Re: Extending configure.ac to check for C++ header only libraries

[Kai Torben Ohlhus]

On 5/22/20 5:25 PM, "Markus Mützel" wrote:
> Am 22. Mai 2020 um 09:40 Uhr schrieb "Kai Torben Ohlhus":
>> On 5/21/20 10:51 AM, Abdallah Elshamy wrote:
>>> Greetings,
>>>
>>> I was trying to add RapidJSON [1] to Octave. At first, I thought about
>>> using AC_CHECK_LIB (after some reading in configure.ac
>>> I noticed that Octave uses the custom macro
>>> OCTAVE_CHECK_LIB which I think is the same thing with some extra
>>> parameters.) But, since RapidJSON [1] is a header-only library, I
>>> thought about using AC_CHECK_HEADER. Which option is more preferable? 
>>>
>>> Also, should I stop the configure script (with AC_MSG_ERROR for example)
>>> If the library is not found or should I disable the functions ? (I think
>>> the latter is better. If it is, any source on how to do this will be
>>> helpful.)
>>>
>>> Thanks in advance,
>>> Abdallah
>>>
>>
>> Dear Abdallah,
>> Dear maintainers,  (please read below)
>>
>> Good question.  RapidJSON should be an optional dependency, thus a
>> warning shall be issued, if not available.  Later in your implementation
>> you wrap your code with
>>
>>    #if defined (HAVE_RAPIDJSON)
>>
>>      // RapidJSON dependent code
>>
>>    #else
>>
>>      err_disabled_feature ("RapidJSON", "JSON library");
>>
>>    #endif
>>
>> to avoid unexpected usage.
>>
>> For the rest, I think OCTAVE_CHECK_LIB is the right way
>> to go.  This macro is documented here [2] and does everything needed,
>> but a little too much :(
>>
>> I tested a small code (AC_LANG_PUSH thanks to StackOverflow [3]):
>>
>>
>>   ### Check for RAPIDJSON (C++ header only) library.
>>
>>   AC_LANG_PUSH([C++])
>>   OCTAVE_CHECK_LIB(rapidjson, RAPIDJSON,
>>     [RAPIDJSON library not found.  Octave will not be able to read and
>>      write JSON files.],
>>     [rapidjson/rapidjson.h])
>>   AC_LANG_POP([C++])
>>
>>
>> but it is still not working, as the default macro wants to check for a
>> linkable library, i.e. "-lrapidjson".  This of course fails.
>>
>> For now you can assume, that `#include "rapidjson/rapidjson.h"` will
>> work and ignore the "configure.ac" part.  Later when merging your
>> contribution, this becomes important.
>>
>>
>>
>> Dear maintainers,
>>
>> Stated the problem above: How do we properly check for a C++ header only
>> library?
>>
>> 1. General solution: Copy OCTAVE_CHECK_LIB to OCTAVE_CHECK_HEADER_LIB
>> and to remove the unnecessary linking check parts.
>>
>> 2. RapidJSON solution: Copy necessary parts from OCTAVE_CHECK_LIB.
>>
>> 3. Did I miss a better option? ;-)
> 
> I think that for header only libraries, you should use AC_CHECK_HEADERS - 
> like Abdallah suggested.
> See e.g. the checks for (alternative) termio headers:
> ## Find a termio header to include.
> 
> AC_CHECK_HEADERS([termios.h], have_termios_h=yes, have_termios_h=no)
> AC_CHECK_HEADERS([termio.h], have_termio_h=yes, have_termio_h=no)
> AC_CHECK_HEADERS([sgtty.h], have_sgtty_h=yes, have_sgtty_h=no)
> AC_CHECK_HEADERS([conio.h], have_conio_h=yes, have_conio_h=no)
> 
> Is there a reason this won't work for RapidJSON?
> 
> Markus
> 


Thank you for looking into it Markus.  By using AC_CHECK_HEADERS only,
we do not get the configure options like

   ./configure --with-rapidjson-includedir=

automatically, right?  Thus, copying the AC_CHECK_HEADERS macro and
other necessary parts from OCTAVE_CHECK_LIB seems like my second
suggestion seems more appropriate.

Kai