15.2 Working With External Software

Some packages require, or can optionally use, other software packages that are already installed. The user can give configure command line options to specify which such external software to use. The options have one of these forms:

--with-package[=arg]
--without-package

For example, --with-gnu-ld means work with the GNU linker instead of some other linker. --with-x means work with The X Window System.

The user can give an argument by following the package name with ‘=’ and the argument. Giving an argument of ‘no’ is for packages that are used by default; it says to not use the package. An argument that is neither ‘yes’ nor ‘no’ could include a name or number of a version of the other package, to specify more precisely which other package this program is supposed to work with. If no argument is given, it defaults to ‘yes’. --without-package is equivalent to --with-package=no.

Normally configure scripts complain about --with-package options that they do not support. See Option Checking, for details, and for how to override the defaults.

For each external software package that may be used, configure.ac should call AC_ARG_WITH to detect whether the configure user asked to use it. Whether each package is used or not by default, and which arguments are valid, is up to you.

Macro: AC_ARG_WITH (package, help-string, [action-if-given], [action-if-not-given])

If the user gave configure the option --with-package or --without-package, run shell commands action-if-given. If neither option was given, run shell commands action-if-not-given. The name package indicates another software package that this program should work with. It should consist only of alphanumeric characters, dashes, plus signs, and dots.

The option’s argument is available to the shell commands action-if-given in the shell variable withval, which is actually just the value of the shell variable named with_package, with any non-alphanumeric characters in package changed into ‘_’. You may use that variable instead, if you wish.

Note that action-if-not-given is not expanded until the point that AC_ARG_WITH was expanded. If you need the value of with_package set to a default value by the time argument parsing is completed, use m4_divert_text to the DEFAULTS diversion (see m4_divert_text) (if done as an argument to AC_ARG_WITH, also provide non-diverted text to avoid a shell syntax error).

The argument help-string is a description of the option that looks like this:

  --with-readline         support fancy command line editing

help-string may be more than one line long, if more detail is needed. Just make sure the columns line up in ‘configure --help’. Avoid tabs in the help string. The easiest way to provide the proper leading whitespace is to format your help-string with the macro AS_HELP_STRING (see Pretty Help Strings).

The following example shows how to use the AC_ARG_WITH macro in a common situation. You want to let the user decide whether to enable support for an external library (e.g., the readline library); if the user specified neither --with-readline nor --without-readline, you want to enable support for readline only if the library is available on the system.

AC_ARG_WITH([readline],
  [AS_HELP_STRING([--with-readline],
    [support fancy command line editing @<:@default=check@:>@])],
  [],
  [: m4_divert_text([DEFAULTS], [with_readline=check])])

LIBREADLINE=
AS_IF([test "x$with_readline" != xno],
  [AC_CHECK_LIB([readline], [main],
    [AC_SUBST([LIBREADLINE], ["-lreadline -lncurses"])
     AC_DEFINE([HAVE_LIBREADLINE], [1],
               [Define if you have libreadline])
    ],
    [if test "x$with_readline" != xcheck; then
       AC_MSG_FAILURE(
         [--with-readline was given, but test for readline failed])
     fi
    ], -lncurses)])

The next example shows how to use AC_ARG_WITH to give the user the possibility to enable support for the readline library, in case it is still experimental and not well tested, and is therefore disabled by default.

AC_ARG_WITH([readline],
  [AS_HELP_STRING([--with-readline],
    [enable experimental support for readline])],
  [],
  [with_readline=no])

LIBREADLINE=
AS_IF([test "x$with_readline" != xno],
  [AC_CHECK_LIB([readline], [main],
    [AC_SUBST([LIBREADLINE], ["-lreadline -lncurses"])
     AC_DEFINE([HAVE_LIBREADLINE], [1],
               [Define if you have libreadline])
    ],
    [AC_MSG_FAILURE(
       [--with-readline was given, but test for readline failed])],
    [-lncurses])])

The last example shows how to use AC_ARG_WITH to give the user the possibility to disable support for the readline library, given that it is an important feature and that it should be enabled by default.

AC_ARG_WITH([readline],
  [AS_HELP_STRING([--without-readline],
    [disable support for readline])],
  [],
  [with_readline=yes])

LIBREADLINE=
AS_IF([test "x$with_readline" != xno],
  [AC_CHECK_LIB([readline], [main],
    [AC_SUBST([LIBREADLINE], ["-lreadline -lncurses"])
     AC_DEFINE([HAVE_LIBREADLINE], [1],
               [Define if you have libreadline])
    ],
    [AC_MSG_FAILURE(
       [readline test failed (--without-readline to disable)])],
    [-lncurses])])

These three examples can be easily adapted to the case where AC_ARG_ENABLE should be preferred to AC_ARG_WITH (see Package Options).