Makefile上面会有个变量区:
VRE_D : make VRE_D=1 or true (必须得有等号,因为它不是target,另外ifdef,只要有值就是true)
OPT: make OPT=add(得等于固定的值)
其实更多使用方法是直接赋值;在变量区调用echo会出错,why???
# 注释顶格写,不然会被输出
不加@ ,会把这条指令也输出出来
#firstly,serval method of assignment, ?= assignmen when no value
#:= reassigned
VAR ?= "defined already"
ifdef VRE_D
VRE = "hello world"
endif
ifeq ($(OPT),define)
VRE ?= "hello world, first\n"
endif
ifeq ($(OPT), add)
VRE += "Kelly"
endif
ifeq ($(OPT),recover)
VRE := "hello world , again !!!"
endif
all:
@echo $(VRE)
@echo $(VAR)
#@ls /dev