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.