Hi Boye,
* Boye Annfelt Høverstad wrote on Fri, May 12, 2006 at 09:57:10AM CEST:
>
> The macro AC_CHECK_LIB seems to malfunction if it is first used inside a
> conditionally executed block.
>
> My relatively uninformed guess is that the first occurrence of
> AC_CHECK_LIB is translated in the configure script to something that
> defines certain variables. If the condition guarding the first call to
> AC_CHECK_LIB is false, the variables are never initialized correctly and
> the subsequent call to AC_CHECK_LIB will fail.
Your guess is pretty close. :-)
> AC_PREREQ(2.50)
> AC_INIT([testprogram], [1], [name])
> AC_CONFIG_AUX_DIR([config])
> AM_INIT_AUTOMAKE([1.9 foreign])
> AC_CONFIG_SRCDIR([main.cpp])
> AC_CONFIG_HEADER([config.h])
>
> dnl AC_CHECK_LIB([BOGUS], [dummy])
>
> if test "x$TEST" = "xtrue"; then
> AC_MSG_RESULT([The test is true])
> AC_CHECK_LIB([BOGUS], [dummy])
> else
> AC_MSG_RESULT([The test is false])
> AC_CHECK_LIB([BOGUS], [dummy])
> fi
The problem here is that AC_CHECK_LIB internally uses some other macros,
and some of those macro require other macros to be called before that.
Autoconf has a mechanism to ensure those requirements are met (once):
AC_REQUIRE. The mechanism works a bit differently from what you would
expect, though: if you call a macro M1, and that requires another macro
M2, Autoconf checks if it has already expanded M2 anywhere; if not, it
expands M2 right before M1. So that will lead to
if ... then
...
expansion of M2
expansion of AC_CHECK_LIB
else
...
expansion of AC_CHECK_LIB
fi
See how the fact that M2 will be expanded only once interacts badly with
shell conditionals?
With the next Autoconf version, there will be a way out that is a bit
cleaner:
AS_IF([test "x$TEST" = xtrue],
[AC_MSG_RESULT([The test is true])
AC_CHECK_LIB([BOGUS], [dummy])],
[AC_MSG_RESULT([The test is false])
AC_CHECK_LIB([BOGUS], [dummy])])
as that will then lead to
expansion of M2
if ... then
...
expansion of AC_CHECK_LIB
else
...
expansion of AC_CHECK_LIB
fi
AS_IF already exists in Autoconf 2.59, but only in 2.60 will it have the
property that it will cause expansion of required macros from macros
inside its arguments to happen _before_ the expansion of the if clause.
*snip*
> AC_CONFIG_FILES([Makefile])
> AC_PROG_CXX
> AC_OUTPUT
The checks for compilers should be done early in the configure.ac
script. I think a suggested order is mentioned in the manual, though.
There is another potential issue here: do you want the library tests to
be done with the C or the C++ compiler? If the former, you should
AC_PROG_CC
AC_PROG_CXX
before doing the library checks, if the latter, you should do at least
AC_PROG_CXX
AC_LANG([C++])
but for some other tests a C compiler may be required anyway, so it's
prudent to check for it as well.
> A workaround is to put an AC_CHECK_LIB before the conditional block, for
> instance by uncommenting the dnl-line in the script above.
Yep. But you don't need that anymore now.
> A note about this in the manual could perhaps be an easy fix to the
> problem.
Well, it's definitely not easy to see all of this unless you read the
whole manual and also have some experience, I agree. OTOH, if you had
followed the order of
info Autoconf "configure.ac Layout"
this wouldn't have hit you. :-)
Cheers,
Ralf
