Makefile 四種等號用法

Lazy Set VARIABLE = value Normal setting of a variable - values within it are recursively expanded when the variable is used, not when it's declared (that means it fetches/parses the value only when you use the variable) Immediate Set VARIABLE := value Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time. (that means it fetches the value immediately, and store the content to the variable) Set If Absent VARIABLE ?= value Setting of a variable only if it doesn't have a value Append VARIABLE += value Appending the supplied value to the existing value (or setting to that value if the variable didn't exist)