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Re: Call the AC_CHECK_HEADER macro on a condition
From: |
YuGiOhJCJ Mailing-List |
Subject: |
Re: Call the AC_CHECK_HEADER macro on a condition |
Date: |
Wed, 13 Apr 2016 02:22:29 +0200 |
On Tue, 12 Apr 2016 11:05:51 -0400
Nick Bowler <address@hidden> wrote:
> Hi,
>
> On 2016-04-12, YuGiOhJCJ Mailing-List <address@hidden>
> 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.