没有配方的makefile模式规则

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：

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:

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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