What does $$@ and the pipe symbol in Makefile stand for?

Question

In the following Makefile definition:

1. What does the $$@ in the second last line stand for?
2. What about the | symbol in the middle line?

define KERNEL_RULE $(DESTDIR)/$(1) : kernel_modules $(DEST_DIR)/$(1) : $(DESTDIR)/$(1) | $(DEST_DIR)     cp $(DESTDIR)/$(1) $$@ endef 

Answer 1

(You have a lamentable choice of variable names; let's change DESTDIR to SOURCE_DIR and leave DEST_DIR alone.)

Suppose you were writing an ordinary rule:

$(DEST_DIR)/foo : $(SOURCE_DIR)/foo     cp $(SOURCE_DIR)/foo $(DEST_DIR)/foo 

That works, but the redundancy is troublesome. Sooner or later you'll change $(DEST_DIR)/foo in the preq but forget to change it in the rule. And the rule is hard to read. So we put in an automatic variable:

$(DEST_DIR)/foo : $(SOURCE_DIR)/foo     cp $(SOURCE_DIR)/foo $@ 

When this rule runs, $@ will be expanded to the name of the target, $(DEST_DIR)/foo. (We can do even better than that, but let's stop there.)

Now we want to make sure that $(DEST_DIR) exists before this rule runs, but we don't want it to be a prerequisite exactly, because the absence of that directory shouldn't be enough to cause this rule to run. So we make it an order-only prerequisite:

$(DEST_DIR)/foo : $(SOURCE_DIR)/foo | $(DEST_DIR)     cp $(SOURCE_DIR)/foo $@ 

Now we want many rules like this, for different targets, and instead of doing it the smart way, we'll use a "canned recipe", sort of a template for creating rules on the fly.

# This won't work define KERNEL_RULE $(SOURCE_DIR)/$(1) : kernel_modules $(DEST_DIR)/$(1) : $(SOURCE_DIR)/$(1) | $(DEST_DIR)     cp $(SOURCE_DIR)/$(1) $@ endef 

The problem is that when we evaluate this definition, $@ will be expanded, and since it isn't a rule yet, it will expand to nothing. So we change it to $$@:

# This will work define KERNEL_RULE $(SOURCE_DIR)/$(1) : kernel_modules $(DEST_DIR)/$(1) : $(SOURCE_DIR)/$(1) | $(DEST_DIR)     cp $(SOURCE_DIR)/$(1) $$@ endef 

When Make calls this definition, $$@ expands to $@, then if/when it runs the rule, $@ will expand to the name of the target.