GNU make:在递归扩展变量之前添加? [英] GNU make: prepend a recursively expanded variable?

问题描述

在GNU Makefile中，有两种类型的变量:

In a GNU Makefile, there are two types of variables:

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

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

可以使用以下方法将其附加到递归扩展的变量上:

One may append to a recursively expanded variable using:

  IMMEDIATE += DEFERRED or IMMEDIATE

对于append运算符'+ ='，认为右侧 如果先前已将变量设置为简单变量，则立即执行 (':='或':: =')，否则推迟.

For the append operator, '+=', the right-hand side is considered immediate if the variable was previously set as a simple variable (':=' or '::='), and deferred otherwise.

是否可以在之前添加递归扩展变量?

Is there any way to prepend to a recursively expanded variable?

我的激励性示例是在$(LDLIBS)中的其他库之前引入一个新库:

My motivating example is to introduce a new library ahead of other ones in $(LDLIBS):

  # 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)

推荐答案

我遇到了同样的问题.我的解决方案很笨拙，并使用$(value)和$(eval)函数.

I've came across the same problem. My solution is quite hacky, and makes use of $(value) and $(eval) functions.

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

What was important for me, it preserves flavor (recursive) of the variable, so that neither prepended variable, nor original variable is expanded during this action:

# 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))

及其执行:

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

其他说明:

• 我的问题实际上与GNU make在附加时添加空格分隔符这一事实有关.
• 棘手的解决方案，但没有针对此问题的本地GNU make功能.

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