GNU make manual 翻译( 一百四十六)

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5.7.3 Communicating Options to a Sub-`make'
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Flags such as `-s' and `-k' are passed automatically to the sub-`make'
through the variable `MAKEFLAGS'.  This variable is set up
automatically by `make' to contain the flag letters that `make'
received.  Thus, if you do `make -ks' then `MAKEFLAGS' gets the value
`ks'.

   As a consequence, every sub-`make' gets a value for `MAKEFLAGS' in
its environment.  In response, it takes the flags from that value and
processes them as if they had been given as arguments.  *Note Summary
of Options: Options Summary.

   Likewise variables defined on the command line are passed to the
sub-`make' through `MAKEFLAGS'.  Words in the value of `MAKEFLAGS' that
contain `=', `make' treats as variable definitions just as if they
appeared on the command line.  *Note Overriding Variables: Overriding.

   The options `-C', `-f', `-o', and `-W' are not put into `MAKEFLAGS';
these options are not passed down.

   The `-j' option is a special case (*note Parallel Execution:
Parallel.).  If you set it to some numeric value `N' and your operating
system supports it (most any UNIX system will; others typically won't),
the parent `make' and all the sub-`make's will communicate to ensure
that there are only `N' jobs running at the same time between them all.
Note that any job that is marked recursive (*note Instead of Executing
Recipes: Instead of Execution.)  doesn't count against the total jobs
(otherwise we could get `N' sub-`make's running and have no slots left
over for any real work!)

   If your operating system doesn't support the above communication,
then `-j 1' is always put into `MAKEFLAGS' instead of the value you
specified.  This is because if the `-j' option were passed down to
sub-`make's, you would get many more jobs running in parallel than you
asked for.  If you give `-j' with no numeric argument, meaning to run
as many jobs as possible in parallel, this is passed down, since
multiple infinities are no more than one.

5.7.3 向子make通信的选项
-------------------------------------------

诸如 -s 或者 -k 这样的标志会通过变量 MAKEFLAGS自动地传递到子make中。这个变量是由make自动设置的，用来包含make所接受到的标志。因此如果你使用的 make -ks ，那么MKAKEFLAGS中就包含了值ks。

作为一个序列，每一个子make在其环境中获得一个MAKEFLAGS的值。作为回应，它拿到这些标志，然后处理它们，就好像它们作为参数一般。*Note Summary of Options: Options Summary

 类似的，在命令行中的定义的变量会通过 MAKEFLAGS 传递到子make中。对于在MAKEFLAGS中包含 = 的单词，make 把它当作变量定义对待，就如同从命令行中出现一样。 *Note Overriding Varaables:Overriding 

选项 -C ，-f ，-o 和 -W 不会放到 MAKEFLAGS里，这些选项不会向下传递。

-j选项是一个特殊的情况(*note Parallel Execution: Parallel.) 如果你把它设置为某些数值N并且你的操作系统支持这么做(很多UNIX系统支持，但是其他系统不会)，父make和所有的子make将会通信以确保只有N个任务在所有make之间同时运行。

 请注意任何被标记为 递归(*note Instead of Executing Recipes: Instead of Execution.)的任务不会记入到总的任务数(否则我们没有办法获得'N'个 子make 运行并且会没有槽留给剩下的真正的工作)中。

如果你的操作系统不支持这种通信，那么 -j1 而不是你指定的值将会总是传递给 MAKEFLAGS里。这是因为 如果-j 选项向下传递到 子make里， 你会获得比你请求的还要夺得任务并行执行。如果你给出没有参数的 -j，意味着尽可能地并行运行任务，这会被传递下去，因为多个无穷大也未必大过一个无穷大。

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