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Secondary Expansion of Implicit Rules
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As `make' searches for an implicit rule, it substitutes the stem and then performs secondary expansion for every rule with a matching target pattern. The value of the automatic variables is derived in the same fashion as for static pattern rules. As an example:
.SECONDEXPANSION:
foo: bar
foo foz: fo%: bo%
%oo: $$< $$^ $$+ $$*
When the implicit rule is tried for target `foo', `$$<' expands to `bar', `$$^' expands to `bar boo', `$$+' also expands to `bar boo', and `$$*' expands to `f'.
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对隐式规则的二次扩展
当 make 搜寻一条 隐式规则，它替换枝干部分，并对每一条符合目的模式的规则进行二次扩展。
自动变量的值也像静态模式规则那样被推导处理。
例如：
.SECONDEXPANSION:
foo: bar
foo foz: fo%: bo%
%oo: $$< $$^ $$+ $$*
当隐式规则用于目的 foo时，$$< 扩展为 bar, $$^ 扩展为 bar boo， $$+也扩展为 bar boo，$$*扩展为 f。
后文待续
本文转自健哥的数据花园博客园博客，原文链接：http://www.cnblogs.com/gaojian/archive/2012/09/18/2690000.html，如需转载请自行联系原作者
