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继续翻译6.9 Undefining Variables========================If you want to clear a variable, setting its value to empty is usuallysufficient. Expanding such a variable will yield the same result (emptystring) regardless of whether it was set or not. However, if you areusing the `flavor' (*note Flavor Fu 阅读全文
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上例子foo:=foobar=barall: @echo $(flavor $(foo)) @echo $(flavor $(bar))结果：simplerecursive结束 阅读全文
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继续翻译 You may nest `define' directives: `make' will keep track of nesteddirectives and report an error if they are not all properly closed with`endef'. Note that lines beginning with the recipe prefix characterare considered part of a recipe, so any `define' or `endef' stringsappe 阅读全文
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上例子define funcfoo: @echo "at foo"endefall: foo @echo "final"$(eval $(call func, foo,abc.c))先不整那些函数参数传递之类的幺蛾子，做一个个简单的例子，运行 make结果是：at foofinal然后再变化下：define func$1: @echo "at foo"endefall: foo @echo "final"$(eval $(call func, foo,abc.c))这次，传递 foo 作为 $1，得到结果相同。基本 阅读全文
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上例子reverse=$(2) $(1)foo=$(call reverse, a, b)all: @echo $(foo)运行结果: makeb a结束 阅读全文
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继续翻译6.8 Defining Multi-Line Variables=================================Another way to set the value of a variable is to use the `define'directive. This directive has an unusual syntax which allows newlinecharacters to be included in the value, which is convenient fordefining both canned sequences 阅读全文
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上例子override gao=abcgao=12345all: @echo $(gao)运行的结果，makeabcoverride 指令比其他的变量赋值优先级要高。结束 阅读全文
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上例子override gao += abcall: @echo $(gao)如果执行 make gao=12345结果为：12345 abc直接写gao += abcall: @echo $(gao)执行 make gao=12345结果是：12345当然，如果直接 make，结果就是 abc 了。结束 阅读全文
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继续翻译6.7 The `override' Directive============================If a variable has been set with a command argument (*note OverridingVariables: Overriding.), then ordinary assignments in the makefile areignored. If you want to set the variable in the makefile even thoughit was set with a command argu 阅读全文
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继续翻译 When you add to a variable's value with `+=', `make' actsessentially as if you had included the extra text in the initialdefinition of the variable. If you defined it first with `:=', makingit a simply-expanded variable, `+=' adds to that simply-expandeddefinition, and expan 阅读全文
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继续翻译6.6 Appending More Text to Variables====================================Often it is useful to add more text to the value of a variable alreadydefined. You do this with a line containing `+=', like this: objects += another.oThis takes the value of the variable `objects', and adds the text 阅读全文
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继续翻译6.5 Setting Variables=====================To set a variable from the makefile, write a line starting with thevariable name followed by `=' or `:='. Whatever follows the `=' or`:=' on the line becomes the value. For example, objects = main.o foo.o bar.o utils.odefines a variable n 阅读全文
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继续翻译6.4 How Variables Get Their Values==================================Variables can get values in several different ways: * You can specify an overriding value when you run `make'. *Note Overriding Variables: Overriding. * You can specify a value in the makefile, either with an assignment... 阅读全文
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上例子首先，在linux 环境中，如此设置:#test=1234567#export test#echo $test1234567#然后编辑 Makefileall: @echo $(test)运行结果:#make 1234567#结束 阅读全文
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上例子gao:=123all: @echo $(gao)如果运行 make ,结果是 123如果运行 make gao=456, 结果是456那么如果改变一下呢。比如我们想要：即便你在命令行给出了变量的值，我也不想放弃呢？修改例子：override gao:=123all: @echo $(gao)如果运行 make ,结果是 123因为有 override 的存在，make gao=456 的结果仍然是 123结束 阅读全文
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继续翻译 The only restriction on this sort of use of nested variablereferences is that they cannot specify part of the name of a functionto be called. This is because the test for a recognized function nameis done before the expansion of nested references. For example, ifdef do_sort func := sort ... 阅读全文
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继续翻译 A computed variable name need not consist entirely of a singlevariable reference. It can contain several variable references, aswell as some invariant text. For example, a_dirs := dira dirb 1_dirs := dir1 dir2 a_files := filea fileb 1_files := file1 file2 ifeq "$(use_a)"... 阅读全文
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继续翻译 The previous example shows two levels of nesting, but any number oflevels is possible. For example, here are three levels: x = y y = z z = u a := $($($(x)))Here the innermost `$(x)' expands to `y', so `$($(x))' expands to`$(y)' which in turn expands to `z'; now we have `$(z) 阅读全文