A CIL Tutorial - Department of Computer Science - ETH Zürich

Chapter 8
Dependant Type Qualiers
Of course, there are more colors than just red, green, and blue. Furthermore, the color of a type
might depend on some other data in the program. In this example we introduce type qualiers
parameterized by arbitrary C expressions. Since types may depend on runtime data, type-checking
must happen partially at runtime, which we will accomplish by instrumenting the code with the
appropriate checks. Furthermore, the information we know about the color of a type may be
imprecise, or rather, for example, a function may accept a range of colors rather than only one
exact color. We introduce type qualiers for expressing this.
First, we'll dene OCaml types for representing the type qualiers. Then, we'll dene functions
for extracting these qualiers from C type attributes. This will involve keeping a typing context
that maps strings to expressions so that the identiers in type attributes may be translated into
CIL variables. With the ability to extract colors from types, we will then be able to perform type
checking as in the previous example. However, instead of generating a compile-time answer about
correct typing, we must generate a list of instructions that test type compatibility at runtime.
It will be possible to determine at compile time that some of these checks will always succeed.
Writing an optimization pass to remove these checks is left as an exercise.
OCaml Style Note: It would be better style to put each of these parts in
separate OCaml modules: one for dening OCaml types; one for compiling the
color qualiers in type attributes into CIL expressions; one for setting up the
runtime checks; and one containing the visitors used for type checking.
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