File: make.info,  Node: Match-Anything Rules,  Next: Canceling Rules,  Prev: Pattern Match,  Up: Pattern Rules

10.5.5 Match-Anything Pattern Rules
-----------------------------------

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 prerequisite.

   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 prerequisites actually exist.  Prerequisites 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
non-terminal.  A non-terminal match-anything rule cannot apply to a
prerequisite of an implicit rule, or 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 non-terminal 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 non-terminal
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 non-terminal match-anything rules will not be
considered.  These dummy rules have no prerequisites and no recipes, 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 (*note Old-Fashioned
Suffix Rules: Suffix Rules.).