Dynamic-Dependency Parameters

A dynamic-dependency parameter has meaning only on the dependency line in a makefile. The $$@ refers to the current ``thing'' to the left of the : symbol (which is $@). Also the form $$(@F) exists, which allows access to the file part of $@. Thus, in the following example:

cat: $$@.c

the dependency is translated at execution time to the string cat.c. This is useful for building a large number of executable files, each of which has only one source file. For instance, the operating system software command directory could have a makefile such as:

CMDS = cat dd echo date cmp comm chown 

$(CMDS): $$@.c 
        	$(CC) $(CFLAGS) $? -o $@

Obviously, this is a subset of all the single file programs. For multiple file programs, a directory is usually allocated and a separate makefile is made. For any particular file that has a peculiar compilation procedure, a specific entry must exist in the makefile.

The second useful form of the dependency parameter is $$(@F). It represents the filename part of $$@. Again, it is evaluated at execution time. Its usefulness becomes evident when trying to maintain the /usr/include directory from makefile in the /usr/src/head directory. Thus, the /usr/src/head/makefile would look like:

INCDIR = /usr/include 

INCLUDES = \ 
        	$(INCDIR)/stdio.h \ 
        	$(INCDIR)/pwd.h \ 
        	$(INCDIR)/dir.h \ 
        	$(INCDIR)/a.out.h 

$(INCLUDES): $$(@F) 
        	cp $? $@ 
        	chmod 0444 $@

This would completely maintain the /usr/include directory whenever one of the above files in /usr/src/head was updated.