GNU make manual 翻译( 一百八十二)

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`ifdef VARIABLE-NAME'
     The `ifdef' form takes the _name_ of a variable as its argument,
     not a reference to a variable.  The value of that variable has anon-empty value, the TEXT-IF-TRUE is effective; otherwise, theTEXT-IF-FALSE, if any, is effective.  Variables that have neverbeen defined have an empty value.  The text VARIABLE-NAME isexpanded, so it could be a variable or function that expands tothe name of a variable.  For example:bar = truefoo = barifdef $(foo)frobozz = yesendifThe variable reference `$(foo)' is expanded, yielding `bar', whichis considered to be the name of a variable.  The variable `bar' isnot expanded, but its value is examined to determine if it isnon-empty.Note that `ifdef' only tests whether a variable has a value.  Itdoes not expand the variable to see if that value is nonempty.Consequently, tests using `ifdef' return true for all definitionsexcept those like `foo ='.  To test for an empty value, use`ifeq ($(foo),)'.  For example,
bar =foo = $(bar)ifdef foofrobozz = yeselsefrobozz = noendifsets `frobozz' to `yes', while:foo =ifdef foofrobozz = yeselsefrobozz = noendifsets `frobozz' to `no'.

`ifdef VARIABLE-NAME'

`ifdef' 把变量的名字作为它的参数，并不是对变量的一个参照。此变量的值有一个非空值。此时TEXT-IF-TRUE is effective 有效；否则，若TEXT-IF-FALSE存在，则TEXT-IF-FALSE有效。从未定义的变量拥有一个空值。VARIABLE-NAME 文本会被扩展，因此它可以是一个变量或者函数的扩展，例如:

bar = true

foo = bar
ifdef $(foo)
frobozz = yes
endif

变量参照$(foo)被扩展，转化为bar，它可以视为一个变量的名字。变量bar不再扩展，但是它的值会用来被检查看其是否为空。

请注意 ifdef 只测试是否一个变量有一个值。它并不扩展词比变量来看它是否为非空。此外，用ifdef来测试会返回真，只有像foo=那种测试才会是例外。为了测试一个空值，可以使用 ifeq($(foo),)。例如:

bar =
foo = $(bar)
ifdef foo
frobozz = yes
else
frobozz = no
endif

设置 `frobozz' 为 `yes', 而:

foo =
ifdef foo
frobozz = yes
else
frobozz = no
endif

设置 `frobozz' 为 `no'.

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