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GNU Info (make.info)Using ImplicitUsing 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 supply. 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. Note: Catalogue of Implicit 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. Note: 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 prerequisite is considered a target whose rule specifies nothing, so implicit rule search happens for it. Note: Implicit Rule Search Algorithm, 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 (Note: 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 (Note: Defining Empty Commands.). automatically generated by info2www version 1.2.2.9 |