OpenWRT构建系统中的Makefile问题

Question

以下代码取自OpenWRT项目。有人可以给出一个抽象的描述吗？提前致谢！

# 
# Copyright (C) 2007 OpenWrt.org
#
# This is free software, licensed under the GNU General Public License v2.
# See /LICENSE for more information.
#

SUBTARGETS:=clean download prepare compile install update refresh prereq dist distcheck

subtarget-default = $(filter-out ., \
    $(if $($(1)/builddirs-$(2)),$($(1)/builddirs-$(2)), \
    $(if $($(1)/builddirs-default),$($(1)/builddirs-default), \
    $($(1)/builddirs))))

define subtarget
  $(call warn_eval,$(1),t,T,$(1)/$(2): $($(1)/) $(foreach bd,$(call subtarget-default,$(1),$(2)),$(1)/$(bd)/$(2)))

endef


lastdir=$(word $(words $(subst /, ,$(1))),$(subst /, ,$(1)))
diralias=$(if $(findstring $(1),$(call lastdir,$(1))),,$(call lastdir,$(1)))

# Parameters: <subdir>
define subdir
  $(call warn,$(1),d,D $(1))
  $(foreach bd,$($(1)/builddirs),
    $(call warn,$(1),d,BD $(1)/$(bd))
    $(foreach target,$(SUBTARGETS),
      $(foreach btype,$(buildtypes-$(bd)),
        $(call warn_eval,$(1)/$(bd),t,T,$(1)/$(bd)/$(btype)/$(target): $(if $(QUILT),,$($(1)/$(bd)/$(btype)/$(target)) $(call $(1)//$(btype)/$(target),$(1)/$(bd)/$(btype))))
          +$$(SUBMAKE) -C $(1)/$(bd) $(btype)-$(target) $(if $(findstring $(bd),$($(1)/builddirs-ignore-$(btype)-$(target))), || $(call MESSAGE,   ERROR: $(1)/$(bd) [$(btype)] failed to build.))
        $$(if $(call debug,$(1)/$(bd),v),,.SILENT: $(1)/$(bd)/$(btype)/$(target))
        $(if $(call diralias,$(bd)),$(call warn_eval,$(1)/$(bd),l,T,$(1)/$(call diralias,$(bd))/$(btype)/$(target): $(1)/$(bd)/$(btype)/$(target)))
      )
      $(call warn_eval,$(1)/$(bd),t,T,$(1)/$(bd)/$(target): $(if $(QUILT),,$($(1)/$(bd)/$(target)) $(call $(1)//$(target),$(1)/$(bd))))
        $(if $(BUILD_LOG),@mkdir -p $(BUILD_LOG_DIR)/$(1)/$(bd))
        $(foreach variant,$(if $(BUILD_VARIANT),$(BUILD_VARIANT),$(if $($(1)/$(bd)/variants),$($(1)/$(bd)/variants),__default)),
            +$(if $(BUILD_LOG),set -o pipefail;) $$(SUBMAKE) -C $(1)/$(bd) $(target) BUILD_VARIANT="$(filter-out __default,$(variant))" $(if $(BUILD_LOG),SILENT= 2>&1 | tee $(BUILD_LOG_DIR)/$(1)/$(bd)/$(target).txt) $(if $(findstring $(bd),$($(1)/builddirs-ignore-$(target))), || $(call MESSAGE,   ERROR: $(1)/$(bd) failed to build$(if $(filter-out __default,$(variant)), (build variant: $(variant))).))
        )
        $$(if $(call debug,$(1)/$(bd),v),,.SILENT: $(1)/$(bd)/$(target))

      # legacy targets
      $(call warn_eval,$(1)/$(bd),l,T,$(1)/$(bd)-$(target): $(1)/$(bd)/$(target))
      # aliases
      $(if $(call diralias,$(bd)),$(call warn_eval,$(1)/$(bd),l,T,$(1)/$(call diralias,$(bd))/$(target): $(1)/$(bd)/$(target)))
    )
  )
  $(foreach target,$(SUBTARGETS),$(call subtarget,$(1),$(target)))
endef

# Parameters: <subdir> <name> <target> <depends> <config options> <stampfile location>
define stampfile
  $(1)/stamp-$(3):=$(if $(6),$(6),$(STAGING_DIR))/stamp/.$(2)_$(3)$(if $(5),_$(call confvar,$(5)))
  $$($(1)/stamp-$(3)): $(TMP_DIR)/.build $(4)
    @+$(SCRIPT_DIR)/timestamp.pl -n $$($(1)/stamp-$(3)) $(1) $(4) || \
        $(MAKE) $$($(1)/flags-$(3)) $(1)/$(3)
    @mkdir -p $$$$(dirname $$($(1)/stamp-$(3)))
    @touch $$($(1)/stamp-$(3))

  $$(if $(call debug,$(1),v),,.SILENT: $$($(1)/stamp-$(3)))

  .PRECIOUS: $$($(1)/stamp-$(3)) # work around a make bug

  $(1)//clean:=$(1)/stamp-$(3)/clean
  $(1)/stamp-$(3)/clean: FORCE
    @rm -f $$($(1)/stamp-$(3))

endef

Answer 1

subdir 模板生成子目录的标准 make 目标。

它以目录名称作为第一个参数来调用，例如在 package/Makefile 中它是“package”
然后，它将变量 package/builddirs 视为具有有效构建目标的子目录列表，这些目标包含在 SUBTARGETS 变量中。

为了运行包“foo”的“compile”目标，它允许您运行“make package/foo/compile”，它将使用正确的环境变量递归调用 make 以构建子目录。

该模板还支持一些可以通过环境变量启用的可选功能，例如将输出记录到文件、忽略依赖项等。
它还支持选择性调试输出。

stampfile 模板只是一个小的 makefile 目标包装器，用于通过在其周围保留递归时间戳检查的 stampfile 来加速构建