85 lines
3.9 KiB
Makefile
85 lines
3.9 KiB
Makefile
# $NetBSD: var-scope-cmdline.mk,v 1.2 2023/04/07 05:54:16 rillig Exp $
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#
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# Tests for variables specified on the command line.
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#
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# Variables that are specified on the command line override those from the
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# global scope.
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#
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# For performance reasons, the actual implementation is more complex than the
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# above single-sentence rule, in order to avoid unnecessary lookups in scopes,
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# which before var.c 1.586 from 2020-10-25 calculated the hash value of the
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# variable name once for each lookup. Instead, when looking up the value of
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# a variable, the search often starts in the global scope since that is where
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# most of the variables are stored. This conflicts with the statement that
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# variables from the cmdline scope override global variables, since after the
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# common case of finding a variable in the global scope, another lookup would
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# be needed in the cmdline scope to ensure that there is no overriding
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# variable there.
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#
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# Instead of this costly lookup scheme, make implements it in a different
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# way:
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#
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# Whenever a global variable is created, this creation is ignored if
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# there is a cmdline variable of the same name.
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#
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# Whenever a cmdline variable is created, any global variable of the
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# same name is deleted.
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#
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# Whenever a global variable is deleted, nothing special happens.
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#
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# Deleting a cmdline variable is not possible.
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#
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# These 4 rules provide the guarantee that whenever a global variable exists,
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# there cannot be a cmdline variable of the same name. Therefore, after
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# finding a variable in the global scope, no additional lookup is needed in
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# the cmdline scope.
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#
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# The above ruleset provides the same guarantees as the simple rule "cmdline
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# overrides global". Due to an implementation mistake, the actual behavior
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# was not entirely equivalent to the simple rule though. The mistake was
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# that when a cmdline variable with '$$' in its name was added, a global
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# variable was deleted, but not with the exact same name as the cmdline
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# variable. Instead, the name of the global variable was expanded one more
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# time than the name of the cmdline variable. For variable names that didn't
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# have a '$$' in their name, it was implemented correctly all the time.
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#
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# The bug was added in var.c 1.183 on 2013-07-16, when Var_Set called
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# Var_Delete to delete the global variable. Just two months earlier, in var.c
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# 1.174 from 2013-05-18, Var_Delete had started to expand the variable name.
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# Together, these two changes made the variable name be expanded twice in a
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# row. This bug was fixed in var.c 1.835 from 2021-02-22.
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#
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# Another bug was the wrong assumption that "deleting a cmdline variable is
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# not possible". Deleting such a variable has been possible since var.c 1.204
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# from 2016-02-19, when the variable modifier ':@' started to delete the
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# temporary loop variable after finishing the loop. It was probably not
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# intended back then that a side effect of this seemingly simple change was
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# that both global and cmdline variables could now be undefined at will as a
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# side effect of evaluating a variable expression. As of 2021-02-23, this is
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# still possible.
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#
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# Most cmdline variables are set at the very beginning, when parsing the
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# command line arguments. Using the special target '.MAKEFLAGS', it is
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# possible to set cmdline variables at any later time.
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#
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# See also:
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# varcmd.mk
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# varname-makeflags.mk
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# A normal global variable, without any cmdline variable nearby.
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VAR= global
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.info ${VAR}
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# The global variable is "overridden" by simply deleting it and then
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# installing the cmdline variable instead. Since there is no obvious way to
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# undefine a cmdline variable, there is no need to remember the old value
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# of the global variable could become visible again.
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#
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# See varmod-loop.mk for a non-obvious way to undefine a cmdline variable.
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.MAKEFLAGS: VAR=makeflags
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.info ${VAR}
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# If Var_SetWithFlags should ever forget to delete the global variable,
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# the below line would print "global" instead of the current "makeflags".
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.MAKEFLAGS: -V VAR
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