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, which is `cc -c -g foo.c'. The command line starts with a tab to identify it as a command.

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• How to decide whether `foo.o' is out of date: it is out of date if it does not exist, or if either `foo.c' or `defs.h' is more recent than it.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 command does not explicitly mention `defs.h', but we presume that `foo.c' includes it, and that that is why `defs.h' was added to the prerequisites.