Linux v6.6.1 - variable

# SPDX-License-Identifier: GPL-2.0

# Shorthand
warning = $(warning-if,y,$(1))

# Simply expanded variable.
X := 1
SIMPLE := $(X)
X := 2
$(warning,SIMPLE = $(SIMPLE))

# Recursively expanded variable.
X := 1
RECURSIVE = $(X)
X := 2
$(warning,RECURSIVE = $(RECURSIVE))

# Append something to a simply expanded variable.
Y := 3
SIMPLE += $(Y)
Y := 4
$(warning,SIMPLE = $(SIMPLE))

# Append something to a recursively expanded variable.
Y := 3
RECURSIVE += $(Y)
Y := 4
$(warning,RECURSIVE = $(RECURSIVE))

# Use += operator to an undefined variable.
# This works as a recursively expanded variable.
Y := 3
UNDEFINED_VARIABLE += $(Y)
Y := 4
$(warning,UNDEFINED_VARIABLE = $(UNDEFINED_VARIABLE))

# You can use variable references for the lefthand side of assignment statement.
X := A
Y := B
$(X)$(Y) := 5
$(warning,AB = $(AB))

# User-defined function.
greeting = $(1), my name is $(2).
$(warning,$(greeting,Hello,John))

# The number of arguments is not checked for user-defined functions.
# If some arguments are optional, it is useful to pass fewer parameters.
# $(2) will be blank in this case.
$(warning,$(greeting,Hello))

# Unreferenced parameters are just ignored.
$(warning,$(greeting,Hello,John,ignored,ignored))