Re: [Xen-devel] [PATCH v2 1/3] livepach: Add .livepatch.hooks functions and test-case

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.08.16 at 12:56, <ian.jackson@xxxxxxxxxxxxx> wrote:
> Jan Beulich writes ("Re: [Xen-devel] [PATCH v2 1/3] livepach: Add 
> .livepatch.hooks functions and test-case"):
>> On 10.08.16 at 11:46, <JBeulich@xxxxxxxx> wrote:
>> > Odd. I've tried this simple example:
>> > 
>> > typedef int fn_t(void);
> ...
>> >    const fn_t**cfn;
> 
> Ie,
>         const int **cfn(void);
> 
>> >    for(i = 0; !rc && i < ps->n; ++i)
>> >            rc = ps->cfn[i]();
> 
> From `(gcc-4)Function Attributes':
> 
>   `const'
>        Many functions do not examine any values except their arguments,
>        and have no effects except the return value.  Basically this is
>        just slightly more strict class than the `pure' attribute below,
>        since function is not allowed to read global memory.
> 
>        Note that a function that has pointer arguments and examines the
>        data pointed to must _not_ be declared `const'.  Likewise, a
>        function that calls a non-`const' function usually must not be
>        `const'.  It does not make sense for a `const' function to return
>        `void'.
> 
>        The attribute `const' is not implemented in GCC versions earlier
>        than 2.5.  An alternative way to declare that a function has no
>        side effects, which works in the current version and in some older
>        versions, is as follows:
> 
>             typedef int intfn ();
> 
>             extern const intfn square;
> 
>        This approach does not work in GNU C++ from 2.6.0 on, since the
>        language specifies that the `const' must be attached to the return
>        value.
> 
> Ie, gcc has always treated a function marked const as having no
> unexpected inputs and no side effects.

Oh, I've always assumed that would be __attribute__((const))
only, but what you quote above proves me wrong.

Jan


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