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4.1 Rule Example

Here is an example of a rule:

foo.o : foo.c defs.h       # module for twiddling the frobs
        cc -c -g foo.c

Its target is foo.o and its prerequisites are foo.c and defs.h. It has one command in the recipe: ‘cc -c -g foo.c’. The recipe starts with a tab to identify it as a recipe.

This rule says two things:

  • How to decide whether foo.o is out of date: it is out of date if it does not exist, or if either foo.c or defs.h is more recent than it.
  • How to update the file foo.o: by running cc as stated. The recipe does not explicitly mention defs.h, but we presume that foo.c includes it, and that is why defs.h was added to the prerequisites.