Using Implicit Rules

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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 dont 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 the following.

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 dependencies (the source files). You would want to write a rule for foo.o with no command lines if you need to specify additional dependencies (such as header files) which the implicit rule cannot supply.

Each implicit rule has a target pattern and dependency 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 dependencies 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 dependency files are supposed to exist. See Catalogue of implicit rules for a catalogue of all the predefined implicit rules.

Previously, we said an implicit rule applies if the required dependencies exist or can be made. A file can be made if it is mentioned explicitly in the makefile as a target or a dependency, or if an implicit rule can be recursively found for how to make it. When an implicit dependency is the result of another implicit rule, we say that chaining is occurring. See Chains of implicit rules.

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 dependency is considered a target whose rule specifies nothing, so implicit rule search happens for it. See Implicit rule search algorithm for the details of how the search is done.

Note:
Explicit dependencies do not influence implicit rule search. For example, consider the explicit rule: foo.o: foo.p.

The dependency 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 (see Catalogue of implicit 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 (see Using empty commands).

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