scripts/Makefile.host 分析【fixdep、conf】

文章目录

1. 目标 $(obj)/fixdep
- 1.1 cmd_host-csingle函数分析：
2. 目标 $(obj)/conf
- 2.1 cmd_host-cmulti函数分析：
- 2.2 生成 $(obj)/conf.o 文件：
- 2.3 生成 $(obj)/zconf.tab.o 文件：

1  	# SPDX-License-Identifier: GPL-2.0
2  	# ==========================================================================
3  	# Building binaries on the host system
4  	# Binaries are used during the compilation of the kernel, for example
5  	# to preprocess a data file.
6  	#
7  	# Both C and C++ are supported, but preferred language is C for such utilities.
8  	#
9  	# Sample syntax (see Documentation/kbuild/makefiles.txt for reference)
10 	# hostprogs-y := bin2hex
11 	# Will compile bin2hex.c and create an executable named bin2hex
12 	#
13 	# hostprogs-y    := lxdialog
14 	# lxdialog-objs := checklist.o lxdialog.o
15 	# Will compile lxdialog.c and checklist.c, and then link the executable
16 	# lxdialog, based on checklist.o and lxdialog.o
17 	#
18 	# hostprogs-y      := qconf
19 	# qconf-cxxobjs   := qconf.o
20 	# qconf-objs      := menu.o
21 	# Will compile qconf as a C++ program, and menu as a C program.
22 	# They are linked as C++ code to the executable qconf
23 
24 	__hostprogs := $(sort $(hostprogs-y) $(hostprogs-m))
25 	host-cshlib := $(sort $(hostlibs-y) $(hostlibs-m))
26 	host-cxxshlib := $(sort $(hostcxxlibs-y) $(hostcxxlibs-m))
27 
28 	# C code
29 	# Executables compiled from a single .c file
30 	host-csingle	:= $(foreach m,$(__hostprogs), \
31 				$(if $($(m)-objs)$($(m)-cxxobjs)$($(m)-sharedobjs),,$(m)))
32 
33 	# C executables linked based on several .o files
34 	host-cmulti	:= $(foreach m,$(__hostprogs),\
35 			   $(if $($(m)-cxxobjs),,$(if $($(m)-objs),$(m))))
36 
37 	# Shared object libraries
38 	host-shared	:= $(foreach m,$(__hostprogs),\
39 			   $(if $($(m)-sharedobjs),$(m))))
40 
41 	# Object (.o) files compiled from .c files
42 	host-cobjs	:= $(sort $(foreach m,$(__hostprogs),$($(m)-objs)))
43 
44 	# C++ code
45 	# C++ executables compiled from at least one .cc file
46 	# and zero or more .c files
47 	host-cxxmulti	:= $(foreach m,$(__hostprogs),$(if $($(m)-cxxobjs),$(m)))
48 
49 	# C++ Object (.o) files compiled from .cc files
50 	host-cxxobjs	:= $(sort $(foreach m,$(host-cxxmulti),$($(m)-cxxobjs)))
51 
52 	# Object (.o) files used by the shared libaries
53 	host-cshobjs	:= $(sort $(foreach m,$(host-cshlib),$($(m:.so=-objs))))
54 	host-cxxshobjs	:= $(sort $(foreach m,$(host-cxxshlib),$($(m:.so=-objs))))
55 
56 	host-csingle	:= $(addprefix $(obj)/,$(host-csingle))
57 	host-cmulti	:= $(addprefix $(obj)/,$(host-cmulti))
58 	host-cobjs	:= $(addprefix $(obj)/,$(host-cobjs))
59 	host-cxxmulti	:= $(addprefix $(obj)/,$(host-cxxmulti))
60 	host-cxxobjs	:= $(addprefix $(obj)/,$(host-cxxobjs))
61 	host-cshlib	:= $(addprefix $(obj)/,$(host-cshlib))
62 	host-cxxshlib	:= $(addprefix $(obj)/,$(host-cxxshlib))
63 	host-cshobjs	:= $(addprefix $(obj)/,$(host-cshobjs))
64 	host-cxxshobjs	:= $(addprefix $(obj)/,$(host-cxxshobjs))
65 	host-shared	:= $(addprefix $(obj)/,$(host-shared))
66 
67 	#####
68 	# Handle options to gcc. Support building with separate output directory
69 
70 	_hostc_flags   = $(KBUILD_HOSTCFLAGS)   $(HOST_EXTRACFLAGS)   \
71 					 $(HOSTCFLAGS_$(basetarget).o)
72 	_hostcxx_flags = $(KBUILD_HOSTCXXFLAGS) $(HOST_EXTRACXXFLAGS) \
73 					 $(HOSTCXXFLAGS_$(basetarget).o)
74 
75 	ifeq ($(KBUILD_SRC),)
76 	__hostc_flags	= $(_hostc_flags)
77 	__hostcxx_flags	= $(_hostcxx_flags)
78 	else
79 	__hostc_flags	= -I$(obj) $(call flags,_hostc_flags)
80 	__hostcxx_flags	= -I$(obj) $(call flags,_hostcxx_flags)
81 	endif
82 
83 	hostc_flags    = -Wp,-MD,$(depfile) $(__hostc_flags)
84 	hostcxx_flags  = -Wp,-MD,$(depfile) $(__hostcxx_flags)
85 
86 	#####
87 	# Compile programs on the host
88 
89 	# Create executable from a single .c file
90 	# host-csingle -> Executable
91 	quiet_cmd_host-csingle 	= HOSTCC  $@
92 		  cmd_host-csingle	= $(HOSTCC) $(hostc_flags) -o $@ $< \
93 			$(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(@F))
94 	$(host-csingle): $(obj)/%: $(src)/%.c FORCE
95 		$(call if_changed_dep,host-csingle)
96 
97 	# Link an executable based on list of .o files, all plain c
98 	# host-cmulti -> executable
99 	quiet_cmd_host-cmulti	= HOSTLD  $@
100		  cmd_host-cmulti	= $(HOSTCC) $(KBUILD_HOSTLDFLAGS) -o $@ \
101				  $(addprefix $(obj)/,$($(@F)-objs)) \
102				  $(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(@F))
103	$(host-cmulti): FORCE
104		$(call if_changed,host-cmulti)
105	$(call multi_depend, $(host-cmulti), , -objs)
106
107	# Create .o file from a single .c file
108	# host-cobjs -> .o
109	quiet_cmd_host-cobjs	= HOSTCC  $@
110		  cmd_host-cobjs	= $(HOSTCC) $(hostc_flags) -c -o $@ $<
111	$(host-cobjs): $(obj)/%.o: $(src)/%.c FORCE
112		$(call if_changed_dep,host-cobjs)
113
114	# Link an executable based on list of .o files, a mixture of .c and .cc
115	# host-cxxmulti -> executable
116	quiet_cmd_host-cxxmulti	= HOSTLD  $@
117		  cmd_host-cxxmulti	= $(HOSTCXX) $(KBUILD_HOSTLDFLAGS) -o $@ \
118				  $(foreach o,objs cxxobjs,\
119				  $(addprefix $(obj)/,$($(@F)-$(o)))) \
120				  $(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(@F))
121	$(host-cxxmulti): FORCE
122		$(call if_changed,host-cxxmulti)
123	$(call multi_depend, $(host-cxxmulti), , -objs -cxxobjs)
124
125	# Create .o file from a single .cc (C++) file
126	quiet_cmd_host-cxxobjs	= HOSTCXX $@
127		  cmd_host-cxxobjs	= $(HOSTCXX) $(hostcxx_flags) -c -o $@ $<
128	$(host-cxxobjs): $(obj)/%.o: $(src)/%.cc FORCE
129		$(call if_changed_dep,host-cxxobjs)
130
131	# Compile .c file, create position independent .o file
132	# host-cshobjs -> .o
133	quiet_cmd_host-cshobjs	= HOSTCC  -fPIC $@
134		  cmd_host-cshobjs	= $(HOSTCC) $(hostc_flags) -fPIC -c -o $@ $<
135	$(host-cshobjs): $(obj)/%.o: $(src)/%.c FORCE
136		$(call if_changed_dep,host-cshobjs)
137
138	# Compile .c file, create position independent .o file
139	# Note that plugin capable gcc versions can be either C or C++ based
140	# therefore plugin source files have to be compilable in both C and C++ mode.
141	# This is why a C++ compiler is invoked on a .c file.
142	# host-cxxshobjs -> .o
143	quiet_cmd_host-cxxshobjs	= HOSTCXX -fPIC $@
144		  cmd_host-cxxshobjs	= $(HOSTCXX) $(hostcxx_flags) -fPIC -c -o $@ $<
145	$(host-cxxshobjs): $(obj)/%.o: $(src)/%.c FORCE
146		$(call if_changed_dep,host-cxxshobjs)
147
148	# Link a shared library, based on position independent .o files
149	# *.o -> .so shared library (host-cshlib)
150	quiet_cmd_host-cshlib	= HOSTLLD -shared $@
151		  cmd_host-cshlib	= $(HOSTCC) $(HOSTLDFLAGS) -shared -o $@ \
152				  $(addprefix $(obj)/,$($(@F:.so=-objs))) \
153				  $(HOST_LOADLIBES) $(HOSTLDLIBS_$(@F))
154	$(host-cshlib): FORCE
155		$(call if_changed,host-cshlib)
156	$(call multi_depend, $(host-cshlib), .so, -objs)
157
158	# Link a shared library, based on position independent .o files
159	# *.o -> .so shared library (host-cxxshlib)
160	quiet_cmd_host-cxxshlib	= HOSTLLD -shared $@
161		  cmd_host-cxxshlib	= $(HOSTCXX) $(HOSTLDFLAGS) -shared -o $@ \
162				  $(addprefix $(obj)/,$($(@F:.so=-objs))) \
163				  $(HOST_LOADLIBES) $(HOSTLDLIBS_$(@F))
164	$(host-cxxshlib): FORCE
165		$(call if_changed,host-cxxshlib)
166	$(call multi_depend, $(host-cxxshlib), .so, -objs)
167
168	targets += $(host-csingle)  $(host-cmulti) $(host-cobjs)\
169		   $(host-cxxmulti) $(host-cxxobjs) $(host-shared) \
170		   $(host-cshlib) $(host-cshobjs) $(host-cxxshlib) $(host-cxxshobjs)
171

1. 目标 $(obj)/fixdep

由于顶层 Makefile 的482行执行make -f ./scripts/Makefile.build obj=scripts/basic<见执行依赖scripts_basic>，又在 Makefile.build 中第57行引用scripts/basic/Makefile<见1.1>，并且其中有如下定义：
12 hostprogs-y	:= fixdep
13 always		:= $(hostprogs-y)
16 $(addprefix $(obj)/,$(filter-out fixdep,$(always))): $(obj)/fixdep
$(hostprogs-m)、$(hostlibs-y)、$(hostlibs-m)、$(hostcxxlibs-y)、$(hostcxxlibs-m) 未定义，所以24-65行展开为:
24 __hostprogs   := fixdep
25 host-cshlib 	 :=
26 host-cxxshlib :=
......
30 host-csingle	 := fixdep
34 host-cmulti	 := 
38 host-shared	 :=
42 host-cobjs	 := 
47 host-cxxmulti := 
50 host-cxxobjs	 := 
53 host-cshobjs  :=
54 host-cxxshobjs:=
......
56 host-csingle	 := $(obj)/$(host-csingle)
57 host-cmulti	 := 
58 host-cobjs	 := 
59 host-cxxmulti := 
60 host-cxxobjs	 :=
61 host-cshlib	 :=
62 host-cxxshlib := 
63 host-cshobjs	 :=
64 host-cxxshobjs:=
65 host-shared	 :=
第94行中的 $(obj) 和 $(src) 在 scripts/Makefile.build 的第7-15行中定义，所以最终： $(obj) = $(src)
addprefix函数、sort函数、foreach函数
7  prefix := tpl
8  src := $(patsubst $(prefix)/%,%,$(obj))
9  ifeq ($(obj),$(src))
10 prefix := spl
11 src := $(patsubst $(prefix)/%,%,$(obj))
12 ifeq ($(obj),$(src))
13 prefix := .
14 endif
15 endif
但是从 1.3的96行可知，这里只需要生成依赖 $(obj)/fixdep 即可，所以匹配第94-95行：
94 $(host-csingle): $(obj)/%: $(src)/%.c FORCE
95 	  $(call if_changed_dep,host-csingle)
$(host-csingle) 在上面的 56 行定义，第94-95行最终展开为：
94 $(obj)/fixdep: $(obj)/%: $(obj)/%.c FORCE
95 	  $(call if_changed_dep,host-csingle)
这里目标$(obj)/fixdep的生成，依赖$(src)/%.c，其目录下存在此类文件【fixdep.c】，经过 if_changed_dep函数分析可知，打印和执行 cmd_host-csingle 函数，在执行的时候目标 $(obj)/fixdep 就已经生成完成，并通过 $(obj)/fixdep 工具将实现目标 $(obj)/fixdep 所用的命令、源码、头文件等信息全部写入到 $(obj)/.fixdep.cmd文件中。

最终：打印和执行 cmd_host-csingle 函数【即生成 scripts/basic/fixdep 工具】，将实现目标 $(obj)/fixdep 所用的命令、源码、头文件等信息全部写入到 scripts/basic/.fixdep.cmd文件中。
92 cmd_host-csingle = cc -Wp,-MD,scripts/basic/.fixdep.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer   -std=gnu11     -o scripts/basic/fixdep scripts/basic/fixdep.c   
94 scripts/basic/fixdep: scripts/basic/fixdep: scripts/basic/fixdep.c FORCE
95 	  $(cmd_host-csingle)

1.1 cmd_host-csingle函数分析：

91 quiet_cmd_host-csingle 	= HOSTCC  $@
92       cmd_host-csingle	= $(HOSTCC) $(hostc_flags) -o $@ $< \
93 	  	$(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(@F))
第92行：$(HOSTCC) 在顶层 Makefile 的282行定义 HOSTCC = cc
第92行：$(hostc_flags) 在83行定义：
70 _hostc_flags   = $(KBUILD_HOSTCFLAGS)   $(HOST_EXTRACFLAGS)   \
71                 $(HOSTCFLAGS_$(basetarget).o)
72 _hostcxx_flags = $(KBUILD_HOSTCXXFLAGS) $(HOST_EXTRACXXFLAGS) \
73                 $(HOSTCXXFLAGS_$(basetarget).o)
74
75 ifeq ($(KBUILD_SRC),)
76 __hostc_flags	= $(_hostc_flags)
77 __hostcxx_flags	= $(_hostcxx_flags)
78 else
79 __hostc_flags	= -I$(obj) $(call flags,_hostc_flags)
80 __hostcxx_flags	= -I$(obj) $(call flags,_hostcxx_flags)
81 endif
82
83 hostc_flags    = -Wp,-MD,$(depfile) $(__hostc_flags)
84 hostcxx_flags  = -Wp,-MD,$(depfile) $(__hostcxx_flags)
......
83 hostc_flags    = -Wp,-MD,$(depfile) $(__hostc_flags)
第83行：$(depfile) 在scripts/Kbuild.include 【在顶层Makefile的396行引用】的20行定义：
最终： depfile = $(obj)/.$(notdir $@).cmd
dir函数、notdir 函数、subst 函数
5  comma   := ,
15 dot-target = $(dir $@).$(notdir $@)
20 depfile = $(subst $(comma),_,$(dot-target).d)
第83行：$(__hostc_flags)，当输入make O=dir时，KBUILD_SRC=dir，由于这里没有O=，所以第75行条件成立，执行第76行，第76行中的 $(_hostc_flags) 在第70行中定义，其中的 $(HOST_EXTRACFLAGS) 和$(HOSTCFLAGS_$(basetarget).o) 未定义为空，$(KBUILD_HOSTCFLAGS)在顶层 Makefile 中定义为:-Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -std=gnu11
最终： hostc_flags = -Wp,-MD,scripts/basic/.fixdep.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -std=gnu11

第93行：@F介绍，$(KBUILD_HOSTLDLIBS) 和$(HOSTLDLIBS_$(@F) 未定义为空

最终：
cmd_host-csingle = cc -Wp,-MD,$(obj).$(notdir $@).cmd -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer   -std=gnu11     -o $@ $<

2. 目标 $(obj)/conf

由于顶层 Makefile 的564行执行make -f ./scripts/Makefile.build obj=scripts/kconfig rpi_3_32b_defconfig<见实现%config>，又在 Makefile.build 的57行引用scripts/kconfig/Makefile<见2.1>，并且其中有如下定义：
170 hostprogs-y := conf
184 hostprogs-y	+= nconf
194 hostprogs-y	+= mconf
206 hostprogs-y	+= qconf
222 hostprogs-y	+= gconf
......
195 lxdialog		:= checklist.o inputbox.o menubox.o textbox.o util.o yesno.o
173 conf-objs		:= conf.o  zconf.tab.o
185 nconf-objs		:= nconf.o zconf.tab.o nconf.gui.o
196 mconf-objs		:= mconf.o zconf.tab.o $(addprefix lxdialog/, $(lxdialog))
208 qconf-objs		:= zconf.tab.o
223 gconf-objs		:= gconf.o zconf.tab.o
207 qconf-cxxobjs	:= qconf.o
$(hostprogs-m)、$(hostlibs-y)、$(hostlibs-m)、$(hostcxxlibs-y)、$(hostcxxlibs-m) 未定义，所以24-65行展开为:
24 __hostprogs   := conf gconf mconf nconf qconf
25 host-cshlib 	 :=
26 host-cxxshlib :=
......
30 host-csingle	 :=
34 host-cmulti	 := conf gconf mconf nconf
38 host-shared	 :=
42 host-cobjs	 := conf.o gconf.o lxdialog/checklist.o lxdialog/inputbox.o lxdialog/menubox.o lxdialog/textbox.o lxdialog/util.o lxdialog/yesno.o mconf.o nconf.gui.o nconf.o zconf.tab.o
47 host-cxxmulti := qconf
50 host-cxxobjs	 := qconf.o
53 host-cshobjs  :=
54 host-cxxshobjs:=
......
56 host-csingle	 :=
57 host-cmulti	 := $(obj)/conf $(obj)/gconf $(obj)/mconf $(obj)/nconf
58 host-cobjs	 := $(obj)/conf.o gconf.o $(obj)/lxdialog/checklist.o $(obj)/lxdialog/inputbox.o $(obj)/lxdialog/menubox.o $(obj)/lxdialog/textbox.o $(obj)/lxdialog/util.o $(obj)/lxdialog/yesno.o $(obj)/mconf.o $(obj)/nconf.gui.o $(obj)/nconf.o $(obj)/zconf.tab.o
59 host-cxxmulti := $(obj)/qconf
60 host-cxxobjs	 := $(obj)/qconf.o
61 host-cshlib	 :=
62 host-cxxshlib := 
63 host-cshobjs	 :=
64 host-cxxshobjs:=
65 host-shared	 :=
$(obj) 和 $(src) 在 scripts/Makefile.build 的第7-15行中定义，所以最终： $(obj) = $(src)
addprefix函数、sort函数、foreach函数
7  prefix := tpl
8  src := $(patsubst $(prefix)/%,%,$(obj))
9  ifeq ($(obj),$(src))
10 prefix := spl
11 src := $(patsubst $(prefix)/%,%,$(obj))
12 ifeq ($(obj),$(src))
13 prefix := .
14 endif
15 endif
但是从 2.6的109行可知，这里只需要生成依赖 $(obj)/conf 即可，所以这里只会匹配下列目标：
100 cmd_host-cmulti	= $(HOSTCC) $(KBUILD_HOSTLDFLAGS) -o $@ \
101 	$(addprefix $(obj)/,$($(@F)-objs)) \
102 	$(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(@F))
103 $(host-cmulti): FORCE
104 	$(call if_changed,host-cmulti)
$(host-cmulti) 在上面的 57 行定义，cmd_host-cmulti 的展开见 < 2.1 cmd_host-cmulti函数分析>，展开后为：
100 cmd_host-cmulti = cc -o $@  $(obj)/$($(@F)-objs)
103 $(obj)/conf: FORCE
104 	$(call if_changed,host-cmulti)
从上面可知 $(conf-objs) 在173行中定义conf-objs := conf.o zconf.tab.o,所以最终展开为：
100 cmd_host-cmulti = cc -o $@  $(obj)/conf.o  $(obj)/zconf.tab.o
103 $(obj)/conf: FORCE
104 	$(call if_changed,host-cmulti)
编译生成目标文件需要 $(obj)/conf.o 和 $(obj)/zconf.tab.o 存在，这两个文件的生成见下面的 <2.2> <2.3>。

最终：if_changed函数

<2.2>： 编译生成 $(obj)/conf.o文件。
打印和执行 $(cmd_host-cobjs)，并将实现 scripts/Kconfig/conf.o 所用的命令、源码、头文件等信息全部写入到 scripts/kconfig/.conf.o.cmd 文件中。
<2.3>： 编译生成 $(obj)/zconf.tab.c、 $(obj)/flex.lex.c、 $(obj)/zconf.tab.o 文件。
2.1打印和执行$(cmd_bison)，并将cmd_scripts/kconfig/zconf.tab.c := $(cmd_bison)的内容输入到 scripts/kconfig/.zconf.tab.c.cmd 文件中。
2.2. 打印和执行$(cmd_flex)，并将cmd_scripts/kconfig/flex.lex.c := $(cmd_flex)的内容输入到 scripts/kconfig/.zconf.lex.c.cmd 文件中。
2.3. 打印和执行 $(cmd_host-cobjs)，并将实现目标 $(obj)/zconf.tab.o 所用的命令、源码、头文件等信息全部写入到 scripts/kconfig/.zconf.tab.o.cmd 文件中。
编译生成 $(obj)/conf 文件。
打印和执行 $(cmd_host-cmulti)，并将 cmd_scripts/Kconfig/conf := $(cmd_host-cmulti) 的内容输入到 scripts/Kconfig/.conf.cmd 文件中。

回显 5-9 行:
5   cc -Wp,-MD,scripts/kconfig/.conf.o.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer   -std=gnu11     -c -o scripts/kconfig/conf.o scripts/kconfig/conf.c
6   bison -oscripts/kconfig/zconf.tab.c -t -l scripts/kconfig/zconf.y
7   flex -oscripts/kconfig/zconf.lex.c -L scripts/kconfig/zconf.l
8   cc -Wp,-MD,scripts/kconfig/.zconf.tab.o.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer   -std=gnu11    -Iscripts/kconfig -c -o scripts/kconfig/zconf.tab.o scripts/kconfig/zconf.tab.c
9   cc   -o scripts/kconfig/conf scripts/kconfig/conf.o scripts/kconfig/zconf.tab.o   

2.1 cmd_host-cmulti函数分析：

100 cmd_host-cmulti	= $(HOSTCC) $(KBUILD_HOSTLDFLAGS) -o $@ \
101 	  $(addprefix $(obj)/,$($(@F)-objs)) \
102 	  $(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(@F))
addprefix、@F解释
第100行：$(HOSTCC) 在顶层 Makefile 的282行定义 HOSTCC = cc
第100行：$(KBUILD_HOSTLDFLAGS) 在顶层 Makefile 的定义 KBUILD_HOSTLDFLAGS=
第102行：$(KBUILD_HOSTLDLIBS) 在顶层 Makefile 的定义 KBUILD_HOSTLDLIBS=
第102行：$($(HOSTLDLIBS_$(@F)) 在顶层 Makefile 的定义 $(HOSTLDLIBS_$(@F)=

最终：
cmd_host-cmulti = cc -o $@  $(obj)/$($(@F)-objs)

2.2 生成 $(obj)/conf.o 文件：

因为要生成 $(obj)/conf.o 文件，会匹配 scripts/Makefile.host 中的下列目标：
100 cmd_host-cobjs	= $(HOSTCC) $(hostc_flags) -c -o $@ $<
101 $(host-cobjs): $(obj)/%.o: $(src)/%.c FORCE
102	    $(call if_changed_dep,host-cobjs)
由上面的分析可知：

$(host-cobjs) 在第58行定义，
$(obj) = $(src)
$(HOSTCC) = cc
$(hostc_flags)的定义分析，见 <1.1 cmd_host-csingle函数分析>

最终：打印和执行 $(cmd_host-cobjs)，并将实现目标 $(obj)/conf.o 所用的命令、源码、头文件等信息全部写入到 scripts/kconfig/.conf.o.cmd 文件中。【生成 conf.o 文件】
100 cmd_host-cobjs	= cc -Wp,-MD,scripts/kconfig/.conf.o.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer   -std=gnu11     -c -o $@ $<
101 $(obj)/conf.o: $(obj)/%.o: $(src)/%.c FORCE
102	    $(call if_changed_dep,host-cobjs)
if_changed_dep函数，其中的$(src)/%.c在源码中存在(conf.c)，

2.3 生成 $(obj)/zconf.tab.o 文件：

因为要生成 $(obj)/zconf.tab.o 文件，会匹配 scripts/Makefile.host 中的下列目标：
58 host-cobjs := $(obj)/conf.o gconf.o $(obj)/lxdialog/checklist.o $(obj)/lxdialog/inputbox.o $(obj)/lxdialog/menubox.o $(obj)/lxdialog/textbox.o $(obj)/lxdialog/util.o $(obj)/lxdialog/yesno.o $(obj)/mconf.o $(obj)/nconf.gui.o $(obj)/nconf.o $(obj)/zconf.tab.o
......
101 $(host-cobjs): $(obj)/%.o: $(src)/%.c FORCE
102	    $(call if_changed_dep,host-cobjs)
由上面的分析可知： $(host-cobjs) 在第58行定义，只需要生成目标 $(obj)/zconf.tab.o，所以展开后为：
101 $(obj)/zconf.tab.o: $(obj)/%.o: $(src)/%.c FORCE
102	    $(call if_changed_dep,host-cobjs)
由于在 scripts/Kconfig/Makefile 文件也定义了 $(obj)/zconf.tab.o 文件生成的依赖关系，如下：
230 $(obj)/zconf.tab.o: $(obj)/zconf.lex.c
所以需要生成2个C文件，分别为：$(obj)/zconf.lex.c 和 $(obj)/zconf.tab.c，这两个文件不存在，需要优先生成这两个文件。

生成2个C文件：
由于顶层 Makefile 的564行执行make -f ./scripts/Makefile.build obj=XXX，在 ./scripts/Makefile.build 文件的71行引用了 scripts/Makefile.lib 文件，并且在此文件中有如下依赖关系：
211 cmd_flex = $(LEX) -o$@ -L $<
213 $(obj)/%.lex.c: $(src)/%.l FORCE
214 	$(call if_changed,flex)
......
219 cmd_bison = $(YACC) -o$@ -t -l $<
221 $(obj)/%.tab.c: $(src)/%.y FORCE
222 	$(call if_changed,bison)
$(LEX) 和 $(YACC) 在顶层 Makefile 中的定义如下：
405 LEX		= flex
406 YACC	= bison
展开后为：
211 cmd_flex = flex -o$@ -L $<
213 $(obj)/%.lex.c: $(src)/%.l FORCE
214 	$(call if_changed,flex)
......
219 cmd_bison = bison  -o$@ -t -l $<
221 $(obj)/%.tab.c: $(src)/%.y FORCE
222 	$(call if_changed,bison)
经过 if_changed函数分析可知:

打印和执行$(cmd_flex)，并将cmd_scripts/kconfig/flex.lex.c := $(cmd_flex)的内容输入到 scripts/kconfig/.zconf.lex.c.cmd 文件中。
打印和执行$(cmd_bison)，并将cmd_scripts/kconfig/zconf.tab.c := $(cmd_bison)的内容输入到 scripts/kconfig/.zconf.tab.c.cmd 文件中。

最终： flex & bison 基础概述

打印和执行$(cmd_flex)，并将cmd_scripts/kconfig/flex.lex.c := $(cmd_flex)的内容输入到 scripts/kconfig/.zconf.lex.c.cmd 文件中。【生成 flex.lex.c 文件】
打印和执行$(cmd_bison)，并将cmd_scripts/kconfig/zconf.tab.c := $(cmd_bison)的内容输入到 scripts/kconfig/.zconf.tab.c.cmd 文件中。【生成 zconf.tab.c 文件】
打印和执行 $(cmd_host-cobjs)，并将实现目标 $(obj)/zconf.tab.o 所用的命令、源码、头文件等信息全部写入到 scripts/kconfig/.zconf.tab.o.cmd 文件中。【生成 zconf.tab.o 文件】
100 cmd_host-cobjs	= cc -Wp,-MD,scripts/kconfig/.zconf.tab.o.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer   -std=gnu11     -c -o $@ $<
101 $(obj)/zconf.tab.o: $(obj)/%.o: $(src)/%.c FORCE
102	    $(call if_changed_dep,host-cobjs)
if_changed_dep函数