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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.03.12 at 10:02, Roger Pau MonnÃ<roger.pau@xxxxxxxxxxxxx> wrote:
> 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 ();
>>  ;
>>  return 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.

Okay - ac_fn_c_check_header_mongrel just doesn't tell me anything.
Sorry for being dense.

Jan

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