1# $NetBSD: varmod-undefined.mk,v 1.8 2022/08/06 21:26:05 rillig Exp $ 2# 3# Tests for the :U variable modifier, which returns the given string 4# if the variable is undefined. 5# 6# See also: 7# directive-for.mk 8# varmod-defined.mk 9 10# The pattern ${:Uword} is heavily used when expanding .for loops. 11# 12# This is how an expanded .for loop looks like. 13# .for word in one 14# . if ${word} != one 15.if ${:Uone} != one 16# . error ${word} 17. error ${:Uone} 18# . endif 19.endif 20# .endfor 21 22# The variable expressions in the text of the :U modifier may be arbitrarily 23# nested. 24 25.if ${:U${:Unested}${${${:Udeeply}}}} != nested 26. error 27.endif 28 29# The nested variable expressions may contain braces, and these braces don't 30# need to match pairwise. In the following example, the :S modifier uses '{' 31# as delimiter, which confuses both editors and humans because the opening 32# and closing braces don't match anymore. It's syntactically valid though. 33# For more similar examples, see varmod-subst.mk, mod-subst-delimiter. 34 35.if ${:U${:Uvalue:S{a{X{}} != vXlue 36. error 37.endif 38 39# The escaping rules for the :U modifier (left-hand side) and condition 40# string literals (right-hand side) are completely different. 41# 42# In the :U modifier, the backslash only escapes very few characters, all 43# other backslashes are retained. 44# 45# In condition string literals, the backslash always escapes the following 46# character, no matter whether it would be necessary or not. 47# 48# In both contexts, \n is an escaped letter n, not a newline; that's what 49# the .newline variable is for. 50# 51# Whitespace at the edges is preserved, on both sides of the comparison. 52# 53.if ${:U \: \} \$ \\ \a \b \n } != " : } \$ \\ \\a \\b \\n " 54. error 55.endif 56 57# Even after the :U modifier has been applied, the expression still remembers 58# that it originated from an undefined variable, and the :U modifier can 59# be used to overwrite the value of the expression. 60# 61.if ${UNDEF:Uvalue:S,a,X,} != "vXlue" 62. error 63.elif ${UNDEF:Uvalue:S,a,X,:Uwas undefined} != "was undefined" 64. error 65.endif 66 67all: 68 @:; 69