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# $NetBSD: varname-empty.mk,v 1.7 2020/10/23 17:53:01 rillig Exp $
#
# Tests for the special variable with the empty name.
#
# There is no variable named "" at all, and this fact is used a lot in
# variable expressions of the form ${:Ufallback}. These expressions are
# based on the variable named "" and use the :U modifier to assign a
# fallback value to the expression (but not to the variable).
#
# This form of expressions is used to implement value substitution in the
# .for loops. Another use case is in a variable assignment of the form
# ${:Uvarname}=value, which allows for characters in the variable name that
# would otherwise be interpreted by the parser, such as whitespace, ':',
# '=', '$', backslash.
#
# The only places where a variable is assigned a value are Var_Set and
# Var_Append, and these places protect the variable named "" from being
# defined. This is different from read-only variables, as that flag can
# only apply to variables that are defined. The variable named "" must
# never be defined though.
#
# See also:
# The special variables @F or ^D, in var-class-local.mk
# Until 2020-08-22 it was possible to assign a value to the variable with
# the empty name, leading to all kinds of unexpected effects in .for loops
# and other places that assume that ${:Ufallback} expands to "fallback".
# The bug in Var_Set was that only expanded variables had been checked for
# the empty name, but not the direct assignments with an empty name.
?= default
= assigned # undefined behavior until 2020-08-22
+= appended
:= subst
!= echo 'shell-output'
.if ${:Ufallback} != "fallback"
. error
.endif
${:U}= assigned indirectly
.if ${:Ufallback} != "fallback"
. error
.endif
# Before 2020-08-22, the simple assignment operator '=' after an empty
# variable name had an off-by-one bug in Parse_DoVar. The code that was
# supposed to "skip to operator character" started its search _after_ the
# assignment operator, assuming that the variable name would be at least
# one character long. It then looked for the next occurrence of a '=', which
# could be several lines away or not occur at all. While looking for the
# '=', some whitespace was nulled out, leading to out-of-bounds write.
= assigned # undefined behavior until 2020-08-22
# The .for loop expands the expression ${i} to ${:U1}, ${:U2} and so on.
# This only works if the variable with the empty name is guaranteed to
# be undefined.
.for i in 1 2 3
NUMBERS+= ${i}
.endfor
all:
@echo out: ${:Ufallback}
@echo out: ${NUMBERS}
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