Re: [Xen-devel] [PATCH] autoconf: add missing compression checks for libxc

[Roger Pau Monnà <roger.pau@xxxxxxxxxxxxx>]

2012/3/7 Jan Beulich <JBeulich@xxxxxxxx>:
>>>> On 06.03.12 at 18:27, Roger Pau MonnÃ<roger.pau@xxxxxxxxxxxxx> wrote:
>> 2012/3/6 Jan Beulich <JBeulich@xxxxxxxx>:
>>>>>> On 06.03.12 at 16:01, Roger Pau MonnÃ<roger.pau@xxxxxxxxxxxxx> wrote:
>>>> 2012/3/6 Jan Beulich <JBeulich@xxxxxxxx>:
>>>>> ... are you certain that checking for these symbols in the respective
>>>>> libraries suffices? The old check really was verifying corresponding
>>>>> header files' presence.
>>>>
>>>> I think it's best to do it this way, what this tests do is try to
>>>> compile a sample program that uses the library, which is much more
>>>> closer to the reality than just checking for a header existence. In a
>>>> strange case you might have the header files, but maybe the library is
>>>> broken, so we can not use it. What I can do if you want is check for
>>>> the header files also, something like this?
>>>
>>> Hmm, no - I was rather thinking of a real usability test (which implies
>>> existence), i.e. include the header in the test source snippet. This is
>>> what really matches reality.
>>
>> The last patch does this more or less, it's just that not on the same
>> test. If headers are found and are usable then we perform a library
>> check, basically it does the same as the previous test found in
>> tools/libxc/Makefile but also checks that the library is usable for
>> linking.
>
> Not really - this is an example of what is being tried to get compiled
> and linked:
>
> #ifdef __cplusplus
> extern "C"
> #endif
> char BZ2_bzDecompressInit ();
> int
> main ()
> {
> return BZ2_bzDecompressInit ();
> Â;
> Âreturn 0;
> }
>
> - no inclusion of the header in question, using a bogus prototype
> instead.

This is done by AC_CHECK_LIB, the AC_CHECK_HEADER macro, which calls
the function "ac_fn_c_check_header_mongrel", tries to compile a sample
test that includes the requested header. So I think both tests are
done, it's just that not at the same time.

> Jan

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