Gnu make - Makefile variable assignment

Questions

Can anybody give a clear explanation of how variable assignment really works in Makefiles.

What is the difference between :

 VARIABLE = value
 VARIABLE ?= value
 VARIABLE := value
 VARIABLE += value

http://www.gnu.org/software/make/manual/make.html#Reading-Makefiles http://www.gnu.org/software/make/manual/make.html#Reading-Makefiles

Answers

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

Immediate Set

VARIABLE := value

Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time.

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)

Source

License : cc by-sa 3.0

http://stackoverflow.com/questions/448910/makefile-variable-assignment

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