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KDIR ?= $(shell uname -r) What is the meaning of ?=?
I have understood the difference between :=, += and = from another thread available in Stack Overflow, but unable to find the explanation for ?=.
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?= indicates to set the KDIR variable only if it's not set/doesn't have a value. For example: KDIR ?= "foo" KDIR ?= "bar" test: echo $(KDIR)
The simple answer is that %.o is a target that matches any file ending in .o. "%.o: %. c" means that any file ending in .o depends on the same filename ending in . c to be present.
$$ means be interpreted as a $ by the shell. the $(UNZIP_PATH) gets expanded by make before being interpreted by the shell.
If there is a wildcard symbol (%) in the pattern, this will match any number of characters, and these characters are copied into the replacement string at the place of the % in the second argument.
?= indicates to set the KDIR variable only if it's not set/doesn't have a value.
For example:
KDIR ?= "foo" KDIR ?= "bar" test: echo $(KDIR) Would print "foo"
GNU manual: http://www.gnu.org/software/make/manual/html_node/Setting.html
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Thanks to Simon and R.T. for their quick and correct response.
Also, I have found the GNU manual that explains everything in detail: http://www.gnu.org/software/make/manual/html_node/Setting.html
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