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Re: Variable expansin in AC_PREFIX_DEFAULT

From: Eric Blake
Subject: Re: Variable expansin in AC_PREFIX_DEFAULT
Date: Wed, 22 Mar 2017 08:38:42 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 03/22/2017 03:20 AM, Joakim Jalap wrote:
> Hello autoconf gurus!
> I am trying to write some autotools support for installing a latex
> package. On CTAN I found some autoconf macros (which I had to modify a
> bit) which I use to find for example latex(1) and pdflatex(1). So far so
> good. This CTAN package also has a macro to find the path to the tex
> installation, AC_TEXMF_PATH. This finds the correct path for me at

It's poor practice to name a macro in the AC_ namespace if it is not
actually owned by autoconf; you may want to choose a different namespace
to make it obvious where the macro originates from.

> /usr/share/texmf-dist. Now I want to put this as the default prefix so
> that configure will "do the right thing" by default, but this is where I
> fail.
> This is my
> ----------------------------------------
> AC_INIT([package], [0.1], [blah])
> AC_PREREQ([2.60])
> AM_INIT_AUTOMAKE([foreign])
> AC_PREFIX_DEFAULT([$texmfpath])
> AC_MSG_NOTICE([$texmfpath])
> ----------------------------------------
> AC_TEXMF_PATH exports `texmfpath' and does AC_SUBST on it. (You can find
> all the macros here:
> The problem is with the AC_PREFIX_DEFAULT. It sets the default prefix to
> empty. But the AC_MSG_NOTICE macro on the line below prints the correct
> path! So somehow $texmfpath is expanded to the correct value by
> AC_MSG_NOTICE, but it is expanded to nothing on the line above. If I do
> AC_PREFIX_DEFAULT(["some_path"]) then the default prefix is correctly
> set to "some_path".

Indeed, thanks to the magic of m4 diversions, AC_PREFIX_DEFAULT expands
to code that appears very early in configure, and using a shell variable
that early in the run is tricky; while AC_MSG_NOTICE expands to code
that runs at the place where the macro was used.

> Is it not possible to set the default prefix in this way? What other
> good ways are there to set the default prefix to something discovered at
> configure time?

Is AC_PREFIX_PROGRAM any better for your use case?

> I'm not subscribed to the list so if somebody answers, please keep me on
> cc :)

That's GNU list policy anyways.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library

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