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Re: About dynamic configure options.

From: Nick Bowler
Subject: Re: About dynamic configure options.
Date: Thu, 13 Dec 2012 14:25:53 -0500
User-agent: Mutt/1.5.21 (2010-09-15)

On 2012-12-13 10:20 -0700, Eric Blake wrote:
> On 12/13/2012 05:58 AM, Stefano Lattarini wrote:
> > On 12/11/2012 12:57 AM, Eduardo Costa wrote:
> >> Is it possible at all to have a configure option whose help message can 
> >> expand a
> >> variable, or can otherwise accept the output of a command at 
> >> configure-time?
> >>
> >> For example, imagine this as part of the output of `./configure':
> >>
> >> --with-user=user        Who to complain to (default X)
> >>
> >> Where X could be, say, the output of the command `whoami'.
> >>
> >> This might be done by usual means (couldn't do it) or just with some
> >> trickery to to inject the string manually at the end of some section
> >> (say HELP_CANON or other), and giving the output of the command on a
> >> new line, so it doesn't get inside the `cat << LABEL ... LABEL'
> >> constructs that seem to be used to output options and help messages.
> Unfortunately, it looks like the current setup of autoconf is pretty
> hard-coded to constant strings determined at m4-time; making it use
> shell variables for dynamic output would require quite a bit of patching
> (maybe by introducing new macros such as AC_ARG_WITH_UNQUOTED, to take
> care of the tweaks needed to close the quoted heredoc, open an unquoted
> heredoc to do the substitution, then reopen the quoted heredoc for the
> rest of the script).  It sounds like a useful request, though.

FWIW it's not too hard to hack it:

  AC_ARG_WITH([user], [_ACEOF
    # Eschew error handling!
    cat <<EOF dnl
                               [Who to complain to (default $me)])])dnl
    cat <<\_ACEOF])

but that is, of course, relying on internal details of how Autoconf
generates the code to produce --help output.  But there remain other
problems: firstly, the --help text appears quite early in the configure
output -- much earlier than the position of AC_ARG_WITH in
would imply.  This may make it hard to put anything useful in the help
text, without computing it directly in AC_ARG_WITH_UNQUOTED or whatever
(as done in the above example).

Secondly, regarding this specific example, I don't know if whoami is
universally available.  So determining the default username is probably
within the realm of a "full" configure test, with at least one invocation
of AC_CHECK_PROGS.  Such a test is almost certainly too much to do before
printing --help output.

Normally it is sufficient to just write the --help text as a static
string explaining how the default is determined.  In the case of a
username like above, I would probably do something like:

  --with-user=user        Who to complain to (default: current user)

and then make the configure script print a line such as:

  Checking which user will receive complaints... nbowler

Nick Bowler, Elliptic Technologies (

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