( Using Implicit

Info Catalog ( Implicit Rules ( Implicit Rules ( Catalogue of Rules
 10.1 Using Implicit Rules
 To allow `make' to find a customary method for updating a target file,
 all you have to do is refrain from specifying commands yourself.  Either
 write a rule with no command lines, or don't write a rule at all.  Then
 `make' will figure out which implicit rule to use based on which kind
 of source file exists or can be made.
    For example, suppose the makefile looks like this:
      foo : foo.o bar.o
              cc -o foo foo.o bar.o $(CFLAGS) $(LDFLAGS)
 Because you mention `foo.o' but do not give a rule for it, `make' will
 automatically look for an implicit rule that tells how to update it.
 This happens whether or not the file `foo.o' currently exists.
    If an implicit rule is found, it can supply both commands and one or
 more prerequisites (the source files).  You would want to write a rule
 for `foo.o' with no command lines if you need to specify additional
 prerequisites, such as header files, that the implicit rule cannot
    Each implicit rule has a target pattern and prerequisite patterns.
 There may be many implicit rules with the same target pattern.  For
 example, numerous rules make `.o' files: one, from a `.c' file with the
 C compiler; another, from a `.p' file with the Pascal compiler; and so
 on.  The rule that actually applies is the one whose prerequisites
 exist or can be made.  So, if you have a file `foo.c', `make' will run
 the C compiler; otherwise, if you have a file `foo.p', `make' will run
 the Pascal compiler; and so on.
    Of course, when you write the makefile, you know which implicit rule
 you want `make' to use, and you know it will choose that one because you
 know which possible prerequisite files are supposed to exist.  
 Catalogue of Implicit Rules Catalogue of Rules, for a catalogue of all
 the predefined implicit rules.
    Above, we said an implicit rule applies if the required
 prerequisites "exist or can be made".  A file "can be made" if it is
 mentioned explicitly in the makefile as a target or a prerequisite, or
 if an implicit rule can be recursively found for how to make it.  When
 an implicit prerequisite is the result of another implicit rule, we say
 that "chaining" is occurring.   Chains of Implicit Rules Chained
    In general, `make' searches for an implicit rule for each target, and
 for each double-colon rule, that has no commands.  A file that is
 mentioned only as a prerequisite is considered a target whose rule
 specifies nothing, so implicit rule search happens for it.  
 Implicit Rule Search Algorithm Implicit Rule Search, for the details
 of how the search is done.
    Note that explicit prerequisites do not influence implicit rule
 search.  For example, consider this explicit rule:
      foo.o: foo.p
 The prerequisite on `foo.p' does not necessarily mean that `make' will
 remake `foo.o' according to the implicit rule to make an object file, a
 `.o' file, from a Pascal source file, a `.p' file.  For example, if
 `foo.c' also exists, the implicit rule to make an object file from a C
 source file is used instead, because it appears before the Pascal rule
 in the list of predefined implicit rules ( Catalogue of Implicit
 Rules Catalogue of Rules.).
    If you do not want an implicit rule to be used for a target that has
 no commands, you can give that target empty commands by writing a
 semicolon ( Defining Empty Commands Empty Commands.).
Info Catalog ( Implicit Rules ( Implicit Rules ( Catalogue of Rules
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