VSU_introIntroduction to Verified Software Units

Looking back: single-module programs

You've now seen how to verify programs that use many of C's data structures (arrays, pointers, structs, integers) and control structures (functions, if-then-else, loops), and abstraction structures (data abstraction, module interfaces). The capstone exercise (hash tables) demonstrated all of these at once, in addition to a key concept: layering the proof using a functional model, so that proofs about the properties of the functional model (Hashfun) cleanly separate from the implementation proof (Verif_hash) showing that the C program refines the functional model.

Next: modular verification of modular programs

VST's facility for verifying modular programs is called "Verified Software Units," designed and implemented by Lennart Beringer.
A program module in C
  • is implemented in one or more .c files;
  • exports some set of functions (an API) described in a .h file
  • (perhaps) has some private functions that are not exported
  • imports from other modules, various sets of functions described in .h files
  • (perhaps) manipulates data structures whose representations should be kept private from client modules
  • (perhaps) has global variables of its own that (perhaps) other modules should not directly manipulate
Although the C programming language has the notion of static functions and variables that are "global" only within a .c file, and not exported from that .c file, we will not rely on this feature;
  • in part because a single "module" may be implemented in more than one .c file, to which the static keyword does not generalize;
  • in part because we will use Verifiable C's "VSU" facility to enforce this kind of locality.
Therefore the private functions and private variables of a module will be extern in the C sense.
The C programs you have already seen in previous chapters -- stack, strlib, hash -- are already written as program modules in this sense. But the VST verifications in Verif_stack, Verif_hash, don't enforce data abstraction, and don't show you how to link the modules together. That is a job for VSU - Verified Software Units -- and the next chapters show how to verify those same .c programs in the VSU style.
VSU verification of a module (for example, "Stack") usually comprises these separate files:
  • Model_stack.v, the functional model, just in the sense that hash.v is the functional model for our hash table. But the functional model for stacks is so simple -- just Coq lists -- that we will merge that into Spec_stack.v.
  • Spec_stack.v, the API specification of the stack module. This is the one that clients import; it contains (for example) funspecs and the names of abstract data types (ADTs). The important thing is that it contains nothing about the implementation of the function bodies, or the representations of ADTs, or any mention of internal functions that are not meant to be exported through the API.
  • VSU_stack.v is the proof that the implementation of the stack module, stack.c, correctly implements the API spec in Spec_stack.v.

A Stack/Triang program to verify

As an exercise in VSU verification, we will use the stack.c program that was introduced in Verif_stack and Verif_triang. But first we'll break it up into modules:


  #include <stddef.h>
  extern void * malloc (size_t n);
  extern void free (void *p);
  extern void exit(int n);


  struct stack;
  struct stack *newstack(void);
  void push (struct stack *p, int i);
  int pop (struct stack *p);


  int triang (int n);


  #include "triang2.h"
  int main(void) { return triang(10); }
With, of course, stack2.c and triang2.c that implement modules corresponding to stack2.h and triang2.h.

Let's verify!

And with that preamble, you can move on to the next chapter, Spec_stack.


VSU is still a new feature, and some of the VSU tactics don't give very good error diagnostics when something goes wrong.

Next Chapter: Spec_stack

(* 2021-08-11 15:21 *)