GNU Make工具（三） 文本转换函数 $origin(...)

Functions for Transforming Text

origin

$(origin variable)

这个函数作用是返回一个表明变量 varible 是如何定义的字符串。

字符串包括

• undefined：未定义的变量
• default：比如 CC CXX Variables Used by Implicit Rules
• environment：环境变量
• environment override：make -e 选项所覆盖的环境变量
• file：在makefile中定义的变量
• command line：在命令行定义的变量
• override：makefile 中通过 override 得到的变量
• automatic：makefile中保留的变量，如 @，^

DEFINED_IN_FILE="good"
LOGNAME=""

override OVERRIDE="override you"
OVERRIDE="assign again success"

all: clean
        @echo "Okay, this is $(CC)"

ifeq "$(origin CC)" "default"
  CC         = clang
endif


clean:
        @echo "The XX is never defined: $(origin XX)"
        @echo "The CXX is default: $(origin CXX)"
        @echo "PATH is defined in the environment: $(origin PATH)"
        @echo "LOGNAME is override by environment variable: $(origin LOGNAME)"
        @echo "DEFINED_IN_FILE is defined in the makefile: $(origin DEFINED_IN_FILE)"
        @echo "COMMAND_LINE type is: $(origin COMMAND_LINE)"
        @echo "OVERRIDE is $(OVERRIDE), override type: $(origin OVERRIDE)"
        @echo "@ is the automatic variable: $(origin @)"

运行 make OVERRIDE="hhh" COMMAND_LINE=1 -e，-e 选项引入了环境变量，覆盖makefile中定义的同名变量 LOGNAME。