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doc: mention how to set early defaults

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Jonathan Lebon reported an issue to me off-list about a regression
in libvirt's configure script, which I traced to a patch that
rearranged code that was checking $with_library compared to the
AC_ARG_WITH that actually set $with_library [1].  As the whole point
of the libvirt patch was to refactor code to make maintenance
easier by hiding the AC_ARG_WITH in a helper macro for a net
reduction in lines, it makes sense to actually document how to
check what value a variable has prior to the AC_ARG_WITH usage.

Alas, although the functionality for this has been present in
autoconf for ages, the documentation has been lacking.

[1] libvirt.org/git/?p=libvirt.git;a=commitdiff;h=654c709

* doc/autoconf.texi (Diversion support) <m4_divert_text>: Add
anchor.
(External Software) <AC_ARG_WITH>: Demonstrate how to use DEFAULTS
diversion, for earlier defaults.
(Package Options) <AC_ARG_ENABLE>: Likewise.
* THANKS: Add Jonathan Lebon.

Signed-off-by: Eric Blake <eblake@redhat.com>
This commit is contained in:
Eric Blake 2013-09-05 14:14:55 -06:00
parent 63f3c78cdb
commit befa52738c
2 changed files with 19 additions and 1 deletions
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1
THANKS
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@ -210,6 +210,7 @@ John Interrante interran@uluru.stanford.edu
John R. Cary cary@txcorp.com
John W. Eaton jwe@bevo.che.wisc.edu
Jonathan Kamens jik@kamens.brookline.ma.us
Jonathan Lebon jlebon@redhat.com
Josef Tran josef@timetrackertechnology.com
Josef Vukovic josefvukovic@googlemail.com
Joseph S. Myers jsm28@cam.ac.uk

19
doc/autoconf.texi
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@ -11997,6 +11997,7 @@ stack, and issues an error if there is not a matching pop for every
push.
@end defmac
@anchor{m4_divert_text}
@defmac m4_divert_text (@var{diversion}, @ovar{content})
@msindex{divert_text}
Output @var{content} and a newline into @var{diversion}, without
@ -22085,6 +22086,14 @@ actually just the value of the shell variable named
@var{package} changed into @samp{_}. You may use that variable instead,
if you wish.
Note that @var{action-if-not-given} is not expanded until the point that
@code{AC_ARG_WITH} was expanded. If you need the value of
@code{with_@var{package}} set to a default value by the time argument
parsing is completed, use @code{m4_divert_text} to the @code{DEFAULTS}
diversion (@pxref{m4_divert_text}) (if done as an argument to
@code{AC_ARG_WITH}, also provide non-diverted text to avoid a shell
syntax error).
The argument @var{help-string} is a description of the option that
looks like this:
@example
@ -22111,7 +22120,7 @@ AC_ARG_WITH([readline],
[AS_HELP_STRING([--with-readline],
[support fancy command line editing @@<:@@default=check@@:>@@])],
[],
[with_readline=check])
[: m4_divert_text([DEFAULTS], [with_readline=check])])
LIBREADLINE=
AS_IF([test "x$with_readline" != xno],
@ -22238,6 +22247,14 @@ actually just the value of the shell variable named
if you wish. The @var{help-string} argument is like that of
@code{AC_ARG_WITH} (@pxref{External Software}).
Note that @var{action-if-not-given} is not expanded until the point that
@code{AC_ARG_ENABLE} was expanded. If you need the value of
@code{enable_@var{feature}} set to a default value by the time argument
parsing is completed, use @code{m4_divert_text} to the @code{DEFAULTS}
diversion (@pxref{m4_divert_text}) (if done as an argument to
@code{AC_ARG_ENABLE}, also provide non-diverted text to avoid a shell
syntax error).
You should format your @var{help-string} with the macro
@code{AS_HELP_STRING} (@pxref{Pretty Help Strings}).