1 # $NetBSD: var-class-cmdline.mk,v 1.5 2021/02/23 21:59:31 rillig Exp $
3 # Tests for variables specified on the command line.
5 # Variables that are specified on the command line override those from the
8 # For performance reasons, the actual implementation is more complex than the
9 # above single-sentence rule, in order to avoid unnecessary lookups in scopes,
10 # which before var.c 1.586 from 2020-10-25 calculated the hash value of the
11 # variable name once for each lookup. Instead, when looking up the value of
12 # a variable, the search often starts in the global scope since that is where
13 # most of the variables are stored. This conflicts with the statement that
14 # variables from the cmdline scope override global variables, since after the
15 # common case of finding a variable in the global scope, another lookup would
16 # be needed in the cmdline scope to ensure that there is no overriding
19 # Instead of this costly lookup scheme, make implements it in a different
22 # Whenever a global variable is created, this creation is ignored if
23 # there is a cmdline variable of the same name.
25 # Whenever a cmdline variable is created, any global variable of the
26 # same name is deleted.
28 # Whenever a global variable is deleted, nothing special happens.
30 # Deleting a cmdline variable is not possible.
32 # These 4 rules provide the guarantee that whenever a global variable exists,
33 # there cannot be a cmdline variable of the same name. Therefore, after
34 # finding a variable in the global scope, no additional lookup is needed in
37 # The above ruleset provides the same guarantees as the simple rule "cmdline
38 # overrides global". Due to an implementation mistake, the actual behavior
39 # was not entirely equivalent to the simple rule though. The mistake was
40 # that when a cmdline variable with '$$' in its name was added, a global
41 # variable was deleted, but not with the exact same name as the cmdline
42 # variable. Instead, the name of the global variable was expanded one more
43 # time than the name of the cmdline variable. For variable names that didn't
44 # have a '$$' in their name, it was implemented correctly all the time.
46 # The bug was added in var.c 1.183 on 2013-07-16, when Var_Set called
47 # Var_Delete to delete the global variable. Just two months earlier, in var.c
48 # 1.174 from 2013-05-18, Var_Delete had started to expand the variable name.
49 # Together, these two changes made the variable name be expanded twice in a
50 # row. This bug was fixed in var.c 1.835 from 2021-02-22.
52 # Another bug was the wrong assumption that "deleting a cmdline variable is
53 # not possible". Deleting such a variable has been possible since var.c 1.204
54 # from 2016-02-19, when the variable modifier ':@' started to delete the
55 # temporary loop variable after finishing the loop. It was probably not
56 # intended back then that a side effect of this seemingly simple change was
57 # that both global and cmdline variables could now be undefined at will as a
58 # side effect of evaluating a variable expression. As of 2021-02-23, this is
61 # Most cmdline variables are set at the very beginning, when parsing the
62 # command line arguments. Using the special target '.MAKEFLAGS', it is
63 # possible to set cmdline variables at any later time.
65 # A normal global variable, without any cmdline variable nearby.
69 # The global variable is "overridden" by simply deleting it and then
70 # installing the cmdline variable instead. Since there is no obvious way to
71 # undefine a cmdline variable, there is no need to remember the old value
72 # of the global variable could become visible again.
74 # See varmod-loop.mk for a non-obvious way to undefine a cmdline variable.
75 .MAKEFLAGS: VAR=makeflags
78 # If Var_SetWithFlags should ever forget to delete the global variable,
79 # the below line would print "global" instead of the current "makeflags".