection memory_region_find(MemoryRegion *address_space,
>>>> +                                       target_phys_addr_t addr,
>>>> uint64_t size);
>>> Returning structs by value is a bit unexpected.
>> It's just prejudice, here's the call sequence:
> It's not about whether it's fast or slow.  It's just unexpected.

It's plain C, I don't see why it's so unexpected.

> Instead of returning a NULL pointer on error, you set .mr to NULL if
> an error occurs (which isn't documented by the comment btw). 
> Returning a pointer, or taking a pointer and returning a 0/-1 return
> value makes for a more consistent semantic with the rest of the code
> base.

How can I return a pointer?  If I allocate it, someone has to free it. 
If I pass it as a parameter, we end up with a silly looking calling
convention.  If I return an error code, the caller has to set up an
additional variable.

The natural check is section.size which is always meaningful -
memory_region_find() returns a subrange of the input, which may be
zero.  You're right that I should document it (and I should use it

error compiling committee.c: too many arguments to function

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