VSU_stdlibAxiomatization of malloc/free/exit

Require Import VST.floyd.proofauto.
Require Import VST.floyd.VSU.
Require Import VC.stdlib.
Require Import VC.Spec_stdlib.

Instance CompSpecs : compspecs. make_compspecs prog. Defined.
The VSU for external axiomatized functions is much like any other VSU, except that we use Axioms instead of Definitions and Lemmas.
Because this VSU does not import any APDs, we don't need a Section.
Normally at this point, we would construct an M showing how the mem_mgr predicate is implemented. But here we just axiomatize:
Axiom M: MallocFreeAPD.

Axiom make_mem_mgr: gv, emp |-- mem_mgr M gv.
Normally at this point, we would prove a semax_body lemma for each function that the module exports. But here, we use body_lemma_of_funspec for external functions; and we write this as an axiom:
Axiom body_malloc:
  {Espec: OracleKind} {cs: compspecs} ,
   VST.floyd.library.body_lemma_of_funspec EF_malloc (snd (malloc_spec_sz M)).

Axiom body_free:
  {Espec: OracleKind} {cs: compspecs} ,
   VST.floyd.library.body_lemma_of_funspec EF_free (snd (free_spec_sz M)).

Axiom body_exit:
  {Espec: OracleKind},
    (EF_external "exit" (mksignature (AST.Tint :: nil) AST.Tvoid cc_default))
    (snd (exit_spec)).

Internal functions

There is one internal (nonexported) function, so as usual we must write a funspec for it. That is, the placeholder, which is a useless function whose only purpose was to force clightgen to keep the external declarations for malloc, free, exit. Because nobody will call placeholder(), we can give it a trivial funspec whose precondition is False.
Definition placeholder_spec :=
    (_placeholder, vacuous_funspec (Internal f_placeholder)).

Definining the pieces of a VSU

And now, in the usual way, we can put totether the pieces:
  Definition MF_ASI: funspecs := MallocFreeASI M.
  Definition MF_imported_specs:funspecs := nil.
  Definition MF_internal_specs: funspecs := placeholder_spec::MF_ASI.
  Definition MF_globals: globals mpred:= Spec_stdlib.mem_mgr M.
  Definition MFVprog : varspecs. mk_varspecs prog. Defined.
  Definition MFGprog: funspecs := MF_imported_specs ++ MF_internal_specs.

Constructing the Component and the VSU

This lemma is required for the solve_SF_external (body_malloc) below. The basic purpose is to ensure that the postcondition of the external function ensures a return value that's strictly compatible with the calling conventions specified by the C-language return-type of the function. The particular form of the lemma is derived from the proof goal of solve_SF_external below.
Lemma semax_func_cons_malloc_aux {cs: compspecs} (gv: globals)
         (gx : genviron) (ret : option val) n:
 (EX x : val,
  (PROP ( )
   RETURN (x) SEP (mem_mgr M gv;
             if eq_dec x nullval
             then emp
             else malloc_token_sz M Ews n x × memory_block Ews n x))
    (make_ext_rval gx (rettype_of_type (tptr tvoid)) ret)) &&
  !! Builtins0.val_opt_has_rettype ret
     (rettype_of_type (tptr tvoid))
 |-- !! tc_option_val (tptr tvoid) ret.
 Intros p.
 rewrite <- insert_local.
 rewrite lower_andp.
 apply derives_extract_prop; intro.
 red in H0; unfold_lift in H0; destruct H0.
 destruct ret; try contradiction.
 unfold eval_id in H0; simpl in H0. subst v.
 if_tac. rewrite H0; entailer!.
 renormalize. entailer!.
 simpl. auto.

  Definition MF_E : funspecs := MF_ASI.
Each VSU may have private global variables that constitute its "local state". This VSU_initializer lemma calculates how to reason about their initial values. But the module stdlib.c does not have any global variables, so this initialize lemma is rather trivial here. See VSU_stdlib2.v, lemma initialize, for a more interesting example.
Lemma initialize: VSU_initializer prog MF_globals.
apply make_mem_mgr.
Now that all the subcomponents are ready, we can bundle up the VSU:
Definition MallocFreeVSU: @VSU NullExtension.Espec
         MF_E MF_imported_specs ltac:(QPprog prog) MF_ASI MF_globals.
    mkVSU prog MF_internal_specs.
    - solve_SF_external (@body_malloc NullExtension.Espec CompSpecs).
       apply (semax_func_cons_malloc_aux gv gx ret n).
    - solve_SF_external (@body_free NullExtension.Espec CompSpecs).
    - solve_SF_external (@body_exit NullExtension.Espec).
    - apply initialize.

Next Chapter: VSU_main

(* 2021-08-11 15:21 *)