When a pattern rule's target is just `%', it matches any file name
whatever. We call these rules match-anything rules. They are very
useful, but it can take a lot of time for make to think about them,
because it must consider every such rule for each file name listed either
as a target or as a dependency.
Suppose the makefile mentions `foo.c'. For this target, make
would have to consider making it by linking an object file `foo.c.o',
or by C compilation-and-linking in one step from `foo.c.c', or by
Pascal compilation-and-linking from `foo.c.p', and many other
possibilities.
We know these possibilities are ridiculous since `foo.c' is a C source
file, not an executable. If make did consider these possibilities,
it would ultimately reject them, because files such as `foo.c.o' and
`foo.c.p' would not exist. But these possibilities are so
numerous that make would run very slowly if it had to consider
them.
To gain speed, we have put various constraints on the way make
considers match-anything rules. There are two different constraints that
can be applied, and each time you define a match-anything rule you must
choose one or the other for that rule.
One choice is to mark the match-anything rule as terminal by defining it with a double colon. When a rule is terminal, it does not apply unless its dependencies actually exist. Dependencies that could be made with other implicit rules are not good enough. In other words, no further chaining is allowed beyond a terminal rule.
For example, the built-in implicit rules for extracting sources from RCS
and SCCS files are terminal; as a result, if the file `foo.c,v' does
not exist, make will not even consider trying to make it as an
intermediate file from `foo.c,v.o' or from `RCS/SCCS/s.foo.c,v'.
RCS and SCCS files are generally ultimate source files, which should not be
remade from any other files; therefore, make can save time by not
looking for ways to remake them.
If you do not mark the match-anything rule as terminal, then it is nonterminal. A nonterminal match-anything rule cannot apply to a file name that indicates a specific type of data. A file name indicates a specific type of data if some non-match-anything implicit rule target matches it.
For example, the file name `foo.c' matches the target for the pattern
rule `%.c : %.y' (the rule to run Yacc). Regardless of whether this
rule is actually applicable (which happens only if there is a file
`foo.y'), the fact that its target matches is enough to prevent
consideration of any nonterminal match-anything rules for the file
`foo.c'. Thus, make will not even consider trying to make
`foo.c' as an executable file from `foo.c.o', `foo.c.c',
`foo.c.p', etc.
The motivation for this constraint is that nonterminal match-anything rules are used for making files containing specific types of data (such as executable files) and a file name with a recognized suffix indicates some other specific type of data (such as a C source file).
Special built-in dummy pattern rules are provided solely to recognize certain file names so that nonterminal match-anything rules will not be considered. These dummy rules have no dependencies and no commands, and they are ignored for all other purposes. For example, the built-in implicit rule
%.p :
exists to make sure that Pascal source files such as `foo.p' match a specific target pattern and thereby prevent time from being wasted looking for `foo.p.o' or `foo.p.c'.
Dummy pattern rules such as the one for `%.p' are made for every suffix listed as valid for use in suffix rules (see section Old-Fashioned Suffix Rules).