target : main.o g_a.o g_b.o
        gcc -o $@ $^
        @echo $@
        @echo $^
        
main.o : main.c g_a.h g_b.h
        gcc -o main.o -c main.c
g_a.o : g_a.c g_a.h
        gcc -o g_a.o -c g_a.c
g_b.o : g_b.c g_b.h
        gcc -o g_b.o -c g_b.c
a:
        @echo $@
        @echo $^
.PHONY : clean

src = $(wildcard ./sub/*.c)
dir = $(notdir $(src))
obj = $(patsubst %.c,%.o,$(dir))

all :

注意看输出结果：