File: make.info,  Node: Implicit Rule Search,  Prev: Suffix Rules,  Up: Implicit Rules

10.8 Implicit Rule Search Algorithm
===================================

Here is the procedure 'make' uses for searching for an implicit rule for
a target T.  This procedure is followed for each double-colon rule with
no recipe, for each target of ordinary rules none of which have a
recipe, and for each prerequisite that is not the target of any rule.
It is also followed recursively for prerequisites that come from
implicit rules, in the search for a chain of rules.

   Suffix rules are not mentioned in this algorithm because suffix rules
are converted to equivalent pattern rules once the makefiles have been
read in.

   For an archive member target of the form 'ARCHIVE(MEMBER)', the
following algorithm is run twice, first using the entire target name T,
and second using '(MEMBER)' as the target T if the first run found no
rule.

  1. Split T into a directory part, called D, and the rest, called N.
     For example, if T is 'src/foo.o', then D is 'src/' and N is
     'foo.o'.

  2. Make a list of all the pattern rules one of whose targets matches T
     or N.  If the target pattern contains a slash, it is matched
     against T; otherwise, against N.

  3. If any rule in that list is _not_ a match-anything rule, or if T is
     a prerequisite of an implicit rule, then remove all non-terminal
     match-anything rules from the list.

  4. Remove from the list all rules with no recipe.

  5. For each pattern rule in the list:

       a. Find the stem S, which is the nonempty part of T or N matched
          by the '%' in the target pattern.

       b. Compute the prerequisite names by substituting S for '%'; if
          the target pattern does not contain a slash, append D to the
          front of each prerequisite name.

       c. Test whether all the prerequisites exist or ought to exist.
          (If a file name is mentioned in the makefile as a target or as
          an explicit prerequisite of target T, then we say it ought to
          exist.)

          If all prerequisites exist or ought to exist, or there are no
          prerequisites, then this rule applies.

  6. If no pattern rule has been found so far, try harder.  For each
     pattern rule in the list:

       a. If the rule is terminal, ignore it and go on to the next rule.

       b. Compute the prerequisite names as before.

       c. Test whether all the prerequisites exist or ought to exist.

       d. For each prerequisite that does not exist, follow this
          algorithm recursively to see if the prerequisite can be made
          by an implicit rule.

       e. If all prerequisites exist, ought to exist, or can be made by
          implicit rules, then this rule applies.

  7. If no pattern rule has been found then try step 5 and step 6 again
     with a modified definition of "ought to exist": if a filename is
     mentioned as a target or as an explicit prerequisite of _any_
     target, then it ought to exist.  This check is only present for
     backward-compatibility with older versions of GNU Make: we don't
     recommend relying on it.

  8. If no implicit rule applies, the rule for '.DEFAULT', if any,
     applies.  In that case, give T the same recipe that '.DEFAULT' has.
     Otherwise, there is no recipe for T.

   Once a rule that applies has been found, for each target pattern of
the rule other than the one that matched T or N, the '%' in the pattern
is replaced with S and the resultant file name is stored until the
recipe to remake the target file T is executed.  After the recipe is
executed, each of these stored file names are entered into the data base
and marked as having been updated and having the same update status as
the file T.

   When the recipe of a pattern rule is executed for T, the automatic
variables are set corresponding to the target and prerequisites.  *Note
Automatic Variables::.