1# $NetBSD: varparse-undef-partial.mk,v 1.5 2024/01/07 11:39:04 rillig Exp $ 2 3# When an undefined variable is expanded in a ':=' assignment, only the 4# initial '$' of the expression is skipped by the parser, while 5# the remaining expression is evaluated. In edge cases this can lead to 6# a completely different interpretation of the partially expanded text. 7 8LIST= ${DEF} ${UNDEF} ${VAR.${PARAM}} end 9DEF= defined 10PARAM= :Q 11 12# The expression ${VAR.${PARAM}} refers to the variable named "VAR.:Q", 13# with the ":Q" being part of the name. This variable is not defined, 14# therefore the initial '$' of that whole expression is skipped by the parser 15# (see VarSubstExpr) and the rest of the expression is expanded as usual. 16# 17# The resulting expression is ${VAR.:Q}, which means that the 18# interpretation of the ":Q" has changed from being part of the variable 19# name to being a variable modifier. This is a classical code injection. 20EVAL:= ${LIST} 21.if ${EVAL} != "defined end" 22. error ${EVAL} 23.endif 24 25# Define the possible outcomes, to see which of them gets expanded. 26VAR.= var-dot without parameter 27${:UVAR.\:Q}= var-dot with parameter :Q 28 29# At this point, the variable "VAR." is defined, therefore the expression 30# ${VAR.:Q} is expanded, consisting of the variable name "VAR." and the 31# modifier ":Q". 32.if ${EVAL} != "defined var-dot\\ without\\ parameter end" 33. error ${EVAL} 34.endif 35 36# In contrast to the previous line, evaluating the original LIST again now 37# produces a different result since the variable named "VAR.:Q" is now 38# defined. It is expanded as usual, interpreting the ":Q" as part of the 39# variable name, as would be expected from reading the expression. 40EVAL:= ${LIST} 41.if ${EVAL} != "defined var-dot with parameter :Q end" 42. error ${EVAL} 43.endif 44 45# It's difficult to decide what the best behavior is in this situation. 46# Should the whole expression be skipped for now, or should the inner 47# subexpressions be expanded already? 48# 49# Example 1: 50# CFLAGS:= ${CFLAGS:N-W*} ${COPTS.${COMPILER}} 51# 52# The variable COMPILER typically contains an identifier and the variable is 53# not modified later. In this practical case, it does not matter whether the 54# expression is expanded early, or whether the whole ${COPTS.${COMPILER}} is 55# expanded as soon as the variable COPTS.${COMPILER} becomes defined. The 56# expression ${COMPILER} would be expanded several times, but in this simple 57# scenario there would not be any side effects. 58# 59# TODO: Add a practical example where early/lazy expansion actually makes a 60# difference. 61 62all: 63 @: 64