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* doc/autoconf.texi (Prerequisite Macros): More about AC_REQUIRE.

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Akim Demaille 2000-06-26 09:07:36 +00:00
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@ -1,3 +1,7 @@
2000-06-26 Akim Demaille <akim@epita.fr>
* doc/autoconf.texi (Prerequisite Macros): More about AC_REQUIRE.
2000-06-26 Akim Demaille <akim@epita.fr>
Given better names to the diversions.

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doc/autoconf.texi
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@ -6274,8 +6274,83 @@ call it (without any arguments). Make sure to quote @var{macro-name}
with square brackets. @var{macro-name} must have been defined using
@code{AC_DEFUN} or else contain a call to @code{AC_PROVIDE} to indicate
that it has been called.
@code{AC_REQUIRE} must be used inside an @code{AC_DEFUN}'d macro, it
must not be called from the top level.
@end defmac
@code{AC_REQUIRE} is often misunderstood, it really implements
dependencies between macros in the sense that if a macro depends upon
another, the latter will be expanded @emph{before} the body of the
former. In particular, @samp{AC_REQUIRE(FOO)} is not replaced with the
body of @code{FOO}. For instance, this definition of macros
@example
@group
AC_DEFUN([TRAVOLTA],
[test "$body_temparature_in_celsius" -gt "38" &&
dance_floor=occupied])
AC_DEFUN([NEWTON_JOHN],
[test "$hair_style" = "curly" &&
dance_floor=occupied])
@end group
@group
AC_DEFUN([RESERVE_DANCE_FLOOR],
[if date | grep '^Sat.*pm' >/dev/null 2>&1; then
AC_REQUIRE([TRAVOLTA])
AC_REQUIRE([NEWTON_JOHN])
fi])
@end group
@end example
@noindent
with this @file{configure.in}
@example
AC_INIT
RESERVE_DANCE_FLOOR
if test "$dance_floor" = occupied; then
AC_MSG_ERROR([cannot pick up here, let's move])
fi
@end example
@noindent
will not leave you with a better chance to meet the kindred soul the
other times that the Saturday night since it expands into:
@example
@group
test "$body_temperature_in_Celsius" -gt "38" &&
dance_floor=occupied
test "$hair_style" = "curly" &&
dance_floor=occupied
fi
if date | grep '^Sat.*pm' >/dev/null 2>&1; then
fi
@end group
@end example
This behavior was chosen on purpose: (i) it avoids that messages from
required macros interrupt the messages from the requiring macros, (ii),
it avoids bad surprises when shell conditionals are used, as in:
@example
@group
if ...; then
AC_REQUIRE([SOME_CHECK])
fi
...
SOME_CHECK
@end group
@end example
You are encouraged to put all the @code{AC_REQUIRE}s at the beginning of
the macros. You can use @code{dnl} to avoid the empty line they leave.
An alternative to using @code{AC_DEFUN} is to use @code{define} and call
@code{AC_PROVIDE}. Because this technique does not prevent nested
messages, it is considered obsolete.