Dependency Generation Improvements

22 January 2001

The following is a brief summary of the improvements made to the preprocessor's Makefile dependency generation functionality.

• Consider the simple example of a file test.c that includes the file test.h, and the command line

  	/usr/bin/gcc -MD -c test.c -o subdir/bar.o
    

  With previous versions of CPP, this command line would leave a file test.d in the current directory, and its contents would be

  	test.o: test.c test.h
    

  which is pretty useless. The latest versions of CPP instead do what you probably want - they make the target subdir/bar.o, place the dependency file in the same directory as the object file, and name it after the object file. Thus in the file subdir/bar.d you will find

  	subdir/bar.o: test.c test.h
    

• By default CPP uses the main file name, excluding any path, and appends the object suffix, normally ``.o'', to it to obtain the name of the target for dependency generation. With -MT or -MQ you can specify a target yourself, overriding the default one.

  If you want multiple targets, you can specify them as a single argument to a switch, or use multiple switches. The targets you specify are output in the order they appear on the command line. -MQ is identical to -MT, except that the target name is quoted for Make, but with -MT it isn't. For example, -MT '$(objpfx)foo.o' gives

  	$(objpfx)foo.o: /tmp/foo.c
    

  but -MQ '$(objpfx)foo.o' gives

  	$$(objpfx)foo.o: /tmp/foo.c
    

  The default target is automatically quoted, as if it were given with -MQ.

• When used with -M or -MM, -MF specifies a file to write the dependencies to. This allows the preprocessor to write the preprocessed file to stdout normally. If no -MF switch is given, and one is not implied by -MD or -MMD, CPP sends the rules to stdout and suppresses normal preprocessed output.

• Finally, -MP instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors make gives if you remove header files without updating the Makefile to match.

  This is typical output:-

  test.o: /tmp/test.c /tmp/test.h
  
  /tmp/test.h: