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Re: Call the AC_CHECK_HEADER macro on a condition

From: Thomas Petazzoni
Subject: Re: Call the AC_CHECK_HEADER macro on a condition
Date: Tue, 19 Apr 2016 15:40:00 +0200


On Tue, 12 Apr 2016 11:05:51 -0400, Nick Bowler wrote:

> 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])

I just wanted to thank you for this explanation. I was not affected by
the original issue, but your answer was very useful. Until now, I
thought AS_IF() was just a stupid wrapper around the shell's if clause,
but your answer makes it clear that it is a lot smarter than that.


Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering

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