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(make.info)Implicit Rule Search

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Implicit Rule Search Algorithm
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   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 commands, for each target of ordinary rules none of which have
commands, 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, then
     remove all nonterminal match-anything rules from the list.

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

  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, 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 implicit rule applies, the rule for `.DEFAULT', if any,
     applies.  In that case, give T the same commands that `.DEFAULT'
     has.  Otherwise, there are no commands 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
commands to remake the target file T are executed.  After these
commands are 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 commands of a pattern rule are executed for T, the automatic
variables are set corresponding to the target and prerequisites.  Note:
Automatic Variables.