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From: Eric Blake
Date: Tue, 19 Mar 2019 11:27:38 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1

On 3/19/19 10:19 AM, Reuben Thomas wrote:
> I'm trying to use this macro to indicate which headers should be used for
> e.g. AC_CHECK_SIZEOF, but the argument I supply is just copied verbatim
> into configure, and not used to set ac_includes_default. What am I doing
> wrong?
> For example:
> AC_INIT(foo, 0.1)
> AC_INCLUDES_DEFAULT([#include "foo.h"])
> I just find a line in configure with
> #include "foo.h"
> which is just a comment to m4.

That is behaving as documented.  From the manual:

@defmac AC_INCLUDES_DEFAULT (@ovar{include-directives})
Expand to @var{include-directives} if present and nonempty, otherwise to:

If you want to see the default includes, you have to invoke
AC_INCLUDES_DEFAULT with an empty argument.

The manual does not describe a way to override the shell variable
$ac_includes_default at configure runtime.  Looking at the
implementation in lib/autoconf/headers.m4, the variable is assigned via
m4_divert_text([DEFAULTS]) during AC_CHECK_INCLUDES_DEFAULT (which in
turn is required by a number of places, including when
AC_INCLUDES_DEFAULT is invoked without an argument), if you wanted to
try and override a different setting of that shell variable to be
effective at the point where the rest of configure then relies on the
contents of the variable.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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