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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. 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 Note: 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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