问题
I have a master makefile which has the default values for variables and then a child makefile which includes project specific settings. At the end of the child makefile, I include the master makefile.
I have been using the following code in the master makefile to set default values for a variable
ifndef CC
CC = avr-gcc
endif
And then recently I read that I can also do
CC ?= avr-gcc
So my question is, whether both are same and if yes which one is the recommended way of overriding variables.
回答1:
The second is broadly understood, easier to read and causes less clutter.
The first way, using ifndef / endif is more for instances where you want to do more than just set a variable, like toggling many things depending on if DEBUG is set, or something else.
If you just want to set a variable if it's not already set, then var ?= value is definitely sufficient.
来源：https://stackoverflow.com/questions/18869628/overriding-a-makefile-variable