GNU make中的2种变量#

官网的介绍

1. recursively expanded variable（使用时展开）#

=

example: 

foo = $(bar)
bar = $(ugh)
ugh = Huh?

all:
    echo $(foo)

执行结果：

$ make all

echo Huh?
Huh?

2. Simply expanded variables（定义时展开）#

Variables defined with := in GNU make are expanded when they are defined rather than when they are used.

:=

x := foo
y := $(x) bar
x := later

is equivalent to

y := foo bar
x := later

3. conditional variable（条件变量，假如未定义才定义）#

There is another assignment operator for variables, ‘?=’. This is called a conditional variable assignment operator, because it only has an effect if the variable is not yet defined. This statement:

FOO ?= bar

is exactly equivalent to this (see The origin Function):

ifeq ($(origin FOO), undefined)
  FOO = bar
endif

Note that a variable set to an empty value is still defined, so ‘?=’ will not set that variable.