GNU make manual 翻译(八十六)

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4.4.3 The Function `wildcard'                        
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Wildcard expansion happens automatically in rules.  But wildcard expansion does not normally take place when a variable is set, or inside the arguments of a function.  If you want to do wildcard expansion in such places, you need to use the `wildcard' function, like  this:                        
                        
     $(wildcard PATTERN...)                        
                        
This string, used anywhere in a makefile, is replaced by a space-separated list of names of existing files that match one of the given file name patterns.  If no existing file name matches a pattern,then that pattern is omitted from the output of the `wildcard' function.  Note that this is different from how unmatched wildcards
behave in rules, where they are used verbatim rather than ignored (*note Wildcard Pitfall::).                        
                        
   One use of the `wildcard' function is to get a list of all the C source files in a directory, like this:                        
                        
     $(wildcard *.c)                        
                        
   We can change the list of C source files into a list of object files by replacing the `.c' suffix with `.o' in the result, like this:                        
                        
     $(patsubst %.c,%.o,$(wildcard *.c))                          
(Here we have used another function, `patsubst'.  *Note Functions for String Substitution and Analysis: Text Functions.)                        
                        
   Thus, a makefile to compile all C source files in the directory and then link them together could be written as follows:  
     objects := $(patsubst %.c,%.o,$(wildcard *.c))                        
                        
     foo : $(objects)                        
             cc -o foo $(objects)                        
                        
(This takes advantage of the implicit rule for compiling C programs, so 
there is no need to write explicit rules for compiling the files. *Note The Two Flavors of Variables: Flavors, for an explanation of `:=', which is a variant of `='.)                        

4.4.3 wildcard 函数      
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通配符扩展自动地在规则中发生。但是通配符扩展在变量被设置的时候，不会正常发生，通配符在函数的参数中时，扩展也不会发生。如果你想在此种场合下执行 通配符扩展，你需要使用 wildcard 函数，像下面这样:

$(wildcard PATTERN...)
以上的函数字符串，可以用在makefile的任何一个地方，将会被替换成为 用空格分隔的已存在文件的名称列表，只要它符合某种名称模式。如果没有可以匹配模式的文件，那么此模式被省略。请注意这个和 直接在规则里使用通配符时找不到匹配是的行为不同，那时将会不是被忽略，而是留下一字不差地保留下来。(*note Wildcard Pitfall::)

有一个对 wildcard 函数的用法就是，获得一个 某目录下所有C 源程序的名字列表

如下：
$(wildcard *.c)

我们可以改变 C 源程序文件的列表，使其转化为目标文件的列表，这要通过把 .c的后缀换成 .o 后缀来实现，像如下: 

$(patsubst %.c,%.o,$(wildcard *.c))

(这里我们用到了另一个函数, `patsubst'. *Note Functions for
String Substitution and Analysis: Text Functions.)

因此，我们可以用一个 makefile来编译一个目录下的所有的 C 源程序，并把它们链接起来，像如下:

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

foo : $(objects)
cc -o foo $(objects)

(这种做法，获得了隐式规则编译C 程序的好处, 因此此处也就不需要写编译文件的显式规则了。      
*Note The Two Flavors of Variables: Flavors, for an explanation of
`:=', which is a variant of `='.)

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