Re: [Qemu-devel] [PULL for-1.7 v2 3/6] acpi-build: fix build on glib < 2.22

[Laszlo Ersek <lersek@redhat.com>]

On 11/25/13 21:54, Michael S. Tsirkin wrote:
> On Tue, Nov 26, 2013 at 06:37:43AM +1000, Richard Henderson wrote:
>> On 11/26/2013 06:31 AM, Michael S. Tsirkin wrote:
>>> On Tue, Nov 26, 2013 at 06:24:53AM +1000, Richard Henderson wrote:
>>>> On 11/25/2013 09:48 PM, Michael S. Tsirkin wrote:
>>>>> g_string_vprintf was only introduced in 2.24 so switch to vsnprintf
>>>>> instead.  A bit uglier but name size is fixed at 4 bytes here so it's
>>>>> easy.
>>>>
>>>> You list 2.24 here,
>>>>
>>>>> -    GString *s = g_string_new("");
>>>>> +    /* It would be nicer to use g_string_vprintf but it's only there in 2.22 */
>>>>
>>>> ... 2.22 here.
>>>>
>>>> But https://developer.gnome.org/glib/2.28/glib-Strings.html#g-string-vprintf
>>>>
>>>> says "since 2.14".
>>>>
>>>>> +    char s[] = "XXXX";
>>>>
>>>> char s[5];
>>>>
>>>> Initializing it is a waste of time.
>>>>
>>>>
>>>> r~
>>>
>>> It's sets the length in a nice way.
>>>
>>
>> Then do something like
>>
>>   char s[sizeof("XXXX")];
>>
>> so that the actual initialization doesn't happen.
>>
>>
>> r~
> 
> Why? As an optimization?
> I'm not quite sure this doesn't mean we are using VLA which I'd rather not.
> Would need to look at language spec ... simple initialization is shorter
> and more obviously correct.

No, it's not a VLA.

C99 6.7.5.2 Array declarators, p4:

    [...] If the size is an integer constant expression and the element
    type has a known constant size, the array type is not a variable
    length array type; [...]

6.6 Constant expressions, p6:

    An integer constant expression [...] shall have integer type and
    shall only have operands that are [...] sizeof expressions whose
    results are integer constants, [...]

6.5.3.4 The sizeof operator, p2:

    [...] The result is an integer. If the type of the operand is a
    variable length array type, the operand is evaluated; otherwise,
    the operand is not evaluated and the result is an integer constant.

6.4.5 String literals, p2:

    [...] The multibyte character sequence is then used to initialize
    an array of static storage duration and length just sufficient to
    contain the sequence. [...]

6.7.5.2 Array declarators, p2:

    [...] If an identifier is declared to be an object with static
    storage duration, it shall not have a variable length array type.

6.7.5.2 Array declarators, p10:

    EXAMPLE 4 [...] Array objects declared with the static or extern
    storage-class specifier cannot have a variable length array (VLA)
    type. [...]

So,

    char s[sizeof("XXXX")]
                  ^^^^^^
                  string literal, static storage duration, not a VLA

           ^^^^^^^^^^^^^^
           operand not evaluated, result is integer constant
           ... which qualifies as integer constant expression

    ^^^^^^^^^^^^^^^^^^^^^^
    size is an integer constant expression,
    element type has a known constant size: not a VLA

(Admittedly, EXAMPLE 4 in 6.7.5.2 Array declarators, p10, is informative
(not normative), and 6.7.5.2 Array declarators, p2, speaks about an
"identifier". We don't have an identifier for "XXXX", but I think we can
still derive that static storage duration implies non-variable length
for the array that holds the string.)

Laszlo