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9.4.1 Prerequisite Macros

A macro that you write might need to use values that have previously been computed by other macros. For example, AC_DECL_YYTEXT examines the output of flex or lex, so it depends on AC_PROG_LEX having been called first to set the shell variable LEX.

Rather than forcing the user of the macros to keep track of the dependencies between them, you can use the AC_REQUIRE macro to do it automatically. AC_REQUIRE can ensure that a macro is only called if it is needed, and only called once.

Macro: AC_REQUIRE (macro-name)
If the M4 macro macro-name has not already been called, call it (without any arguments). Make sure to quote macro-name with square brackets. macro-name must have been defined using AC_DEFUN or else contain a call to AC_PROVIDE to indicate that it has been called.

AC_REQUIRE must be used inside an AC_DEFUN'd macro; it must not be called from the top level.

AC_REQUIRE is often misunderstood. It really implements dependencies between macros in the sense that if one macro depends upon another, the latter will be expanded before the body of the former. In particular, `AC_REQUIRE(FOO)' is not replaced with the body of FOO. For instance, this definition of macros:

 
AC_DEFUN([TRAVOLTA],
[test "$body_temperature_in_celsius" -gt "38" &&
  dance_floor=occupied])
AC_DEFUN([NEWTON_JOHN],
[test "$hair_style" = "curly" &&
  dance_floor=occupied])

AC_DEFUN([RESERVE_DANCE_FLOOR],
[if date | grep '^Sat.*pm' >/dev/null 2>&1; then
  AC_REQUIRE([TRAVOLTA])
  AC_REQUIRE([NEWTON_JOHN])
fi])

with this `configure.ac'

 
AC_INIT
RESERVE_DANCE_FLOOR
if test "$dance_floor" = occupied; then
  AC_MSG_ERROR([cannot pick up here, let's move])
fi

will not leave you with a better chance to meet a kindred soul at other times than Saturday night since it expands into:

 
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

This behavior was chosen on purpose: (i) it prevents messages in required macros from interrupting the messages in the requiring macros; (ii) it avoids bad surprises when shell conditionals are used, as in:

 
if ...; then
  AC_REQUIRE([SOME_CHECK])
fi
...
SOME_CHECK

You are encouraged to put all AC_REQUIREs at the beginning of a macro. You can use dnl to avoid the empty lines they leave.


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