make: Chained Rules
10.4 Chains of Implicit Rules
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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.
Info Catalog
make: Implicit Variables
make: Implicit Rules
make: Pattern Rules