先去编译obj-y,以init下的obj-y为例：
obj-y=init/main.o init/version.o init/mounts.o init/noinitramfs.o init/calibrate.o
首先找到将.c编译为.o的规则：
点击(此处)折叠或打开
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linux-2.6.30.4/scripts/Makefile.build
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226 # Built-in and composite module parts
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227 $(obj)/%.o: $(src)/%.c FORCE
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228 $(call cmd,force_checksrc)
- 229 $(call if_changed_rule,cc_o_c)
1. L228,如果KBUILD_CHECKSRC为空，不做任何操作;不为空，没看。
2. L229，编译的具体操作。
2. 下面开始执行L229 $(call if_changed_rule,cc_o_c)，首先转到定义处：
1.1 检查 L228 $(call cmd,force_checksrc)
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linux-2.6.30.4/scripts/Kbuild.include
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154 # echo command.
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155 # Short version is used, if $(quiet) equals `quiet_', otherwise full one.
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156 echo-cmd = $(if $($(quiet)cmd_$(1)),\
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157 echo ' $(call escsq,$($(quiet)cmd_$(1)))$(echo-why)';)
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158
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159 # printing commands
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160 cmd = @$(echo-cmd) $(cmd_$(1))
- cmd 定义在L160处，如果cmd_force_checksrc不为空，则把cmd_force_checksrc这个命令打印出来，然后执行cmd_force_checksrc
- 转到cmd_force_checksrc定义处:
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linux-2.6.30.4/scripts/Makefile.build
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98 # Linus' kernel sanity checking tool
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99 ifneq ($(KBUILD_CHECKSRC),0)
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100 ifeq ($(KBUILD_CHECKSRC),2)
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101 quiet_cmd_force_checksrc = CHECK $<
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102 cmd_force_checksrc = $(CHECK) $(CHECKFLAGS) $(c_flags) $< ;
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103 else
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104 quiet_cmd_checksrc = CHECK $<
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105 cmd_checksrc = $(CHECK) $(CHECKFLAGS) $(c_flags) $< ;
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106 endif
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107 endif
- 由于此时 KBUILD_CHECKSRC=0,所以cmd_force_checksrc为空，L156处的if条件为false，echo不执行，L160处cmd_force_checksrc为空，也不执行。
点击(此处)折叠或打开
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linux-2.6.30.4/scripts/Kbuild.include
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208 if_changed_rule = $(if $(strip $(any-prereq) $(arg-check) ), \
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209 @set -e; \
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210 $(rule_$(1)))
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$(strip $(any-prereq) $(arg-check),如果目标有更新则调用：rule_cc_o_c
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- linux-2.6.30.4/scripts/Makefile.build
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215 define rule_cc_o_c
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216 $(call echo-cmd,checksrc) $(cmd_checksrc) \
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217 $(call echo-cmd,cc_o_c) $(cmd_cc_o_c); \
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218 $(cmd_modversions) \
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219 $(cmd_record_mcount) \
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220 scripts/basic/fixdep $(depfile) $@ '$(call make-cmd,cc_o_c)' > \
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221 $(dot-target).tmp; \
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222 rm -f $(depfile); \
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223 mv -f $(dot-target).tmp $(dot-target).cmd
- 224 endef
rule_cc_o_c
2.1 echo-cmd && cmd_checksrc ：不执行
2.2 echo-cmd && cmd_cc_o_c :打印cmd_cc_o_c的内容，并执行cmd_cc_o_c将.c编译为.o
2.3 cmd_modversions ：不执行
2.4 cmd_record_mcount ：不执行
2.5 生成命令.cmd
下面逐条分析：
2.1-2.2 echo-cmd && cmd_checksrc 和 echo-cmd && cmd_cc_o_c
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- linux-2.6.30.4/scripts/Kbuild.include
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154 # echo command.
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155 # Short version is used, if $(quiet) equals `quiet_', otherwise full one.
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156 echo-cmd = $(if $($(quiet)cmd_$(1)),\
- 157 echo ' $(call escsq,$($(quiet)cmd_$(1)))$(echo-why)';)
- cmd_checksrc为空，不执行
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- linux-2.6.30.4/scripts/Makefile.build
- 178 ifndef CONFIG_MODVERSIONS
- 179 cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $<
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- $(call echo-cmd,cc_o_c)将执行的编译命令打印出来， $(cmd_cc_o_c)执行具体的编译命令。这将会在命令行中看到一坨坨的打印。
点击(此处)折叠或打开
- linux-2.6.30.4/scripts/Makefile.build
- 178 ifndef CONFIG_MODVERSIONS
- 179 cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $<
- 180
- 181 else
- 194 cmd_cc_o_c = $(CC) $(c_flags) -c -o $(@D)/.tmp_$(@F) $<
- 195 cmd_modversions = ...
- 因为此处没有定义 CONFIG_MODVERSIONS， 所以cmd_modversions为空。
2.4 cmd_record_mcount
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- 208 ifdef CONFIG_FTRACE_MCOUNT_RECORD
- 209 cmd_record_mcount = perl $(srctree)/scripts/recordmcount.pl "$(ARCH)" \
- 210 "$(if $(CONFIG_64BIT),64,32)" \
- 211 "$(OBJDUMP)" "$(OBJCOPY)" "$(CC)" "$(LD)" "$(NM)" "$(RM)" "$(MV)" \
- 212 "$(if $(part-of-module),1,0)" "$(@)";
- 213 endif
- CONFIG_FTRACE_MCOUNT_RECORD 没有定义，所以为空
虽然写了这么多，但是有用处的只有两个echo-cmd && cmd_cc_o_c和 mv ，打印cmd_cc_o_c的内容，并执行cmd_cc_o_c将.c编译为.o，把命令保存到.cmd文件中。假设让我自己写出来的Makefile可能也就是$(CC) $(c_flags) -c -o $@ $<这么一句。