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Re: Call the AC_CHECK_HEADER macro on a condition
From: |
Eric Blake |
Subject: |
Re: Call the AC_CHECK_HEADER macro on a condition |
Date: |
Tue, 12 Apr 2016 09:24:21 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1 |
On 04/12/2016 09:05 AM, Nick Bowler wrote:
>
> 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])])
Also, checking for <stdio.h> is pointless these days. You can portably
assume a C89 compiler (and these days, often a C99 compiler), which
guarantees <stdio.h> is present.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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