File: autoconf.info,  Node: Obsoleting Macros,  Next: Coding Style,  Prev: Dependencies Between Macros,  Up: Writing Autoconf Macros

10.4 Obsoleting Macros
======================

Configuration and portability technology has evolved over the years.
Often better ways of solving a particular problem are developed, or
ad-hoc approaches are systematized.  This process has occurred in many
parts of Autoconf.  One result is that some of the macros are now
considered “obsolete”; they still work, but are no longer considered the
best thing to do, hence they should be replaced with more modern macros.
Ideally, ‘autoupdate’ should replace the old macro calls with their
modern implementation.

   Autoconf provides a simple means to obsolete a macro.

 -- Macro: AU_DEFUN (OLD-MACRO, IMPLEMENTATION, [MESSAGE], [SILENT])
     Define OLD-MACRO as IMPLEMENTATION, just like ‘AC_DEFUN’, but also
     declare OLD-MACRO to be obsolete.  When ‘autoupdate’ is run,
     occurrences of OLD-MACRO will be replaced by the text of
     IMPLEMENTATION in the updated ‘configure.ac’ file.

     If a simple textual replacement is not enough to finish the job of
     updating a ‘configure.ac’ to modern style, provide instructions for
     whatever additional manual work is required as MESSAGE.  These
     instructions will be printed by ‘autoupdate’, and embedded in the
     updated ‘configure.ac’ file, next to the text of IMPLEMENTATION.

     Normally, ‘autoconf’ will also issue a warning (in the "obsolete"
     category) when it expands OLD-MACRO.  This warning does not include
     MESSAGE; it only advises the maintainer to run ‘autoupdate’.  If it
     is inappropriate to issue this warning, set the SILENT argument to
     the word ‘silent’.  One might want to use a silent ‘AU_DEFUN’ when
     OLD-MACRO is used in a widely-distributed third-party macro.  If
     that macro's maintainers are aware of the need to update their
     code, it's unnecessary to nag all of the transitive users of
     OLD-MACRO as well.  This capability was added to ‘AU_DEFUN’ in
     Autoconf 2.70; older versions of autoconf will ignore the SILENT
     argument and issue the warning anyway.

     *Caution:* If IMPLEMENTATION contains M4 or M4sugar macros, they
     will be evaluated when ‘autoupdate’ is run, not emitted verbatim
     like the rest of IMPLEMENTATION.  This cannot be avoided with extra
     quotation, because then OLD-MACRO will not work when it is called
     normally.  See the definition of ‘AC_FOREACH’ in ‘general.m4’ for a
     workaround.

 -- Macro: AU_ALIAS (OLD-NAME, NEW-NAME, [SILENT])
     A shorthand version of ‘AU_DEFUN’, to be used when a macro has
     simply been renamed.  ‘autoupdate’ will replace calls to OLD-NAME
     with calls to NEW-NAME, keeping any arguments intact.  No
     instructions for additional manual work will be printed.

     The SILENT argument works the same as the SILENT argument to
     ‘AU_DEFUN’.  It was added to ‘AU_ALIAS’ in Autoconf 2.70.

     *Caution:* ‘AU_ALIAS’ cannot be used when NEW-NAME is an M4 or
     M4sugar macro.  See above.