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var-op-default.mk revision 1.5 
      1 # $NetBSD: var-op-default.mk,v 1.5 2023/11/19 22:32:44 rillig Exp $
      2 #
      3 # Tests for the ?= variable assignment operator, which only assigns
      4 # if the variable is still undefined.
      5 
      6 # The variable VAR is not defined yet.  Therefore it gets the default value
      7 # from the variable assignment.
      8 VAR?=		default value
      9 .if ${VAR} != "default value"
     10 .  error
     11 .endif
     12 
     13 # At this point, the variable 'VAR' is already defined.  The '?=' therefore
     14 # ignores the new variable value, preserving the previous "default value".
     15 VAR?=		ignored
     16 .if ${VAR} != "default value"
     17 .  error
     18 .endif
     19 
     20 # The '?=' operator only checks whether the variable is defined or not.
     21 # An empty variable is defined, therefore the '?=' operator does nothing.
     22 EMPTY=		# empty
     23 EMPTY?=		ignored
     24 .if ${EMPTY} != ""
     25 .  error
     26 .endif
     27 
     28 # The .for loop is described in the manual page as if it would operate on
     29 # variables.  This is not entirely true.  Instead, each occurrence of an
     30 # expression $i or ${i} or ${i:...} is substituted with ${:Uloop-value}.
     31 # This comes very close to the description, the only difference is that
     32 # there is never an actual variable named 'i' involved.
     33 #
     34 # Because there is not really a variable named 'i', the '?=' operator
     35 # performs the variable assignment, resulting in $i == "default".
     36 .for i in loop-value
     37 i?=		default
     38 .endfor
     39 .if ${i} != "default"
     40 .  error
     41 .endif
     42 
     43 # At the point where the '?=' operator checks whether the variable exists,
     44 # it expands the variable name exactly once.  Therefore both 'VAR.param'
     45 # and 'VAR.${param}' expand to 'VAR.param', and the second '?=' assignment
     46 # has no effect.
     47 #
     48 # Since 2000.05.11.07.43.42 it has been possible to use nested
     49 # expressions in variable names, which made make much more versatile.
     50 # On 2008.03.31.00.12.21, this particular case of the '?=' operator has been
     51 # fixed.  Before, the '?=' operator had not expanded the variable name
     52 # 'VAR.${:Uparam}' to see whether the variable already existed.  Since that
     53 # variable didn't exist (and variables with '$' in their name are particularly
     54 # fragile), the variable assignment with "not used" was performed, and only
     55 # during that, the variable name was expanded.
     56 VAR.param=		already defined
     57 VAR.${:Uparam}?=	not used
     58 .if ${VAR.param} != "already defined"
     59 .  error
     60 .endif
     61 
     62 # Now demonstrate that the variable name is indeed expanded exactly once.
     63 # This is tricky to measure correctly since there are many inconsistencies
     64 # in and around the code that expands expressions in the various
     65 # places where expressions can occur.  If in doubt, enable the
     66 # following debug flags to see what happens:
     67 #.MAKEFLAGS: -dcpv
     68 EXPAND_NAME=		EXPAND.$$$$	# The full variable name is EXPAND.$$
     69 PARAM=			$$$$
     70 EXPAND.${PARAM}?=	value with param
     71 .if ${${EXPAND_NAME}} != "value with param"
     72 .  error
     73 .endif
     74 .MAKEFLAGS: -d0
     75 
     76 all:
     77 	@:;
     78