没有配方的makefile模式规则 [英] makefile pattern rules without recipes

问题描述

我正在观察一个有趣的作品行为，我想知道除了gmake中的错误，是否有合理的解释。

假设我们有以下在makefile中：

 ％ - 动物：
 echo$ *是动物
 
％-fox：％-fox-animal 
 
％-wolf：％-wolf-animal 
  

最后两个目标之间的区别是％-wolf没有任何配方，％-fox有一个空的配方（即只是开头有一个标签的行） / p>

当我们尝试执行规则时，会发生什么：

  [root @ cv19 tmp]＃make freddy-animal 
 echofreddy是一个动物
 freddy是一个动物
 [root @ cv19 tmp]＃make freddy-wolf 
 make ：***没有规定使目标freddy-wolf。停止。 
 [root @ cv19 tmp]＃make freddy-fox 
 echofreddy-fox是一个动物
 freddy-fox是一个动物
  

有一个配方的模式规则（虽然是空的）可以工作，但不是没有。我想要以某种方式工作吗？

解决方案

根本没有食谱的图案规则被记录在一起与提供食谱的人不同，甚至是空的。而是取消任何预先存在的隐含规则：

您可以通过定义具有相同目标和先决条件但没有配方的模式规则来取消内置隐式规则。

因此，您的％-wolf模式实际上用于取消任何现有的％ - wolf-animal - > ％ - 狼。而且还没有一个。

I'm observing an interesting behavior of make and I wonder if there is a reasonable explanation to it besides a bug in gmake.

Let's say we have the following in makefile:

%-animal:
        echo "$* is an animal"

%-fox: %-fox-animal

%-wolf: %-wolf-animal

The difference between the last two targets is that "%-wolf" does not have any recipe, and "%-fox" has an empty recipe (i.e. just a line with a tab at the beginning).

When we try to execute the rules, here's what happens:

[root@cv19 tmp]# make freddy-animal
echo "freddy is an animal"
freddy is an animal
[root@cv19 tmp]# make freddy-wolf
make: *** No rule to make target `freddy-wolf'.  Stop.
[root@cv19 tmp]# make freddy-fox
echo "freddy-fox is an animal"
freddy-fox is an animal

i.e.the pattern rule that has a recipe (although an empty one) works, the one that doesn't does not. Am I missing something in the way it's supposed to work?

解决方案

Pattern rules with no recipes at all are documented as meaning something quite different from those providing a recipe, even an empty one. Instead they cancel any pre-existing implicit rule:

You can cancel a built-in implicit rule by defining a pattern rule with the same target and prerequisites, but no recipe.

Thus your "%-wolf" pattern actually serves to cancel any existing implicit rule for %-wolf-animal -> %-wolf. And there wasn't one anyway.

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