Makefile中％和*有什么区别 [英] What is the difference between % and * in a makefile

问题描述

GNU make手册没有擅长解释这一部分，我找不到解释，或者我无法推断其他地方的信息.

The GNU make manual does not excel at explaining this part, I could not find the explanation or I could not infer the information elsewhere.

我意识到%是一种通配符，但是在targets，dependencies和commands的上下文中%和*有什么区别?我在哪里可以使用它，并且到处都具有相同的含义?

I realize % is a kind of wildcard, but what is the difference between % and * in the context of targets, dependencies and commands? Where can I use it and does it have the same meaning everywhere?

target: dependencies ...
    commands

推荐答案

通配符*用于简单地在当前目录中生成匹配文件的列表.模式替换字符%是当前可能存在或不存在的文件的占位符.

The wildcard character * is used to simply generate a list of matching files in the current directory. The pattern substitution character % is a placeholder for a file which may or may not exist at the moment.

要扩展通配符陷阱您已经发现的手册中的示例

To expand on the Wildcard pitfall example from the manual which you had already discovered,

objects = *.o

简单地为变量分配了相当无用的文字值*.o-一个目标文件，该文件可能依赖于一个字面意义为*.c的文件，该文件当然也不存在.这样您会得到一个错误和/或不稳定的行为.

simply assigns to the variable the rather useless literal value *.o -- an object file which presumably depends on a file named literally *.c which of course also does not exist. So you get an error, and/or erratic behavior.

正确的短语表达方式

objects := $(patsubst %.c,%.o,$(wildcard *.c))

make本身在这种情况下不执行通配符扩展，但是，当然，如果将字面值*.o传递给外壳，则在扩展发生时(如果存在匹配项)，因此这可能很难调试. make 将在规则目标中执行通配符扩展，因此您可以说

make itself performs no wildcard expansion in this context, but of course, if you pass the literal value *.o to the shell, that's when expansion happens (if there are matches) and so this can be slightly hard to debug. make will perform wildcard expansion in the target of a rule, so you can say

foo: *.o

并使其完全按照您的预期工作(前提是在评估此依赖项时保证所需文件已存在).

and have it work exactly like you intended (provided the required files are guaranteed to exist at the time this dependency is evaluated).

相比之下，您可以拥有一个带有模式占位符的规则，当make尝试查找可用于生成所需依赖项的配方时，该规则将以任何匹配的名称填充.有内置的规则，例如

By contrast, you can have a rule with a pattern placeholder, which gets filled in with any matching name as make tries to find a recipe which can be used to generate a required dependency. There are built-in rules like

%.o: %.c
        $(CC) $(CCFLAGS) $^ -o $@

(这里近似真实的东西)说:给与%.c匹配的文件，相应的文件%.o可以如下生成."在这里，%是一个占位符，可以用任何东西代替；因此，如果将其应用于现有文件foo.c，则说明如何生成foo.o.

(approximating the real thing here) which say "given a file matching %.c, the corresponding file %.o can be generated as follows." Here, the % is a placeholder which can be replaced by anything; so if it is applied against an existing file foo.c it says how foo.o can be generated.

您可以改写为说*匹配每个匹配文件，而%匹配任何匹配文件.

You could rephrase it to say * matches every matching file while % matches any matching file.

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