(make.info.gz) Remaking Makefiles

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 3.5 How Makefiles Are Remade
 ============================
 
 Sometimes makefiles can be remade from other files, such as RCS or SCCS
 files.  If a makefile can be remade from other files, you probably want
 `make' to get an up-to-date version of the makefile to read in.
 
    To this end, after reading in all makefiles, `make' will consider
 each as a goal target and attempt to update it.  If a makefile has a
 rule which says how to update it (found either in that very makefile or
 in another one) or if an implicit rule applies to it ( Using
 Implicit Rules Implicit Rules.), it will be updated if necessary.
 After all makefiles have been checked, if any have actually been
 changed, `make' starts with a clean slate and reads all the makefiles
 over again.  (It will also attempt to update each of them over again,
 but normally this will not change them again, since they are already up
 to date.)
 
    If you know that one or more of your makefiles cannot be remade and
 you want to keep `make' from performing an implicit rule search on
 them, perhaps for efficiency reasons, you can use any normal method of
 preventing implicit rule lookup to do so.  For example, you can write
 an explicit rule with the makefile as the target, and an empty recipe
 ( Using Empty Recipes Empty Recipes.).
 
    If the makefiles specify a double-colon rule to remake a file with a
 recipe but no prerequisites, that file will always be remade (
 Double-Colon).  In the case of makefiles, a makefile that has a
 double-colon rule with a recipe but no prerequisites will be remade
 every time `make' is run, and then again after `make' starts over and
 reads the makefiles in again.  This would cause an infinite loop:
 `make' would constantly remake the makefile, and never do anything
 else.  So, to avoid this, `make' will *not* attempt to remake makefiles
 which are specified as targets of a double-colon rule with a recipe but
 no prerequisites.
 
    If you do not specify any makefiles to be read with `-f' or `--file'
 options, `make' will try the default makefile names;  What Name to
 Give Your Makefile Makefile Names.  Unlike makefiles explicitly
 requested with `-f' or `--file' options, `make' is not certain that
 these makefiles should exist.  However, if a default makefile does not
 exist but can be created by running `make' rules, you probably want the
 rules to be run so that the makefile can be used.
 
    Therefore, if none of the default makefiles exists, `make' will try
 to make each of them in the same order in which they are searched for
 ( What Name to Give Your Makefile Makefile Names.)  until it
 succeeds in making one, or it runs out of names to try.  Note that it
 is not an error if `make' cannot find or make any makefile; a makefile
 is not always necessary.
 
    When you use the `-t' or `--touch' option ( Instead of
 Executing Recipes Instead of Execution.), you would not want to use an
 out-of-date makefile to decide which targets to touch.  So the `-t'
 option has no effect on updating makefiles; they are really updated
 even if `-t' is specified.  Likewise, `-q' (or `--question') and `-n'
 (or `--just-print') do not prevent updating of makefiles, because an
 out-of-date makefile would result in the wrong output for other targets.
 Thus, `make -f mfile -n foo' will update `mfile', read it in, and then
 print the recipe to update `foo' and its prerequisites without running
 it.  The recipe printed for `foo' will be the one specified in the
 updated contents of `mfile'.
 
    However, on occasion you might actually wish to prevent updating of
 even the makefiles.  You can do this by specifying the makefiles as
 goals in the command line as well as specifying them as makefiles.
 When the makefile name is specified explicitly as a goal, the options
 `-t' and so on do apply to them.
 
    Thus, `make -f mfile -n mfile foo' would read the makefile `mfile',
 print the recipe needed to update it without actually running it, and
 then print the recipe needed to update `foo' without running that.  The
 recipe for `foo' will be the one specified by the existing contents of
 `mfile'.
 

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