1# $NetBSD: cond-short.mk,v 1.20 2023/03/04 13:42:36 rillig Exp $ 2# 3# Demonstrates that in conditions, the right-hand side of an && or || 4# is only evaluated if it can actually influence the result. 5# This is called 'short-circuit evaluation' and is the usual evaluation 6# mode in most programming languages. A notable exception is Ada, which 7# distinguishes between the operators 'And', 'And Then', 'Or', 'Or Else'. 8# 9# Before 2020-06-28, the right-hand side of an && or || operator was always 10# evaluated, which was wrong. In cond.c 1.69 and var.c 1.197 on 2015-10-11, 11# Var_Parse got a new parameter named 'wantit'. Since then it would have been 12# possible to skip evaluation of irrelevant variable expressions and only 13# parse them. They were still evaluated though, the only difference to 14# relevant variable expressions was that in the irrelevant variable 15# expressions, undefined variables were allowed. This allowed for conditions 16# like 'defined(VAR) && ${VAR:S,from,to,} != ""', which no longer produced an 17# error message 'Malformed conditional', but the irrelevant expression was 18# still evaluated. 19# 20# Since the initial commit on 1993-03-21, the manual page has been saying that 21# make 'will only evaluate a conditional as far as is necessary to determine', 22# but that was wrong. The code in cond.c 1.1 from 1993-03-21 looks good since 23# it calls Var_Parse(condExpr, VAR_CMD, doEval,&varSpecLen,&doFree), but the 24# definition of Var_Parse did not call the third parameter 'doEval', as would 25# be expected, but instead 'err', accompanied by the comment 'TRUE if 26# undefined variables are an error'. This subtle difference between 'do not 27# evaluate at all' and 'allow undefined variables' led to the unexpected 28# evaluation. 29# 30# See also: 31# var-eval-short.mk, for short-circuited variable modifiers 32 33# The && operator: 34 35.if 0 && ${echo "unexpected and" 1>&2 :L:sh} 36.endif 37 38.if 1 && ${echo "expected and" 1>&2 :L:sh} 39.endif 40 41.if 0 && exists(nonexistent${echo "unexpected and exists" 1>&2 :L:sh}) 42.endif 43 44.if 1 && exists(nonexistent${echo "expected and exists" 1>&2 :L:sh}) 45.endif 46 47.if 0 && empty(${echo "unexpected and empty" 1>&2 :L:sh}) 48.endif 49 50.if 1 && empty(${echo "expected and empty" 1>&2 :L:sh}) 51.endif 52 53# "VAR U11" is not evaluated; it was evaluated before 2020-07-02. 54# The whole !empty condition is only parsed and then discarded. 55VAR= ${VAR${:U11${echo "unexpected VAR U11" 1>&2 :L:sh}}} 56VAR13= ${VAR${:U12${echo "unexpected VAR13" 1>&2 :L:sh}}} 57.if 0 && !empty(VAR${:U13${echo "unexpected U13 condition" 1>&2 :L:sh}}) 58.endif 59 60VAR= ${VAR${:U21${echo "unexpected VAR U21" 1>&2 :L:sh}}} 61VAR23= ${VAR${:U22${echo "expected VAR23" 1>&2 :L:sh}}} 62.if 1 && !empty(VAR${:U23${echo "expected U23 condition" 1>&2 :L:sh}}) 63.endif 64VAR= # empty again, for the following tests 65 66# The :M modifier is only parsed, not evaluated. 67# Before 2020-07-02, it was wrongly evaluated. 68.if 0 && !empty(VAR:M${:U${echo "unexpected M pattern" 1>&2 :L:sh}}) 69.endif 70 71.if 1 && !empty(VAR:M${:U${echo "expected M pattern" 1>&2 :L:sh}}) 72.endif 73 74.if 0 && !empty(VAR:S,from,${:U${echo "unexpected S modifier" 1>&2 :L:sh}},) 75.endif 76 77.if 0 && !empty(VAR:C,from,${:U${echo "unexpected C modifier" 1>&2 :L:sh}},) 78.endif 79 80.if 0 && !empty("" == "" :? ${:U${echo "unexpected ? modifier" 1>&2 :L:sh}} :) 81.endif 82 83.if 0 && !empty(VAR:old=${:U${echo "unexpected = modifier" 1>&2 :L:sh}}) 84.endif 85 86.if 0 && !empty(1 2 3:L:@var@${:U${echo "unexpected @ modifier" 1>&2 :L:sh}}@) 87.endif 88 89.if 0 && !empty(:U${:!echo "unexpected exclam modifier" 1>&2 !}) 90.endif 91 92# Irrelevant assignment modifiers are skipped as well. 93.if 0 && ${1 2 3:L:@i@${FIRST::?=$i}@} 94.endif 95.if 0 && ${1 2 3:L:@i@${LAST::=$i}@} 96.endif 97.if 0 && ${1 2 3:L:@i@${APPENDED::+=$i}@} 98.endif 99.if 0 && ${echo.1 echo.2 echo.3:L:@i@${RAN::!=${i:C,.*,&; & 1>\&2,:S,., ,g}}@} 100.endif 101.if defined(FIRST) || defined(LAST) || defined(APPENDED) || defined(RAN) 102. warning first=${FIRST} last=${LAST} appended=${APPENDED} ran=${RAN} 103.endif 104 105# The || operator: 106 107.if 1 || ${echo "unexpected or" 1>&2 :L:sh} 108.endif 109 110.if 0 || ${echo "expected or" 1>&2 :L:sh} 111.endif 112 113.if 1 || exists(nonexistent${echo "unexpected or exists" 1>&2 :L:sh}) 114.endif 115 116.if 0 || exists(nonexistent${echo "expected or exists" 1>&2 :L:sh}) 117.endif 118 119.if 1 || empty(${echo "unexpected or empty" 1>&2 :L:sh}) 120.endif 121 122.if 0 || empty(${echo "expected or empty" 1>&2 :L:sh}) 123.endif 124 125# Unreachable nested conditions are skipped completely as well. These skipped 126# lines may even contain syntax errors. This allows to skip syntactically 127# incompatible new features in older versions of make. 128 129.if 0 130. if ${echo "unexpected nested and" 1>&2 :L:sh} 131. endif 132.endif 133 134.if 1 135.elif ${echo "unexpected nested or" 1>&2 :L:sh} 136.endif 137 138 139NUMBER= 42 140INDIR_NUMBER= ${NUMBER} 141INDIR_UNDEF= ${UNDEF} 142 143.if defined(NUMBER) && ${NUMBER} > 0 144.else 145. error 146.endif 147 148# Starting with var.c 1.226 from from 2020-07-02, the following condition 149# triggered a warning: "String comparison operator should be either == or !=". 150# 151# The left-hand side of the '&&' evaluated to false, which should have made 152# the right-hand side irrelevant. 153# 154# On the right-hand side of the '&&', the expression ${INDIR_UNDEF} was 155# defined and had the value '${UNDEF}', but the nested variable UNDEF was 156# undefined. The right hand side "${INDIR_UNDEF}" still needed to be parsed, 157# and in parse-only mode, the "value" of the parsed expression was the 158# uninterpreted variable value, in this case '${UNDEF}'. And even though the 159# right hand side of the '&&' should have been irrelevant, the two sides of 160# the comparison were still parsed and evaluated. Comparing these two values 161# numerically was not possible since the string '${UNDEF}' is not a number, 162# so the comparison fell back to string comparison, which then complained 163# about the '>' operator. 164# 165# This was fixed in cond.c 1.79 from 2020-07-09 by not evaluating irrelevant 166# comparisons. Instead, they are only parsed and then discarded. 167# 168# At that time, there was not enough debug logging to see the details in the 169# -dA log. To actually see it, add debug logging at the beginning and end of 170# Var_Parse. 171.if defined(UNDEF) && ${INDIR_UNDEF} < ${NUMBER} 172. error 173.endif 174# Adding a ':U' modifier to the irrelevant expression didn't help, as that 175# expression was only parsed, not evaluated. The resulting literal string 176# '${INDIR_UNDEF:U2}' was not numeric either, for the same reason as above. 177.if defined(UNDEF) && ${INDIR_UNDEF:U2} < ${NUMBER} 178. error 179.endif 180 181# Enclosing the expression in double quotes changes how that expression is 182# evaluated. In irrelevant expressions that are enclosed in double quotes, 183# expressions based on undefined variables are allowed and evaluate to an 184# empty string. 185# 186# The manual page stated from at least 1993 on that irrelevant conditions were 187# not evaluated, but that was wrong. These conditions were evaluated, the 188# only difference was that undefined variables in them didn't trigger an 189# error. Since numeric conditions are quite rare, this subtle difference 190# didn't catch much attention, as most other conditions such as pattern 191# matches or equality comparisons worked fine and never produced error 192# messages. 193.if defined(UNDEF) && "${INDIR_UNDEF}" < ${NUMBER} 194. error 195.endif 196 197# Since the condition is relevant, the indirect undefined variable is 198# evaluated as usual, resolving nested undefined expressions to an empty 199# string. 200# 201# Comparing an empty string numerically is not possible, however, make has an 202# ugly hack in TryParseNumber that treats an empty string as a valid numerical 203# value, thus hiding bugs in the makefile. 204.if ${INDIR_UNDEF} < ${NUMBER} 205# only due to the ugly hack 206.else 207. error 208.endif 209 210# Due to the quotes around the left-hand side of the '<', the operand is 211# marked as a string, thus preventing a numerical comparison. 212# 213# expect+1: Comparison with '<' requires both operands '' and '42' to be numeric 214.if "${INDIR_UNDEF}" < ${NUMBER} 215. info yes 216.else 217. info no 218.endif 219 220# The right-hand side of '||' is irrelevant and thus not evaluated. 221.if 1 || ${INDIR_NUMBER} < ${NUMBER} 222.else 223. error 224.endif 225 226# The right-hand side of '||' is relevant and thus evaluated normally. 227.if 0 || ${INDIR_NUMBER} < ${NUMBER} 228. error 229.endif 230 231# The right-hand side of '||' evaluates to an empty string, as the variable 232# 'INDIR_UNDEF' is defined, therefore the modifier ':U2' has no effect. 233# Comparing an empty string numerically is not possible, however, make has an 234# ugly hack in TryParseNumber that treats an empty string as a valid numerical 235# value, thus hiding bugs in the makefile. 236.if 0 || ${INDIR_UNDEF:U2} < ${NUMBER} 237# only due to the ugly hack 238.else 239. error 240.endif 241 242 243# The right-hand side of the '&&' is irrelevant since the left-hand side 244# already evaluates to false. Before cond.c 1.79 from 2020-07-09, it was 245# expanded nevertheless, although with a small modification: undefined 246# variables may be used in these expressions without generating an error. 247.if defined(UNDEF) && ${UNDEF} != "undefined" 248. error 249.endif 250 251 252# Ensure that irrelevant conditions do not influence the result of the whole 253# condition. As of cond.c 1.302 from 2021-12-11, an irrelevant function call 254# evaluated to true (see CondParser_FuncCall and CondParser_FuncCallEmpty), an 255# irrelevant comparison evaluated to false (see CondParser_Comparison). 256# 257# An irrelevant true bubbles up to the outermost CondParser_And, where it is 258# ignored. An irrelevant false bubbles up to the outermost CondParser_Or, 259# where it is ignored. 260# 261# If the condition parser should ever be restructured, the bubbling up of the 262# irrelevant evaluation results might show up accidentally. Prevent this. 263DEF= defined 264.undef UNDEF 265 266.if 0 && defined(DEF) 267. error 268.endif 269 270.if 1 && defined(DEF) 271.else 272. error 273.endif 274 275.if 0 && defined(UNDEF) 276. error 277.endif 278 279.if 1 && defined(UNDEF) 280. error 281.endif 282 283.if 0 || defined(DEF) 284.else 285. error 286.endif 287 288.if 1 || defined(DEF) 289.else 290. error 291.endif 292 293.if 0 || defined(UNDEF) 294. error 295.endif 296 297.if 1 || defined(UNDEF) 298.else 299. error 300.endif 301 302 303all: 304