Re: Possible bug: AC_USE_SYSTEM_EXTENSIONS ignores "AS_IF" conditionals

[Eric Blake]

On 09/20/2014 06:19 PM, David A. Wheeler wrote:
> I *think* that AC_USE_SYSTEM_EXTENSIONS has a bug; it seems to ignore 
> conditionals.

Not a bug, but a feature.

> 
> E.G., given this configure.ac:
> ====
> AC_PREREQ([2.63])
> AC_INIT([bug], [0.01])
> AC_CONFIG_HEADERS([config.h])
> AS_IF([test 0 = 1 ],

The whole POINT of AS_IF is to allow macros to hoist pre-req code that
must be run unconditionally to occur before the if statement.

>  [AC_USE_SYSTEM_EXTENSIONS])

and AC_USE_SYSTEM_EXTENSIONS intentionally defines its body to be a
prerequisite that is hoisted outside the body of AS_IF or any other
macro that uses AC_REQUIRE.  It is a feature that unconditional code is
hoisted like this.

> AC_OUTPUT
> ====
> 
> The file "config.h" still enables extensions, even though the test is false.

That's because of the way the macros are designed to be expanded.  You
are NOT writing:

if test 0 = 1; then
   shell code to turn on conditionals
fi

rather, you are writing:

prereqs of any code embedded in the AS_IF, including the
AC_USE_SYSTEM_EXTENSIONS body that unconditionally turns on extensions
if test 0 = 1; then
  any conditional remains (of which AC_USE_SYSTEM_EXTENSIONS has none)
fi

> 
> I'm trying to conditionally use extensions like this:
>  AS_IF([test "$enable_all_system_extensions" = "yes"],
>   [AC_USE_SYSTEM_EXTENSIONS])
> 
> Am I going about this the wrong way?

I don't think what you are asking for is possible.  Either you want to
use extensions (and just use the macro) or you don't.  I don't really
see a use case for wanting extensions on only some of the builds.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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