A CIL Tutorial - Department of Computer Science - ETH Zürich

Chapter 1
The AST
The Concrete Syntax of a program, is the textual representation of the program. An Abstract Syntax
Tree(AST) is the in-memory data-structure that represents the parsed program. Parsing turns the
concrete syntax into an AST. Parsing, the specication of grammars, and the various algorithms
for lexical analysis and parsing, are covered in detail in many traditional textbooks on compilers.
The textbooks by Appel [2] and Muchnick [3] are good starting points.
This chapter of the tutorial explains the structure of CIL's AST for C. The AST is dened in
the Cil module in cil/src/cil.ml between lines 130 and 850. It might seem pretty complex at
rst glance, but you will nd that it is very well documented, and the names of types, elds and
constructors are logically assigned. Also, if you are familiar with other C or C++ front-ends, this
AST is refreshingly straightforward. Finally, one of the key advantages to using CIL is that it makes
a number of transformations that simplify analysis and instrumentation. Biggest among these is the
absence of side-eects in expressions. Function calls and assignments are all lifted out of expressions
into the Cil.instr nodes.
In this example code, we go digging through the AST looking for a function called target, when
we nd it, we lter out assignments to a global variable called deleted. In src/tut2.ml we'll see
how to do this a bit more elegantly.
Coding Hint: When working with CIL, have two windows of source on your
screen. One window is the project you are hacking on. The other window
is cil.ml, so you can refer quickly to the AST type denitions and utility
functions.
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