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• How to decide whether foo.o is out of date: it is out of date if it does not exist, or if either foo.c or defs.h is more recent than it.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.