一、本文说明
本文为linux内核Makefile整体分析的续篇，是依据Linux内核Makefile体系的主要内容编写一个简要的测试工程。Linux内核Makefile体系就好像一只“大鸟”，而这篇测试算是“麻雀”，主要是为了通过动手实战进一步理解Linux内核Makefile体系的原理和特点。
二、源码结构
1、测试源码包括1个顶层目录以及5个子目录。
2、Makefile体系的构成是顶层1个Makefile文件、scripts目录中的4个脚本文件以及add、sub、main三个目录中的Makefile文件。
3、C程序源码是add/add.c、sub/sub.c、main/main.c
三、Makefile重要源码展示
1、顶层Makefile
PHONY :=
MAKE := makesrctree := $(if $(KBUILD_SRC),$(KBUILD_SRC),$(CURDIR))
export srctree AS = $(CROSS_COMPILE)as
LD = $(CROSS_COMPILE)ld
CC = $(CROSS_COMPILE)gcc
CPP = $(CC) -E
AR = $(CROSS_COMPILE)ar
NM = $(CROSS_COMPILE)nmexport AS LD CC CPP AR NM include $(srctree)/scripts/Kbuild.include //该文件中定义有build变量all: targetsrcdir := add sub main
PHONY += $(srcdir)objdir := $(srcdir)
PHONY += $(objdir)target: $(srcdir) //总目标的依赖是srcdir
gcc -o target add/built-in.o sub/built-in.o main/built-in.o //定义总目标的生成规则$(srcdir): //通过srcdir依次生成各个子目录中的*/built-in.o
$(MAKE) $(build)=$@ clean-dirs := $(addprefix _clean_,$(objdir)) //clean-dirs其实与objdir中的目录是一样的，只是为了执行不同的命令（clean-dirs要删除目标文件），所以要clean-dirs的前边要加上前缀以与srcdir=objdir的目标区别
PHONY += $(clean-dirs) clean$(clean-dirs):
$(MAKE) $(clean)=$(patsubst _clean_%,%,$@)clean: $(clean-dirs)
rm -f targetclean := -f scripts/Makefile.clean obj.PHONY: $(PHONY)
2、srcdirs/Makefile.build
# ==========================================================================
# Building
# ==========================================================================src := $(obj)PHONY := __build
__build: //Makefile.build中的总目标include scripts/Kbuild.include# The filename Kbuild has precedence over Makefile
kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src))
kbuild-file := $(if $(wildcard $(kbuild-dir)/Kbuild),$(kbuild-dir)/Kbuild,$(kbuild-dir)/Makefile)
include $(kbuild-file) //包含子目录中的Makefileinclude scripts/Makefile.libifneq ($(strip $(obj-y)),)
builtin-target := $(obj)/built-in.o
endif__build: $(builtin-target)# If the list of objects to link is empty, just create an empty built-in.o
cmd_link_o_target = $(if $(strip $(obj-y)),\
$(LD) $(ld_flags) -r -o $@ $(filter $(obj-y), $^))$(builtin-target): $(obj-y)
$(call if_changed,link_o_target) //调用生成库built-in.o的函数cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $<define rule_cc_o_c
$(cmd_cc_o_c)
endef# Built-in and composite module parts
$(obj)/%.o: $(src)/%.c
$(call if_changed_rule,cc_o_c) //调用生成目标文件.o的函数PHONY += FORCE
FORCE:.PHONY: $(PHONY)
3、srcdirs/Kbuild.include
build := -f scripts/Makefile.build objif_changed = $(cmd_$())//定义函数if_changed # Usage: $(call if_changed_rule,foo)
# Will check if $(cmd_foo) or any of the prerequisites changed,
# and if so will execute $(rule_foo).
if_changed_rule = $(rule_$()) //定义函数if_changed_rule
4、srcdirs/Makefile.lib
c_flags = //定义gcc编译器选项ld_flags = $(LDFLAGS) $(ldflags-y) //定义ld连接器选项
obj-y := $(patsubst %/, %/built-in.o, $(obj-y)) //对obj-y中的目录添加built-in.o
obj-y := $(addprefix $(obj)/,$(obj-y))//再添加obj前缀
5、srcdirs/Makefile.clean
# ==========================================================================
# Cleaning up
# ==========================================================================src := $(obj)PHONY := __clean //此为Makefile.clean中的总目标
__clean:
rm -f $(obj)/*.o //删除目标文件.o
6、add/Makefile
obj-y += add.o
四、当前源码缺陷
1、没有完成依赖的自动生成
2、没有完成生成目标的目录与源码不在同一目录的功能
