.EXPORT_ALL_VARIABLES仅在设为"phony"时有效

问题描述

文档提供:

但是，以下makefile显示，只有将.EXPORT_ALL_VARIABLES设为 phony 目标，然后再将 only 设为目标，它将对makefile产生所需的效果，即导出所有变量.

Makefile(版本1)为:

ifeq "$(MAKELEVEL)" "0"

foo=bar

.DEFAULT:;

all: .EXPORT_ALL_VARIABLES
    @$(MAKE)

else

all:
    @echo 'foo is: $(foo)'

endif

运行，我们得到:

make[1]: Entering directory '/home/myname'
foo is:
make[1]: Leaving directory '/home/myname'

Makefile(版本2)为:

ifeq "$(MAKELEVEL)" "0"

foo=bar

.DEFAULT:;

all: .EXPORT_ALL_VARIABLES
    @$(MAKE)

# This line is added in THIS version.
.PHONY: .EXPORT_ALL_VARIABLES

else

all:
    @echo 'foo is: $(foo)'

endif

运行，我们得到:

make[1]: Entering directory '/home/myname'
foo is: bar
make[1]: Leaving directory '/home/myname'

现在，这两个版本的makefile之间唯一的区别是，在第二个版本中，.EXPORT_ALL_VARIABLES被设置为伪造.

为什么需要耐心"才能工作?

解决方案

为什么需要耐心"才能工作?

不是.您没有将.EXPORT_ALL_VARIABLES声明为目标，而是将其声明为先决条件:

all: .EXPORT_ALL_VARIABLES

这是先决条件，而不是目标.如果您将其声明为目标:

.EXPORT_ALL_VARIABLES:

然后它将起作用，而您不必将其声明为伪造.

一个更准确的问题是，即使未将.EXPORT_ALL_VARIABLES声明为目标，为什么仍将其声明为假冒呢?之所以会发生这种情况，是因为即使没有明确提及被标记为假"的事物，也被假定为目标.这可能是错误，也可能不是错误，具体取决于您如何解释.PHONY的意图.

您最近的问题似乎遵循一种模式:阅读文档，然后编写一个makefile，使其执行与文档中所述内容相似但不相同的操作，观察它是否无法按照说明进行操作，然后问为什么不这样做. /p>

The docs provides:

However, the following makefiles show that only by making .EXPORT_ALL_VARIABLES a phony target, then and only then, will it have the desired effect on the makefile, i.e. to export ALL variables.

Makefile(version 1) is:

ifeq "$(MAKELEVEL)" "0"

foo=bar

.DEFAULT:;

all: .EXPORT_ALL_VARIABLES
    @$(MAKE)

else

all:
    @echo 'foo is: $(foo)'

endif

Running, we get:

make[1]: Entering directory '/home/myname'
foo is:
make[1]: Leaving directory '/home/myname'

Makefile(version 2) is:

ifeq "$(MAKELEVEL)" "0"

foo=bar

.DEFAULT:;

all: .EXPORT_ALL_VARIABLES
    @$(MAKE)

# This line is added in THIS version.
.PHONY: .EXPORT_ALL_VARIABLES

else

all:
    @echo 'foo is: $(foo)'

endif

Running, we get:

make[1]: Entering directory '/home/myname'
foo is: bar
make[1]: Leaving directory '/home/myname'

Now, the only difference between these 2 versions of makefile, is that in the 2nd version, the .EXPORT_ALL_VARIABLES was made phony.

Why is the 'phoniness' needed in order to work?

解决方案

Why is the 'phoniness' needed in order to work?

It's not. You didn't declare .EXPORT_ALL_VARIABLES as a target, you declared it as a prerequisite:

all: .EXPORT_ALL_VARIABLES

That's a prerequisite, not a target. If you declare it as a target:

.EXPORT_ALL_VARIABLES:

then it will work and you won't have to declare it phony.

A more accurate question would be, why does declaring .EXPORT_ALL_VARIABLES as phony work even though it's not declared as a target? It happens because things that are marked phony are assumed to be targets even if they're not explicitly mentioned as such. That may or may not be a bug, depending on how you interpret the intent of .PHONY.

Your questions recently seem to follow a pattern: read the documentation, then write a makefile that does something similar to but not the same as what the documentation says, observe it doesn't work as described, then ask why not.

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