( Phony Targets

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 4.6 Phony Targets
 A phony target is one that is not really the name of a file.  It is
 just a name for some commands to be executed when you make an explicit
 request.  There are two reasons to use a phony target: to avoid a
 conflict with a file of the same name, and to improve performance.
    If you write a rule whose commands will not create the target file,
 the commands will be executed every time the target comes up for
 remaking.  Here is an example:
              rm *.o temp
 Because the `rm' command does not create a file named `clean', probably
 no such file will ever exist.  Therefore, the `rm' command will be
 executed every time you say `make clean'.  
    The phony target will cease to work if anything ever does create a
 file named `clean' in this directory.  Since it has no prerequisites,
 the file `clean' would inevitably be considered up to date, and its
 commands would not be executed.  To avoid this problem, you can
 explicitly declare the target to be phony, using the special target
 `.PHONY' ( Special Built-in Target Names Special Targets.) as
      .PHONY : clean
 Once this is done, `make clean' will run the commands regardless of
 whether there is a file named `clean'.
    Since it knows that phony targets do not name actual files that
 could be remade from other files, `make' skips the implicit rule search
 for phony targets ( Implicit Rules).  This is why declaring a
 target phony is good for performance, even if you are not worried about
 the actual file existing.
    Thus, you first write the line that states that `clean' is a phony
 target, then you write the rule, like this:
      .PHONY: clean
              rm *.o temp
    Another example of the usefulness of phony targets is in conjunction
 with recursive invocations of `make' (for more information, see 
 Recursive Use of `make' Recursion.).  In this case the makefile will
 often contain a variable which lists a number of subdirectories to be
 built.  One way to handle this is with one rule whose command is a
 shell loop over the subdirectories, like this:
      SUBDIRS = foo bar baz
              for dir in $(SUBDIRS); do \
                $(MAKE) -C $$dir; \
    There are a few problems with this method, however.  First, any error
 detected in a submake is not noted by this rule, so it will continue to
 build the rest of the directories even when one fails.  This can be
 overcome by adding shell commands to note the error and exit, but then
 it will do so even if `make' is invoked with the `-k' option, which is
 unfortunate.  Second, and perhaps more importantly, you cannot take
 advantage of `make''s ability to build targets in parallel (
 Parallel Execution Parallel.), since there is only one rule.
    By declaring the subdirectories as phony targets (you must do this as
 the subdirectory obviously always exists; otherwise it won't be built)
 you can remove these problems:
      SUBDIRS = foo bar baz
      .PHONY: subdirs $(SUBDIRS)
      subdirs: $(SUBDIRS)
              $(MAKE) -C $@
      foo: baz
    Here we've also declared that the `foo' subdirectory cannot be built
 until after the `baz' subdirectory is complete; this kind of
 relationship declaration is particularly important when attempting
 parallel builds.
    A phony target should not be a prerequisite of a real target file;
 if it is, its commands are run every time `make' goes to update that
 file.  As long as a phony target is never a prerequisite of a real
 target, the phony target commands will be executed only when the phony
 target is a specified goal ( Arguments to Specify the Goals
    Phony targets can have prerequisites.  When one directory contains
 multiple programs, it is most convenient to describe all of the
 programs in one makefile `./Makefile'.  Since the target remade by
 default will be the first one in the makefile, it is common to make
 this a phony target named `all' and give it, as prerequisites, all the
 individual programs.  For example:
      all : prog1 prog2 prog3
      .PHONY : all
      prog1 : prog1.o utils.o
              cc -o prog1 prog1.o utils.o
      prog2 : prog2.o
              cc -o prog2 prog2.o
      prog3 : prog3.o sort.o utils.o
              cc -o prog3 prog3.o sort.o utils.o
 Now you can say just `make' to remake all three programs, or specify as
 arguments the ones to remake (as in `make prog1 prog3').  Phoniness is
 not inherited: the prerequisites of a phony target are not themselves
 phony, unless explicitly declared to be so.
    When one phony target is a prerequisite of another, it serves as a
 subroutine of the other.  For example, here `make cleanall' will delete
 the object files, the difference files, and the file `program':
      .PHONY: cleanall cleanobj cleandiff
      cleanall : cleanobj cleandiff
              rm program
      cleanobj :
              rm *.o
      cleandiff :
              rm *.diff
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