Dependency List Read Twice

This technique works because make reads the dependency list twice, once as part of its initial reading of the entire makefile, and again as it processes target dependencies. In each pass through the list, it performs macro expansion. Since the dynamic macros aren't defined in the initial reading, unless references to them are delayed until the second pass, they are expanded to null strings.

The string $$ is a reference to the predefined macro `$'. This macro, conveniently enough, has the value `$'; when make resolves it in the initial reading, the string $$@ is resolved to $@. In dependency scan, when the resulting $@ macro reference has a value dynamically assigned to it, make resolves the reference to that value.

Notice that make only evaluates the target-name portion of a target entry in the first pass. A delayed macro reference as a target name produces incorrect results. The makefile:

NONE= none 
all: $(NONE) 

$$(NONE): 
         @: this target's name isn't `none'

produces the following results.

$ make 
make: Fatal error: Don't know how to make target `none'