87 lines
3.3 KiB
Makefile
87 lines
3.3 KiB
Makefile
# $NetBSD: directive-for-if.mk,v 1.1 2021/08/30 17:08:13 rillig Exp $
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#
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# Test for a .for directive that contains an .if directive.
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#
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# Before for.c 1.39 from 2008-12-21, when expanding the variables of a .for
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# loop, their values were placed verbatim in the expanded body. Since then,
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# each variable value expands to an expression of the form ${:Uvalue}.
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#
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# Before that change, the following adventurous code was possible:
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#
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# .for directive in if ifdef ifndef
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# . ${directive} "1" != "0"
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# . endif
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# .endfor
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#
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# A more practical usage of the .for loop that often led to surprises was the
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# following:
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#
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# .for var in VAR1 VAR2 VAR3
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# . if ${var} != "VAR2"
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# . endif
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# .endfor
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#
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# The .for loop body expanded to this string:
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#
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# . if VAR1 != "VAR2"
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# . endif
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#
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# Since bare words were not allowed at the left-hand side of a condition,
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# make complained about a "Malformed conditional", which was surprising since
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# the code before expanding the .for loop body looked quite well.
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#
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# In cond.c 1.48 from 2008-11-29, just a month before the expansion of .for
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# loops changed from plain textual value to using expressions of the form
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# ${:Uvalue}, this surprising behavior was documented in the code, and a
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# workaround was implemented that allowed bare words when they are followed
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# by either '!' or '=', as part of the operators '!=' or '=='.
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#
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# Since cond.c 1.68 from 2015-05-05, bare words are allowed on the left-hand
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# side of a condition, but that applies only to expression of the form
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# ${${cond} :? then : else}, it does not apply to conditions in ordinary .if
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# directives.
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# The following snippet worked in 2005, when the variables from the .for loop
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# expanded to their bare textual value.
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.for directive in if ifdef ifndef
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. ${directive} "1" != "0"
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. endif
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.endfor
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# In 2021, the above code does not generate an error message, even though the
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# code looks clearly malformed. This is due to the '!', which is interpreted
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# as a dependency operator, similar to ':' and '::'. The parser turns this
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# line into a dependency with the 3 targets '.', 'if', '"1"' and the 2 sources
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# '=' and '"0"'. Since that line is not interpreted as an '.if' directive,
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# the error message 'if-less endif' makes sense.
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# In 2005, make complained:
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#
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# .if line: Malformed conditional (VAR1 != "VAR2")
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# .endif line: if-less endif
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# .endif line: Need an operator
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#
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# 2008.11.30.22.37.55 does not complain about the left-hand side ${var}.
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.for var in VAR1 VAR2 VAR3
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. if ${var} != "VAR2"
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_!= echo "${var}" 1>&2; echo # In 2005, '.info' was not invented yet.
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. endif
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.endfor
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# Before for.c 1.39 from 2008-12-21, a common workaround was to surround the
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# variable expression from the .for loop with '"'. Such a string literal
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# has been allowed since cond.c 1.23 from 2004-04-13. Between that commit and
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# the one from 2008, the parser would still get confused if the value from the
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# .for loop contained '"', which was effectively a code injection.
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#
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# Surrounding ${var} with quotes disabled the check for typos though. For
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# ordinary variables, referring to an undefined variable on the left-hand side
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# of the comparison resulted in a "Malformed conditional". Since the .for
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# loop was usually close to the .if clause, this was not a problem in
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# practice.
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.for var in VAR1 VAR2 VAR3
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. if "${var}" != "VAR2"
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. endif
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.endfor
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all:
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