How do you define pattern-specific variables for makefile pattern rules containing a patterned prerequisite?

Question

I understand that you can define pattern-specific variables for rules with no prerequisites, like this:

%.o: var = 2

This will set variable var to 2 for recipes that apply to all targets ending in .o. This is clearly stated in the GNU Make documentation. But how do you define a pattern-specific variable for pattern rules that contain a pattern prerequisite, like this:

%.o: %.c

Say I have this section of makefile:

%.o: %.c
    (recipe here)

%.o: %.b
    (recipe here)

I want to define a pattern-specific variable only for the %.o: %.b rule, but I don't see how to do it (if it's even possible). I'd like to do something like this, but of course it doesn't work:

%.o: %.c
    (recipe here)

%.o: %.b: var = 2
    (recipe here)

Is there a way to do this?

Ivan Baldin Ivan Baldin · Accepted Answer · 2016-07-21T22:15:21

You can only set variable for targets, not rules. %.o: %b is a rule where %.o is a target pattern (hence the "pattern-specific" name).

The usual way to solve this is ether hard coding values in the recipes or using rule-specific flags (maybe CVAR and BVAR in your case).

EDIT: Scratch that. Came up with a workaround.

It can be done by leveraging variables' recursive evaluation.

all: a.o b.o c.o

$(shell touch a.xx b.yy c.zz)

##
# Create rule-specific variable... rules
#
# @param 1 Target.
# @param 2 Prerequisite.
# @param 3 Variable name.
# @param 4 Variable value.
#
rule-var = $(eval $(rule-var-body))
define rule-var-body
$1: private $3 = $$(if $$(<:$2=),$(or $(value rule-var-$1-$3),$(value $3)),$4)
$2: $3 = $4
rule-var-$1-$3 = $$(if $$(<:$2=),$(or $(value rule-var-$1-$3),$(value $3)),$4)
endef

VAR = $(VAR_DEFAULT)

# Declare couple of test values
$(call rule-var,%.o,%.x,VAR,x-value)
$(call rule-var,%.o,%.y,VAR,y-value)
VAR_DEFAULT := z-value

ECHO_RULE_RECIPE = @echo -e '$@: $^\t(VAR = $(VAR))'

%.o: %.x
    $(ECHO_RULE_RECIPE)
%.o: %.y
    $(ECHO_RULE_RECIPE)
%.o: %.z
    $(ECHO_RULE_RECIPE)
%.x: %.xx
    $(ECHO_RULE_RECIPE)
%.y: %.yy
    $(ECHO_RULE_RECIPE)
%.z: %.zz
    $(ECHO_RULE_RECIPE)

The output is:

a.x: a.xx       (VAR = x-value)
a.o: a.x        (VAR = x-value)
b.y: b.yy       (VAR = y-value)
b.o: b.y        (VAR = y-value)
c.z: c.zz       (VAR = z-value)
c.o: c.z        (VAR = z-value)

The brains of the operation is macro rule-var. It will wrap variable value in a prerequisite matching if-else expression. The expression is also saved in rule-var-$1-$3 variable for other rule-specific values.

$$(if $$(<:$2=),$(or $(value rule-var-$1-$3),$(value $3)),$4) deobfuscation:

$$(if $$(<:$2=), will test first prerequisite value ($<) by replacing it's pattern ($2) with empty string.

If pattern doesn't match, use $(or $(value rule-var-$1-$3),$(value $3)). This is a workaround for global variable shadowing. In your example %.o: %.c doesn't have var declared so it should use global value but both rules share the same target, it's not visible. Both are referenced by value and single $ expands the expression during variable substitution phase. So the result is neat and or free.
- Use $(value rule-var-$1-$3) if it's nonzero. That is function has been called before for this target and variable name.
- Otherwise use variable's global value ($(value $3)).
Otherwise use the value provided ($4).

Unfortunately, when inherited, this if-else monstrosity won't expand properly so it's declared as private and fixed with a straightforward second rule.

In this example the following 3 rules will be declared.

%.o: private VAR = $(if $(<:%.y=),$(if $(<:%.x=),$(VAR_DEFAULT),x-value),y-value)
%.y: VAR = y-value
%.x: VAR = x-value

Limitations

Even with a workaround, variable's global counterpart is still shadowed. If you need a default value, assign it before calling rule-var. Global value is copied as a part of the rule-specific variable but not expanded until use.

How do you define pattern-specific variables for makefile pattern rules containing a patterned prerequisite?

2 Answers