On Tue, 12 Apr 2016 11:05:51 -0400 Nick Bowler <nbowler@xxxxxxxxxx> wrote: > Hi, > > On 2016-04-12, YuGiOhJCJ Mailing-List <yugiohjcj-mailinglist@xxxxxxxxxxx> > wrote: > [snip exposition] > > So, I would like to call the AC_CHECK_HEADER macro on a condition: > > AC_INIT([my-project], [20160412]) > > AM_INIT_AUTOMAKE > > AM_PROG_CC_C_O > > if test "x$host" == xavr; then > > AC_CHECK_HEADER([avr/io.h], [], [AC_MSG_ERROR([missing header: > > avr/io.h])]) > > AC_CHECK_HEADER([util/delay.h], [], [AC_MSG_ERROR([missing header: > > util/delay.h])]) > > else > > AC_CHECK_HEADER([stdio.h], [], > > [AC_MSG_ERROR([missing header: stdio.h])]) > > AC_CHECK_HEADER([time.h], [], > > [AC_MSG_ERROR([missing header: time.h])]) > > fi > > AC_CONFIG_FILES([Makefile]) > > AC_OUTPUT > > The basic problem with the above is that AC_PROG_CPP is not > called properly in your configure.ac. Because (simplifying > a bit) AC_CHECK_HEADER requires a the preprocessor, it expands > > AC_REQUIRE([AC_PROG_CPP]). > > Loosely, this means that the first expansion of AC_CHECK_HEADER > will expand AC_PROG_CPP, if it was not already done. In your > original version, these expansions were unconditional and things > worked fine. > > But in your second instance, the first expansion of AC_CHECK_HEADER > expands AC_PROG_CPP inside an "if". The result is that no preprocessor > is checked in the "else" case. You can see this in the configure output, > the following line is only printed in the avr case: > > checking how to run the C preprocessor... avr-gcc -E > > There are several basic solutions: > > - First, you can just expand AC_PROG_CPP directly and unconditionally > before your if. This will ensure the macro is available in both cases. > > - Second is to rewrite your condition using AS_IF, which automatically > "hoists" the dependency AC_PROG_CPP (and any other dependencies) > outside of the if condition. For example: > > AS_IF([test x"$host" = x"avr"], > [AC_CHECK_HEADER([avr/io.h], [], > [AC_MSG_ERROR([missing header: avr/io.h])]) > AC_CHECK_HEADER([util/delay.h], [], > [AC_MSG_ERROR([missing header: util/delay.h])])], > > [AC_CHECK_HEADER([stdio.h], [], > [AC_MSG_ERROR([missing header: stdio.h])]) > AC_CHECK_HEADER([time.h], [], > [AC_MSG_ERROR([missing header: time.h])])]) > > - Third, you can make the checks unconditional but the hard > failures conditional, e.g.: > > AC_CHECK_HEADER([avr/io.h], [], > [if test x"$host" = x"avr"; then > AC_MSG_ERROR([missing header: avr/io.h]) > fi]) > > Normally I would go with AS_IF. > > Hope that helps, > Nick It is a very useful answer with explanations. I am using AS_IF now and it works perfectly. Thank you. _______________________________________________ Autoconf mailing list Autoconf@xxxxxxx https://lists.gnu.org/mailman/listinfo/autoconf
