函数对文件夹变量的引用细节：
经过patsubst，foreach函数处理，文件变量如下：
Makefile:
从第14行开始，"/*.S"字段非注释，但是变了颜色，忽略颜色变化。
1 CROSS_COMPILE ?= arm-linux-gnueabihf- 2 TARGET ?= key 3 4 CC := $(CROSS_COMPILE)gcc 5 LD := $(CROSS_COMPILE)ld 6 OBJCOPY :=$(CROSS_COMPILE)objcopy 7 OBJDUMP :=$(CROSS_COMPILE)objdump 8 9 INCUDIRS := imx6u bsp/beep bsp/clk bsp/delay bsp/key bsp/led 10 11 SRCDIRS := project bsp/beep bsp/clk bsp/delay bsp/key bsp/led 12 13 INCLUDE := $(patsubst %, -I %,$(INCUDIRS)) 14 SFILES :=$(foreach dir,$(SRCDIRS),$(wildcard $(dir)/*.S)) 15 CFILES :=$(foreach dir,$(SRCDIRS),$(wildcard $(dir)/*.c)) 16 SFILENDIR :=$(notdir $(SFILES)) 17 CFILENDIR :=$(notdir $(CFILES)) 18 SOBJS := $(patsubst %, obj/%, $(SFILENDIR:.S=.o)) 19 COBJS := $(patsubst %, obj/%, $(CFILENDIR:.c=.o)) 20 OBJS := $(SOBJS) $(COBJS) 21 22 VPATH := $(SRCDIRS) 23 24 25 .PHONY: clean 26 27 $(TARGET).bin : $(OBJS) 28 $(LD) -Timx6u.lds -o $(TARGET).elf $^ 29 $(OBJCOPY) -O binary -S $(TARGET).elf $@ 30 $(OBJDUMP) -D -m arm $(TARGET).elf > $(TARGET).dis 31 32 $(SOBJS) : obj/%.o:%.S 33 $(CC) -Wall -nostdlib -c -O2 $(INCLUDE) -o $@ $< 34 35 $(COBJS): obj/%.o:%.c 36 $(CC) -Wall -nostdlib -c -O2 $(INCLUDE) -o $@ $< 37 38 clean: 39 rm -rf $(TARGET).elf $(TARGET).bin $(TARGET).dis $(OBJS) load.imx 40 41 print: 42 @echo VPATH=$(VPATH) 43 @echo INCLUDE=$(INCLUDE)