在linux kernel 1.0的net目录下查看子目录及文件结构：
[root@localhost net]# ls -R
.:
ddi.c inet Makefile socket.c Space.c unix
./inet:
arp.c datagram.c dev.h eth.h icmp.h ip.c loopback.c packet.c protocol.c raw.c README route.h skbuff.h sock.h tcp.h udp.c utils.c
arp.h dev.c eth.c icmp.c inet.h ip.h Makefile proc.c protocol.h raw.h route.c skbuff.c sock.c tcp.c timer.c udp.h
./unix:
Makefile proc.c sock.c unix.h
可以看到net由ddi.c，socket.c和Space.c三个文件，inet和unix两个目录，还有Makefile组成。
暂时不理睬inet和unix目录下的文件，看看Makefile的内容：
# Note! Dependencies are done automagically by 'make dep', which also
# removes any old dependencies. DON'T put your own dependencies here
# unless it's something special (ie not a .c file).
#
# Note 2! The CFLAGS definition is now in the main makefile...
# only these two lines should need to be changed to remove inet sockets.
# (and the inet/tcpip.o in net.o)
SUBDIRS := unix inet
SUBOBJS := $(foreach f,$(SUBDIRS),$f/$f.o)
#SUBOBJS的值为 unix/unix.o inet/inet.o
.c.o:
$(CC) $(CFLAGS) -c $<
.s.o:
$(AS) -o $*.o $<
.c.s:
$(CC) $(CFLAGS) -S $<
#老式makefile中的隐式规则定义
#如果编译目标为xx.o:xx.c则自动引用第一个规则，依此类推
OBJS = Space.o ddi.o socket.o
all: subdirs net.o
net.o: $(OBJS) network.a
$(LD) -r -o net.o $(OBJS) network.a
#将Space.o,ddi.o,socket.o和netowrk.a编译成net.o，net.o再编译进内核中。
network.a: $(SUBOBJS)
rm -f $@
ar rc $@ $(SUBOBJS)
ranlib $@
#将unix/unix.o和inet/inet.o打包成network.a
subdirs: dummy
set -e; for i in $(SUBDIRS); do $(MAKE) -C $$i; done
#进入各个子目录分别编译unix.o和inet.o
dep:
$(CPP) -M *.c > .depend
set -e; for i in $(SUBDIRS); do $(MAKE) -C $$i dep; done
dummy:
#假的target
#
# include a dependency file if one exists
#
ifeq (.depend,$(wildcard .depend))
include .depend
endif
"Makefile" 51L, 1023C