Re: How to exit if program is not found

[Eric Blake]

On 06/15/2012 07:16 AM, Timothy Madden wrote:

>>
>> How about the predefined programs ?
>>
>> AC_PROG_YACC might set YACC to 'bison -y', how would I check that 'bison
>> -y' is found or is on path ?

If AC_PROG_YACC set YACC to 'bison -y', then 'bison' is on PATH.  Only
if $YACC ends up as 'yacc' do you run the risk that AC_PROG_YACC didn't
find anything.  But since 'yacc' is required to be on all POSIX-y
systems and is listed as one of the portable programs that the GNU
Coding Standards states that you can assume to exist, this shouldn't
matter in practice.

> 
> I find that the following command
>     command -v bison -y
> works as expected and returns 0 if bison is found or non-zero if there
> is no bison.
> 
> What do you know about the portability of `command` ? Can I use it in my
> configure.ac ?

Not portable.  Sorry.

> 
> Or is there a better way to exit after AC_PROG_YACC if bison is not
> found ? Or to exit if AC_PROG_CXX does not find a C++ compiler ?

Based on the documentation, AC_PROG_YACC looks for bison, and falls back
to yacc (but without checking for yacc).  So the test would be:

AC_PROG_YACC
if test "$YACC" = yacc; then
  AC_MSG_ERROR([this project insists on using bison instead of yacc])
fi

Based on the documentation in current autoconf, AC_PROG_CXX looks for
several compilers, then falls back to 'g++'; but it also sets GXX to yes
if it found a working g++ (as opposed to leaving GXX alone when using
the fallback value); other versions of autoconf and/or combinations with
automake or libtool have set GXX to 'no' rather than 'g++' as a more
useful fallback when no C++ compiler is present.  So that would
translate to something like:

AC_PROG_CXX
if test "$CXX" = no || test "$CXX:$GXX" = "g++:"; then
  AC_MSG_ERROR([no C++ compiler found])
fi

> 
>>
>> And really, is there no support in autoconf itself for this ?
> 
> Is there no flag or option to make the resulting configure exit
> automatically if any of the checks fail ?

No, because blindly failing is not helpful to people that can get by
without yacc or c++.  In other words, the default behavior is useful to
some packages, and the building blocks are in place for you to manually
fail with your own AC_MSG_ERROR if you don't like the default; but
failing by default would prevent the packages that can fall back to
useful behavior without the tools.  It's not that much harder to add
your own AC_MSG_ERROR condition after testing for a tool.

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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