Question

在GNU Makefile中，有两种类型的变量：

# simple variable (immediate evaluation):
VAR := some_assignment

# recursively expanded variable (deferred evaluation):
VAR = some_assignment

可以使用以下命令附加到递归扩展变量：

  IMMEDIATE += DEFERRED or IMMEDIATE

对于追加运算符，＆＃39; + =＆＃39;，考虑右侧如果变量先前已设置为简单变量，则立即生效（＆＃39;：=＆＃39;或＆＃39; :: =＆＃39;），否则延期。

有没有办法 prepend 到递归扩展变量？

我的动机示例是在$(LDLIBS)：

之前引入一个新库

  # Unfortunately, newlib gets added to the end rather than the beginning.
  LDLIBS += $(if $(later_condition),newlib.a)

  # Unfortunately, the expression is evaluated now rather than being deferred.
  LDLIBS := $(if $(later_condition),newlib.a) $(LDLIBS)

Answer 1

我遇到了同样的问题。我的解决方案非常hacky，并使用$（value）和$（eval）函数。

对我来说重要的是，它保留了变量的风味（递归），因此在此操作期间既不会扩展前置变量，也不会扩展原始变量：

# Macro for prepending to a recursively expanded variable.
#
# Usage:
# * Prepending "text" to the VAR variable:
#       $(call prepend,VAR,text)
#
# * Prepending "a word list" to the VAR variable -- remember 
#   to add any trailing separator character (e.g. space):
#       $(call prepend,VAR,a word list )
#
# * Prepending OTHER_VAR variable to the VAR variable -- use $$ 
#   to defer any variable expansions:
#       $(call prepend,VAR,$$(OTHER_VAR))

define prepend
$(eval $(1) = $(2)$(value $(1)))
endef

# Macro for appending to a recursively expanded variable.
#
# Usage:
# * Appending "text" to the VAR variable:
#       $(call append,VAR,text)
#
# * Appending "a word list" to the VAR variable -- remember
#   to add any heading separator character (e.g. space):
#       $(call append,VAR, a word list)
# 
# * Appending OTHER_VAR variable to the VAR variable -- use $$ 
#   to defer any variable expansions:
#       $(call append,VAR,$$(OTHER_VAR))

define append
$(eval $(1) = $(value $(1))$(2))
endef

快速测试用例：

A = A
B = B
VAR = $(A)

$(info before: VAR=$(VAR) | value(VAR)=$(value VAR) | $(flavor VAR))
$(call prepend,VAR,$$(B))
$(info after : VAR=$(VAR) | value(VAR)=$(value VAR) | $(flavor VAR))

执行：

before: VAR=A | value(VAR)=$(A) | recursive
after : VAR=BA | value(VAR)=$(B)$(A) | recursive
make: *** No targets.  Stop.

附加说明：

我的问题实际上与GNU make在追加时添加空白分隔符有关。
Hacky解决方案，但没有针对此类问题的本机GNU make功能。

Answer 2

使用此：

# Expand a variable, properly escaping '$' characters.
expand = $(if $(findstring simple,$(flavor $1)),$(subst $$,$$$$,$($1)),$(value $1))

# Prepend to a variable, preserving flavor.
prepend = \
    $(if $(findstring simple,$(flavor $1)), \
    $(eval $1 := $2 $(call expand,$1)), \
    $(eval $1 = $2 $(call expand,$1)))

x := 1 $$ 2
$(call prepend,x,X)
$(info $x)

x = 1 $$ 2
$(call prepend,x,X)
$(info $x)

$(value)不够好，因为在$(eval)上下文中，$个字符将被解析为变量引用。

GNU make：前置一个递归扩展变量？

2 个答案: