( Chained Rules

Info Catalog ( Implicit Variables ( Implicit Rules ( Pattern Rules
 10.4 Chains of Implicit Rules
 Sometimes a file can be made by a sequence of implicit rules.  For
 example, a file `N.o' could be made from `N.y' by running first Yacc
 and then `cc'.  Such a sequence is called a "chain".
    If the file `N.c' exists, or is mentioned in the makefile, no
 special searching is required: `make' finds that the object file can be
 made by C compilation from `N.c'; later on, when considering how to
 make `N.c', the rule for running Yacc is used.  Ultimately both `N.c'
 and `N.o' are updated.
    However, even if `N.c' does not exist and is not mentioned, `make'
 knows how to envision it as the missing link between `N.o' and `N.y'!
 In this case, `N.c' is called an "intermediate file".  Once `make' has
 decided to use the intermediate file, it is entered in the data base as
 if it had been mentioned in the makefile, along with the implicit rule
 that says how to create it.
    Intermediate files are remade using their rules just like all other
 files.  But intermediate files are treated differently in two ways.
    The first difference is what happens if the intermediate file does
 not exist.  If an ordinary file B does not exist, and `make' considers
 a target that depends on B, it invariably creates B and then updates
 the target from B.  But if B is an intermediate file, then `make' can
 leave well enough alone.  It won't bother updating B, or the ultimate
 target, unless some prerequisite of B is newer than that target or
 there is some other reason to update that target.
    The second difference is that if `make' _does_ create B in order to
 update something else, it deletes B later on after it is no longer
 needed.  Therefore, an intermediate file which did not exist before
 `make' also does not exist after `make'.  `make' reports the deletion
 to you by printing a `rm -f' command showing which file it is deleting.
    Ordinarily, a file cannot be intermediate if it is mentioned in the
 makefile as a target or prerequisite.  However, you can explicitly mark
 a file as intermediate by listing it as a prerequisite of the special
 target `.INTERMEDIATE'.  This takes effect even if the file is mentioned
 explicitly in some other way.
    You can prevent automatic deletion of an intermediate file by
 marking it as a "secondary" file.  To do this, list it as a
 prerequisite of the special target `.SECONDARY'.  When a file is
 secondary, `make' will not create the file merely because it does not
 already exist, but `make' does not automatically delete the file.
 Marking a file as secondary also marks it as intermediate.
    You can list the target pattern of an implicit rule (such as `%.o')
 as a prerequisite of the special target `.PRECIOUS' to preserve
 intermediate files made by implicit rules whose target patterns match
 that file's name; see  Interrupts.  
    A chain can involve more than two implicit rules.  For example, it is
 possible to make a file `foo' from `RCS/foo.y,v' by running RCS, Yacc
 and `cc'.  Then both `foo.y' and `foo.c' are intermediate files that
 are deleted at the end.
    No single implicit rule can appear more than once in a chain.  This
 means that `make' will not even consider such a ridiculous thing as
 making `foo' from `foo.o.o' by running the linker twice.  This
 constraint has the added benefit of preventing any infinite loop in the
 search for an implicit rule chain.
    There are some special implicit rules to optimize certain cases that
 would otherwise be handled by rule chains.  For example, making `foo'
 from `foo.c' could be handled by compiling and linking with separate
 chained rules, using `foo.o' as an intermediate file.  But what
 actually happens is that a special rule for this case does the
 compilation and linking with a single `cc' command.  The optimized rule
 is used in preference to the step-by-step chain because it comes
 earlier in the ordering of rules.
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