a question about makefile

[bzhao]

today I met a makefile having a line like:
TMP ?= /tmp

What does the "?" mean here.

Thanks indeed

[zarthon]

From:
http://www.gnu.org/software/make/man...s.html#Flavors



There is another assignment operator for variables, `?='. This is called a conditional variable assignment operator, because it only has an effect if the variable is not yet defined. This statement:

FOO ?= bar

is exactly equivalent to this (see The origin Function):

ifeq ($(origin FOO), undefined)
FOO = bar
endif

[bzhao]

thank you for providing the link, which is good.