counter.mk revision 1.2 
1# $NetBSD: counter.mk,v 1.2 2020/09/23 03:33:55 rillig Exp $
2#
3# Demonstrates that it is not easily possible to let make count
4# the number of times a variable is actually accessed, using the
5# ::= variable modifier.
6#
7# As of 2020-08-02, the counter ends up at having 4 words, even
8# though the NEXT variable is only accessed 3 times.  This is
9# surprising.
10#
11# A hint to this surprising behavior is that the variables don't
12# get fully expanded.  For example, A does not simply contain the
13# value "1" but an additional unexpanded ${COUNTER:...} before it.
14
15.MAKEFLAGS: -dv
16
17RELEVANT=	yes (load-time part)	# just to filter the output
18
19COUNTER=	# zero
20
21NEXT=		${COUNTER::=${COUNTER} a}${COUNTER:[#]}
22
23# This variable is first set to empty and then expanded.
24# See parse.c, function Parse_DoVar, keyword "!Var_Exists".
25A:=		${NEXT}
26B:=		${NEXT}
27C:=		${NEXT}
28
29RELEVANT=	no
30
31all:
32	@: ${RELEVANT::=yes (run-time part)}
33	@echo A=${A:Q} B=${B:Q} C=${C:Q} COUNTER=${COUNTER:[#]:Q}
34	@: ${RELEVANT::=no}
35