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查了GNUmake英文文档, 其意思跟中文文档一样的, 给的示例也一样.
一下是GNUmake 原英文文档对eval函数的解释：
8.8 The eval Function
The eval function is very special: it allows you to define new makefile constructs that are not constant; which are the result of evaluating other variables and functions. The argument to the eval function is expanded, then the results of that expansion are parsed as makefile syntax. The expanded results can define new make variables, targets, implicit or explicit rules, etc.
The result of the eval function is always the empty string; thus, it can be placed virtually anywhere in a makefile without causing syntax errors.
It's important to realize that the eval argument is expanded twice; first by the eval function, then the results of that expansion are expanded again when they are parsed as makefile syntax. This means you may need to provide extra levels of escaping for "$" characters when using eval. The value function (see section 8.7 The value Function) can sometimes be useful in these situations, to circumvent unwanted expansions.
Here is an example of how eval can be used; this example combines a number of concepts and other functions. Although it might seem overly complex to use eval in this example, rather than just writing out the rules, consider two things: first, the template definition (in PROGRAM_template) could need to be much more complex than it is here; and second, you might put the complex, "generic" part of this example into another makefile, then include it in all the individual makefiles. Now your individual makefiles are quite straightforward.
...... 一句话, 仍然不明白. |
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楼主: captivated
发表于 2011-05-23 04:36
