makefile之二

ALL_DEPENDENCY_DIRS := $(patsubst %/,%,$(sort $(ALL_DEPENDENCY_DIRS)))

-include $(wildcard $(ALL_DEPENDENCY_DIRS:%=%/*.d))

以上两句话的意思是：

对ALL_DEPENDENCY_DIRS中的所有成员排序，并截去结尾的斜杠；获取所有成员的路径下以.d结尾的文件，并-include之，忽视可能出现的错误。

$(patsubst pattern,replacement,text)

Finds whitespace-separated words in text that match pattern and replaces them with replacement. Here pattern may contain a ‘%’ which acts as a wildcard, matching any number of any characters within a word. If replacement also contains a ‘%’, the ‘%’ is replaced by the text that matched the ‘%’ in pattern. Only the first ‘%’ in the pattern and replacement is treated this way; any subsequent ‘%’ is unchanged.

‘%’ characters in patsubst function invocations can be quoted with preceding backslashes (‘\’). Backslashes that would otherwise quote ‘%’ characters can be quoted with more backslashes. Backslashes that quote ‘%’ characters or other backslashes are removed from the pattern before it is compared file names or has a stem substituted into it. Backslashes that are not in danger of quoting ‘%’ characters go unmolested. For example, the pattern the\%weird\\%pattern\\ has ‘the%weird\’ preceding the operative ‘%’ character, and ‘pattern\\’ following it. The final two backslashes are left alone because they cannot affect any ‘%’ character.

Whitespace between words is folded into single space characters; leading and trailing whitespace is discarded.

If you want make to simply ignore a makefile which does not exist or cannot be remade, with no error message, use the -include directive instead of include, like this:

     -include filenames...

This acts like include in every way except that there is no error (not even a warning) if any of the filenames (or any prerequisites of any of the filenames) do not exist or cannot be remade.

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.

Substitution references are a simpler way to get the effect of the patsubst function:

          $(var:pattern=replacement)

is equivalent to

          $(patsubst pattern,replacement,$(var))

The second shorthand simplifies one of the most common uses of patsubst: replacing the suffix at the end of file names.

          $(var:suffix=replacement)

is equivalent to

          $(patsubst %suffix,%replacement,$(var))