4. 目标$(timestamp_h)

# 顶层Makefile
$(timestamp_h): $(srctree)/Makefile FORCE
	$(call filechk,timestamp.h)

4.1 $(timestamp_h)依赖分析

目标 timestamp_h依赖$(srctree)/Makefile 和FORCE。假设直接在源目录编译，顶层Makefile存在，依赖FORCE见UBOOT编译--- make xxx_deconfig过程详解(一) 4.3小节 - 依赖 FORCE。

4.2 $(timestamp_h)规则分析

接下来我们看规则$(call filechk,timestamp.h)：

#note：scripts/Kbuild.include
###
# filechk is used to check if the content of a generated file is updated.
# Sample usage:
# define filechk_sample
#	echo $KERNELRELEASE
# endef
# version.h : Makefile
#	$(call filechk,sample)
# The rule defined shall write to stdout the content of the new file.
# The existing file will be compared with the new one.
# - If no file exist it is created
# - If the content differ the new file is used
# - If they are equal no change, and no timestamp update
# - stdin is piped in from the first prerequisite ($<) so one has
#   to specify a valid file as first prerequisite (often the kbuild file)
define filechk
	$(Q)set -e;				\
	$(kecho) '  CHK     $@';		\
	mkdir -p $(dir $@);			\
	$(filechk_$(1)) < $< > $@.tmp;		\
	if [ -r $@ ] && cmp -s $@ $@.tmp; then	\
		rm -f $@.tmp;			\
	else					\
		$(kecho) '  UPD     $@';	\
		mv -f $@.tmp $@;		\
	fi
endef

上述代码中：

• ‘$@’ ：目标timestamp_h (include/generated/timestamp_autogenerated.h)；
• ‘$(1)’ ：第一个参数timestamp.h；
• ‘$ <’ ：第一个依赖$(srctree)/Makefile ；
• ‘filechk_$(1)’ ：filechk_timestamp.h；
  
  

filechk_timestamp.h定义在顶层Makefile中：

#note：顶层Makefile
define filechk_timestamp.h
	(if test -n "$${SOURCE_DATE_EPOCH}"; then \
		SOURCE_DATE="@$${SOURCE_DATE_EPOCH}"; \
		DATE=""; \
		for date in gdate date.gnu date; do \
			$${date} -u -d "$${SOURCE_DATE}" >/dev/null 2>&1 && DATE="$${date}"; \
		done; \
		if test -n "$${DATE}"; then \
			LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_DATE "%b %d %C%y"'; \
			LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_TIME "%T"'; \
			LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_TZ "%z"'; \
			LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_DMI_DATE "%m/%d/%Y"'; \
			LC_ALL=C $${DATE} -u -d "$${SOURCE_DATE}" +'#define U_BOOT_BUILD_DATE 0x%Y%m%d'; \
		else \
			return 42; \
		fi; \
	else \
		LC_ALL=C date +'#define U_BOOT_DATE "%b %d %C%y"'; \
		LC_ALL=C date +'#define U_BOOT_TIME "%T"'; \
		LC_ALL=C date +'#define U_BOOT_TZ "%z"'; \
		LC_ALL=C date +'#define U_BOOT_DMI_DATE "%m/%d/%Y"'; \
		LC_ALL=C date +'#define U_BOOT_BUILD_DATE 0x%Y%m%d'; \
	fi)
endef