2.4. Pattern Rules

The makefile examples we've been looking at are a bit verbose. For a small program of a dozen files or less we may not care, but for programs with hundreds or thousands of files, specifying each target, prerequisite, and command script becomes unworkable. Furthermore, the commands themselves represent duplicate code in our makefile. If the commands contain a bug or ever change, we would have to update all these rules. This can be a major maintenance problem and source of bugs.

Many programs that read one file type and output another conform to standard conventions. For instance, all C compilers assume that files that have a .c suffix contain C source code and that the object filename can be derived by replacing the .c suffix with .o (or .obj for some Windows compilers). In the previous chapter, we noticed that flex input files use the .l suffix and that flex generates .c files.