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. The difference is that the intermediate file is deleted when
make is finished. Therefore, the intermediate file which did not
exist before make also does not exist after make. The
deletion is reported to you by printing a `rm -f' command that
shows what make is doing. (You can list the target pattern of an
implicit rule (such as `%.o') as a dependency of the special
target .PRECIOUS to preserve intermediate files made by implicit
rules whose target patterns match that file's name;
see section Interrupting or Killing make.)
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.