linux_2.6.30.4_Makefile_6--prepare0流程

linux-2.6.30.4/Makefile中，prepare0的依赖执行完之后，执行：
   make -f scripts/Makefile.build obj=.
#执行scripts/Makefile的__build目标，此处只执行always依赖，(KBUILD_BUILTIN=1，但是$(builtin-target) $(lib-target) $(extra-y)都为空，KBUILD_MODULES=0)
always=./include/linux/bounds.h ./include/asm/asm-offsets.h
__build: ./include/linux/bounds.h ./include/asm/asm-offsets.h
    :
./include/linux/bounds.h 与./include/asm/asm-offsets.h 这两个依赖的处理过程基本相似。
下面只分析./include/linux/bounds.h的过程。

linux-2.6.30.4/Kbuild
 11 bounds-file := include/linux/bounds.h

 38 $(obj)/$(bounds-file): kernel/bounds.s Kbuild
 39  $(Q)mkdir -p $(dir $@)
 40  $(call cmd,bounds)

 34 kernel/bounds.s: kernel/bounds.c FORCE
 35  $(Q)mkdir -p $(dir $@)
 36  $(call if_changed_dep,cc_s_c)

   目标include/linux/bounds.h在linux-2.6.30.4/Kbuild中,它的执行流程如下：

1. L34 mkdir -p kernel

2. L36 $(call if_changed_dep, cc_s_c)，如果kernel/bounds.c有更新，则重新编译kernel/bounds.c，并将编译命令显示在终端，最后将编译命令写到文件.bounds.s.cmd.o中

3. kernel/bounds.s依赖完成之后，执行include/linux/bounds.h目标

4. L39 mkdir -p include/linux

5. $(call cmd, bounds) ，生成include/linux/bounds.h头文件

下面分析一下第2步: $(call if_changed_dep,cc_s_c)

linux-2.6.30.4/scripts/Kbuild.include
197 # Execute the command and also postprocess generated .d dependencies file.
198 if_changed_dep = $(if $(strip $(any-prereq) $(arg-check) ), \
199     @set -e; \
200     $(echo-cmd) $(cmd_$(1)); \
201     scripts/basic/fixdep $(depfile) $@ '$(make-cmd)' > $(dot-target).tmp;\
202     rm -f $(depfile); \
203     mv -f $(dot-target).tmp $(dot-target).cmd)

2.1 $(if $(strip $(any-prereq) $(arg-check) ) #如果目标有更新，则执行条件后面的东东

2.2 $(echo-cmd) 打印编译命令

2.3 $(cmd_cc_s_c) 在linux-2.6.30.4/scripts/Makefile.build中定义cmd_cc_s_c = $(CC) $(c_flags) -fverbose-asm -S -o $@ $<，这里面有-S,是将c文件转为.s文件，也就是常说的汇编阶段。

2.4 将kernel/bounds.c的汇编和编译所需的头文件统统写进文件kernel/.bounds.s.d.tmp中

2.5 删除kernel/.bounds.s.d 

2.6 将kernel/.bounds.s.d.tmp 重命名为kernel/.bounds.s.d.cmd

2.1.1 突然想起小学时写作文老师说的总分方式：

linux-2.6.30.4/scripts/Kbuild.include
L189 any-prereq = $(filter-out $(PHONY),$?) $(filter-out $(PHONY) $(wildcard $^),$^)
     #检查目标代码是否有更新，如果有更新返回值不为空
177 arg-check = $(strip $(filter-out $(cmd_$(1)), $(cmd_$@)) \
178                     $(filter-out $(cmd_$@), $(cmd_$(1))) )
    #检查参数是否改变，若有改变，返回值不为空

2.2.1 $(echo-cmd) 打印编译命令

linux-2.6.30.4/scripts/Kbuild.include
  24 # Escape single quote for use in echo statements
  25 escsq = $(subst $(squote),'\$(squote)',$1)

155 # Short version is used, if $(quiet) equals `quiet_', otherwise full one.
156 echo-cmd = $(if $($(quiet)cmd_$(1)),\
157     echo '  $(call escsq,$($(quiet)cmd_$(1)))$(echo-why)';)
linux-2.6.30.4/Makefile控制quiet的值，此处为空，1是指cc_s_c,如果cmd_cc_s_c执行成功，就打印$(cmd_cc_s_c)即具体的编译过程到命令行。
这地方有点想不明白，if后面 $(cmd_cc_s_c)要执行一次，$(echo-cmd)执行完后又有一个$(cmd_$(1))，难不成$(cmd_cc_s_c)要执行两次？
噢，想错了！下面是GNU `make'对if的说明 “$(if condition,then-part[,else-part]) The first argument, condition, first has all preceding and trailing whitespace stripped, then is expanded. If it expands to any non-empty string, then the condition is considered to be true. If it expands to an empty string, the condition is considered to be false.” 对于第一个参数：condition,首先去掉前面和尾部的空格，然后把条件语句展开。如果展开后是一个不为空的string,那么这个条件就被认为是true,反之是一个空string, 认为是false.

2.3.1 $(cmd_cc_s_c) 即$(CC) $(c_flags) -fverbose-asm -S -o $@ $<，具体的编译过程

2.4.1 scripts/basic/fixdep这个东东在make menuconfig/silentconfig中己经生成，具体是什么作用没有看，感觉是把.c文件需要的include重定向到另一个文件中。

5.1 $(call cmd, bounds)的过程

linux-2.6.30.4/scripts/Kbuild.include
154 # echo command.
155 # Short version is used, if $(quiet) equals `quiet_', otherwise full one.
156 echo-cmd = $(if $($(quiet)cmd_$(1)),\
157     echo ' $(call escsq,$($(quiet)cmd_$(1)))$(echo-why)';)
158
159 # printing commands
160 cmd = @$(echo-cmd) $(cmd_$(1))

cmd = @$(echo-cmd) $(cmd_$(1))

先是打印命令这个，然后执行$(cmd_bounds)

 linux-2.6.30.4/Kbuild
 17 define cmd_bounds
 18 (set -e; \
 19      echo "#ifndef __LINUX_BOUNDS_H__"; \
 20      echo "#define __LINUX_BOUNDS_H__"; \
 21      echo "/*"; \
 22      echo " * DO NOT MODIFY."; \
 23      echo " *"; \
 24      echo " * This file was generated by Kbuild"; \
 25      echo " *"; \
 26      echo " */"; \
 27      echo ""; \
 28      sed -ne $(sed-y) $<; \
 29      echo ""; \
 30      echo "#endif" ) > $@
 31 endef

$@ 是 include/linux/bounds.h

$< 是 kernel/bounds.s 

这个宏简单一点就是: echo "something" >　include/linux/bounds.h,将一些文本写入到include/linux/bounds.h中

linux-2.6.30.4/Kbuild
 54 define sed-y
 55 "/^->/{s:->#\(.*\):/* \1 */:; \
 56     s:^->\([^ ]*\) [\$$#]*\([^ ]*\) \(.*\):#define \1 \2 /* \3 */:; \
 57     s:->::; p;}"
 58 endef

 sed将kernel/bounds.s中以->开头的行进行处理，将行

->NR_PAGEFLAGS #21 __NR_PAGEFLAGS   @           替换为

#define NR_PAGEFLAGS 21 /* __NR_PAGEFLAGS   @ */