Question

I have a makefile from Intel in which there is some “?=”.
Like

COMPILER ?= $(GCC_PATH)g++

But

EXECUTABLE = run

What is the difference between ?= and = and when do I have to use the first one instead of the second one ?

Thank you very much.

Answer 1

Quoth the fine documentation:

If you’d like a variable to be set to a value only if it’s not already set, then you can use the shorthand operator ‘?=’ instead of ‘=’.