6. Subprograms

6.1. Subprogram Declarations

We distinguish the declaration view introduced by a subprogram_declaration from the implementation view introduced by a subprogram_body or an expression_function_declaration. For subprograms that are not declared by a subprogram_declaration, the subprogram_body or expression_function_declaration also introduces a declaration view which may be in SPARK even if the implementation view is not.

Rules are imposed in SPARK to ensure that the execution of a function call does not modify any variables declared outside of the function. It follows as a consequence of these rules that the evaluation of any SPARK expression is side-effect free.

We also introduce the notion of a global item, which is a name that denotes a global object or a state abstraction (see Abstract_State Aspects). Global items are presented in Global aspects (see Global Aspects).

An entire object is an object which is not a subcomponent of a larger containing object. More specifically, an entire object is an object declared by an object_declaration (as opposed to, for example, a slice or the result object of a function call) or a formal parameter of a subprogram. In particular, a component of a protected unit is not an entire object.

An object O1 is said to be a reachable element of an object O2 if

  • O1 is a part of O2; or

  • O1 is a reachable element of the object designated by (the value of) an access-valued part of O2.

Static Semantics

  1. The exit value of a global item or parameter of a subprogram is its value immediately following the successful call of the subprogram.

  2. The entry value of a global item or parameter of a subprogram is its value at the call of the subprogram.

  1. An output of a subprogram is a global item or parameter whose final value, or the final value of any of its reachable elements, may be updated by a successful call to the subprogram. The result of a function is also an output. A global item or parameter which is an external state with the property Async_Readers => True, and for which intermediate values are written during an execution leading to a successful call, is also an output even if the final state is the same as the initial state. (see External State). [On the contrary, a global item or parameter is not an output of the subprogram if it is updated only on paths that lead to an explicit raise_statement or to a pragma Assert (statically_False) or to a call to a subprogram marked No_Return.]

  1. An input of a subprogram is a global item or parameter whose initial value (or that of any of its reachable elements) may be used in determining the exit value of an output of the subprogram. For a global item or parameter which is an external state with Async_Writers => True, each successive value read from the external state is also an input of the subprogram (see External State). As a special case, a global item or parameter is also an input if it is mentioned in a null_dependency_clause in the Depends aspect of the subprogram (see Depends Aspects).

  2. An output of a subprogram is said to be fully initialized by a call if all parts of the output are initialized as a result of any successful execution of a call of the subprogram. In the case of a parameter X of a class-wide type T’Class, this set of “all parts” is not limited to the (statically known) parts of T. For example, if the underlying dynamic tag of X is T2’Tag, where T2 is an extension of T that declares a component C, then C would be included in the set. In this case, this set of “all parts” is not known statically. [In order to fully initialize such a parameter, it is necessary to use some form of dispatching assignment. This can be done by either a direct (class-wide) assignment to X, passing X as an actual out-mode parameter in a call where the formal parameter is of a class-wide type, or passing X as a controlling out-mode parameter in a dispatching call.] The meaning of “all parts” in the case of a parameter of a specific tagged type is determined by the applicable Extensions_Visible aspect (see Extensions_Visible Aspects). [A state abstraction cannot be fully initialized by initializing individual constituents unless its refinement is visible.]

Legality Rules

  1. A function declaration shall not have a parameter_specification with a mode of out or in out. This rule also applies to a subprogram_body for a function for which no explicit declaration is given. A function shall have no outputs other than its result.

  2. A subprogram parameter of mode in shall not be an output of its subprogram unless the type of the parameter is an access type and the subprogram is not a function.

Verification Rules

  1. At the point of a call, all inputs of the callee except for those that have relaxed initialization (see Relaxed Initialization) shall be fully initialized. Similarly, upon return from a call all outputs of the callee except for those that have relaxed initialization shall be fully initialized.

6.1.1. Preconditions and Postconditions

Legality Rules

  1. The corresponding expression for an inherited Pre’Class or Post’Class of an inherited subprogram S of a tagged type T shall not call a non-inherited primitive function of type T.

[The notion of corresponding expression is defined in Ada RM 6.1.1(18/4) as follows: If a Pre’Class or Post’Class aspect is specified for a primitive subprogram S of a tagged type T, or such an aspect defaults to True, then a corresponding expression also applies to the corresponding primitive subprogram S of each descendant of T.]

[The rationale for this rule is that, otherwise, if the contract applicable to an inherited subprogram changes due to called subprograms in its contract being overridden, then the inherited subprogram would have to be re-verified for the derived type. This rule forbids the cases that require re-verification.]

  1. The Pre aspect shall not be specified for a primitive operation of a type T at a point where T is tagged. [Pre’Class should be used instead to express preconditions.]

[The rationale for this rule is that, otherwise, the combination of dynamic semantics and verification rules below would force an identical Pre’Class each time Pre is used on a dispatching operation.]

  1. A subprogram_renaming_declaration shall not declare a primitive operation of a tagged type.

[Consider

package Outer is
   type T is tagged null record;
   package Nested is
      procedure Op (X : T) with Pre => ..., Post => ... ;
      -- not a primitive, so Pre/Post specs are ok
   end Nested;
   procedure Renamed_Op (X : T) renames Nested.Op; -- illegal
end Outer;

Allowing this example in SPARK would introduce a case of a dispatching operation which is subject to a Pre (and Post) aspect specification. This rule is also intended to avoid problematic interactions between the Pre/Pre’Class/Post/Post’Class aspects of the renamed subprogram and the Pre’Class/Post’Class inheritance associated with the declaration of a primitive operation of a tagged type.

Note that a dispatching subprogram can be renamed as long as the renaming does not itself declare a dispatching operation. Note also that this rule would never apply to a renaming-as-body.]

Verification Rules

For a call on a nondispatching operation, a verification condition is introduced (as for any run-time check) to ensure that the specific precondition check associated with the statically denoted callee will succeed. Upon entry to such a subprogram, the specific preconditions of the subprogram may then be assumed.

For a call (dispatching or not) on a dispatching operation, a verification condition is introduced (as for any run-time check) to ensure that the class-wide precondition check associated with the statically denoted callee will succeed.

The verification condition associated with the specific precondition of a dispatching subprogram is imposed on the callee, as opposed to on callers of the subprogram. Upon entry to a subprogram, the class-wide preconditions of the subprogram may be assumed. Given this, the specific preconditions of the subprogram must be proven.

The callee is responsible for discharging the verification conditions associated with any postcondition checks, class-wide or specific. The success of these checks may then be assumed by the caller.

In the case of an overriding dispatching operation whose Pre’Class attribute is explicitly specified, a verification condition is introduced to ensure that the specified Pre’Class condition is implied by the Pre’Class condition of the overridden inherited subprogram(s). Similarly, in the case of an overriding dispatching operation whose Post’Class attribute is explicitly specified, a verification condition is introduced to ensure that the specified Post’Class condition implies the Post’Class condition of the overridden inherited subprogram(s). [These verification conditions do not correspond to any run-time check. They are intended to, in effect, require users to make explicit the implicit disjunction/conjunction of class-wide preconditions/postconditions that is described in Ada RM 6.1.1.]

6.1.2. Subprogram Contracts

In order to extend Ada’s support for specification of subprogram contracts (e.g., the Pre and Post) by providing more precise and/or concise contracts, the SPARK aspects, Global, Depends, and Contract_Cases are defined.

Legality Rules

  1. The Global, Depends and Contract_Cases aspects may be specified for a subprogram with an aspect_specification. More specifically, such aspect specifications are allowed in the same contexts as Pre or Post aspect specifications. [In particular, these aspects may be specified for a generic subprogram but not for an instance of a generic subprogram.]

  2. The Global, Depends and Contract_Cases aspects shall not be specified for an abstract subprogram or a null procedure. Only Global’Class and Depends’Class may be specified for such a subprogram.

See section Contract Cases for further detail on Contract_Case aspects, section Global Aspects for further detail on Global aspects and section Depends Aspects for further detail on Depends aspects.

6.1.3. Contract Cases

The Contract_Cases aspect provides a structured way of defining a subprogram contract using mutually exclusive subcontract cases. The final case in the Contract_Case aspect may be the keyword others which means that, in a specific call to the subprogram, if all the conditions are False this contract_case is taken. If an others contract_case is not specified, then in a specific call of the subprogram exactly one of the guarding conditions should be True.

A Contract_Cases aspect may be used in conjunction with the language-defined aspects Pre and Post in which case the precondition specified by the Pre aspect is augmented with a check that exactly one of the conditions of the contract_case_list is satisfied and the postcondition specified by the Post aspect is conjoined with conditional expressions representing each of the contract_cases. For example:

procedure P (...)
   with Pre  => General_Precondition,
        Post => General_Postcondition,
        Contract_Cases => (A1 => B1,
                           A2 => B2,
                           ...
                           An => Bn);

is short hand for

procedure P (...)
  with Pre  => General_Precondition
                 and then Exactly_One_Of (A1, A2, ..., An),
       Post => General_Postcondition
                 and then (if A1'Old then B1)
                 and then (if A2'Old then B2)
                 and then ...
                 and then (if An'Old then Bn);

where

A1 .. An are Boolean expressions involving the entry values of formal parameters and global objects and

B1 .. Bn are Boolean expressions that may also use the exit values of formal parameters, global objects and results.

Exactly_One_Of(A1,A2...An) evaluates to True if exactly one of its inputs evaluates to True and all other of its inputs evaluate to False.

The Contract_Cases aspect is specified with an aspect_specification where the aspect_mark is Contract_Cases and the aspect_definition must follow the grammar of contract_case_list given below.

Syntax

contract_case_list  ::= (contract_case {, contract_case})
contract_case       ::= condition => consequence
                      | others => consequence

where

consequence ::= Boolean_expression

Legality Rules

  1. A Contract_Cases aspect may have at most one others contract_case and if it exists it shall be the last one in the contract_case_list.

  2. A consequence expression is considered to be a postcondition expression for purposes of determining the legality of Old or Result attribute_references.

Static Semantics

  1. A Contract_Cases aspect is an assertion (as defined in RM 11.4.2(1.1/3)); its assertion expressions are as described below. Contract_Cases may be specified as an assertion_aspect_mark in an Assertion_Policy pragma.

Dynamic Semantics

  1. Upon a call of a subprogram which is subject to an enabled Contract_Cases aspect, Contract_Cases checks are performed as follows:

    • Immediately after the specific precondition expression is evaluated and checked (or, if that check is disabled, at the point where the check would have been performed if it were enabled), all of the conditions of the contract_case_list are evaluated in textual order. A check is performed that exactly one (if no others contract_case is provided) or at most one (if an others contract_case is provided) of these conditions evaluates to True; Assertions.Assertion_Error is raised if this check fails.

    • Immediately after the specific postcondition expression is evaluated and checked (or, if that check is disabled, at the point where the check would have been performed if it were enabled), exactly one of the consequences is evaluated. The consequence to be evaluated is the one corresponding to the one condition whose evaluation yielded True (if such a condition exists), or to the others contract_case (if every condition‘s evaluation yielded False). A check is performed that the evaluation of the selected consequence evaluates to True; Assertions.Assertion_Error is raised if this check fails.

  2. If an Old attribute_reference occurs within a consequence other than the consequence selected for (later) evaluation as described above, then the associated implicit constant declaration (see Ada RM 6.1.1) is not elaborated. [In particular, the prefix of the Old attribute_reference is not evaluated].

Verification Rules

The verification conditions associated with the Contract_Cases runtime checks performed at the beginning of a call are assigned in the same way as those associated with a specific precondition check. More specifically, the verification condition is imposed on the caller or on the callee depending on whether the subprogram in question is a dispatching operation.

Examples

-- This subprogram is specified using a Contract_Cases aspect.
-- The prover will check that the cases are disjoint and
-- cover the domain of X.
procedure Incr_Threshold (X : in out Integer; Threshold : in Integer)
  with Contract_Cases => (X < Threshold  => X = X'Old + 1,
                          X >= Threshold => X = X'Old);

-- This is the equivalent specification not using Contract_Cases.
-- It is noticeably more complex and the prover is not able to check
-- for disjoint cases or that the domain of X is covered.
procedure Incr_Threshold_1 (X : in out Integer; Threshold : in Integer)
  with Pre  => (X < Threshold and not (X >= Threshold))
                  or else (not (X < Threshold) and X >= Threshold),
       Post => (if X'Old < Threshold then X = X'Old + 1
                elsif X'Old >= Threshold then X = X'Old);

-- Contract_Cases can be used in conjunction with pre and postconditions.
procedure Incr_Threshold_2 (X : in out Integer; Threshold : in Integer)
  with Pre  => X in 0 .. Threshold,
       Post => X >= X'Old,
       Contract_Cases => (X < Threshold => X = X'Old + 1,
                          X = Threshold => X = X'Old);

6.1.4. Global Aspects

A Global aspect of a subprogram lists the global items whose values are used or affected by a call of the subprogram.

The Global aspect shall only be specified for the initial declaration of a subprogram (which may be a declaration, a body or a body stub), of a protected entry, or of a task unit. The implementation of a subprogram body shall be consistent with the subprogram’s Global aspect. Similarly, the implementation of an entry or task body shall be consistent with the entry or task’s Global aspect.

Note that a Refined_Global aspect may be applied to a subprogram body when using state abstraction; see section Refined_Global Aspects for further details.

The Global aspect is introduced by an aspect_specification where the aspect_mark is Global and the aspect_definition must follow the grammar of global_specification

For purposes of the rules concerning the Global, Depends, Refined_Global, and Refined_Depends aspects, when any of these aspects are specified for a task unit the task unit’s body is considered to be the body of a nonreturning procedure and the current instance of the task unit is considered to be a formal parameter (of that notional procedure) of mode in out. [For example, rules which refer to the “subprogram body” refer, in the case of a task unit, to the task body.] [Because a task (even a discriminated task) is effectively a constant, one might think that a mode of in would make more sense. However, the current instance of a task unit is, strictly speaking, a variable; for example, it may be passed as an actual out or in out mode parameter in a call.] The Depends and Refined_Depends aspect of a task unit T need not mention this implicit parameter; an implicit specification of “T => T” is assumed, although this may be confirmed explicitly.

Similarly, for purposes of the rules concerning the Global, Refined_Global, Depends, and Refined_Depends aspects as they apply to protected operations, the current instance of the enclosing protected unit is considered to be a formal parameter (of mode in for a protected function, of mode in out otherwise) and a protected entry is considered to be a protected procedure. [For example, rules which refer to the “subprogram body” refer, in the case of a protected entry, to the entry body. As another example, the Global aspect of a subprogram nested within a protected operation might name the current instance of the protected unit as a global in the same way that it might name any other parameter of the protected operation.]

[Note that AI12-0169 modifies the Ada RM syntax for an entry_body to allow an optional aspect_specification immediately before the entry_barrier. This is relevant for aspects such as Refined_Global and Refined_Depends.]

Syntax

global_specification        ::= (moded_global_list {, moded_global_list})
                              | global_list
                              | null_global_specification
moded_global_list           ::= mode_selector => global_list
global_list                 ::= global_item
                              | (global_item {, global_item})
mode_selector               ::= Input | Output | In_Out | Proof_In
global_item                 ::= name
null_global_specification   ::= null

Static Semantics

  1. A global_specification that is a global_list is shorthand for a moded_global_list with the mode_selector Input.

  2. A global_item is referenced by a subprogram if:

    • It denotes an input or an output of the subprogram, or;

    • Its entry value is used to determine the value of an assertion expression within the subprogram, or;

    • Its entry value is used to determine the value of an assertion expression within another subprogram that is called either directly or indirectly by this subprogram.

  3. A null_global_specification indicates that the subprogram does not reference any global_item directly or indirectly.

  4. If a subprogram’s Global aspect is not otherwise specified and either

    • the subprogram is a library-level subprogram declared in a library unit that is declared pure (i.e., a subprogram to which the implementation permissions of Ada RM 10.2.1 apply); or

    • a Pure_Function pragma applies to the subprogram

    then a Global aspect of null is implicitly specified for the subprogram.

Name Resolution Rules

  1. A global_item shall denote an entire object or a state abstraction. [This is a name resolution rule because a global_item can unambiguously denote a state abstraction even if a function having the same fully qualified name is also present].

Legality Rules

  1. The Global aspect may only be specified for the initial declaration of a subprogram (which may be a declaration, a body or a body stub), of a protected entry, or of a task unit.

  2. A global_item occurring in a Global aspect specification of a subprogram shall not denote a formal parameter of the subprogram.

  3. A global_item shall not denote a state abstraction whose refinement is visible. [A state abstraction cannot be named within its enclosing package’s body other than in its refinement. Its constituents shall be used rather than the state abstraction.]

  4. Each mode_selector shall occur at most once in a single Global aspect.

  5. A function subprogram shall not have a mode_selector of Output or In_Out in its Global aspect.

  6. A user-defined equality operation on a record type shall have a Global aspect of null (see Overloading of Operators).

    [This avoids the case where such a record type is a component of another composite type, whose predefined equality operation now depends on variables through the primitive equality operation on its component.]

  7. The global_items in a single Global aspect specification shall denote distinct entities.

  8. If a subprogram is nested within another and if the global_specification of the outer subprogram has an entity denoted by a global_item with a mode_specification of Input or the entity is a formal parameter with a mode of in, then a global_item of the global_specification of the inner subprogram shall not denote the same entity with a mode_selector of In_Out or Output.

Dynamic Semantics

There are no dynamic semantics associated with a Global aspect as it is used purely for static analysis purposes and is not executed.

Verification Rules

  1. For a subprogram that has a global_specification, an object (except a constant without variable inputs) or state abstraction that is declared outside the scope of the subprogram, shall only be referenced within its implementation if it is a global_item in the global_specification.

  2. A global_item shall occur in a Global aspect of a subprogram if and only if it denotes an entity (except for a constant without variable inputs) that is referenced by the subprogram.

  3. Where the refinement of a state abstraction is not visible (see State Refinement) and a subprogram references one or more of its constituents the constituents may be represented by a global_item that denotes the state abstraction in the global_specification of the subprogram. [The state abstraction encapsulating a constituent is known from the Part_Of indicator on the declaration of the constituent.]

  4. Each entity denoted by a global_item in a global_specification of a subprogram that is an input or output of the subprogram shall satisfy the following mode specification rules [which are checked during analysis of the subprogram body]:

    • a global_item that denotes an input but not an output has a mode_selector of Input;

    • a global_item has a mode_selector of Output if:

      • it denotes an output but not an input, other than the use of a discriminant or an attribute related to a property, not its value, of the global_item [examples of attributes that may be used are A’Last, A’First and A’Length; examples of attributes that are dependent on the value of the object and shall not be used are X’Old and X’Loop_Entry] and

      • it does not have relaxed initialization (see Relaxed Initialization);

    • a global_item that denotes an output which is not an input and which has relaxed initialization may have a mode_selector of Output or In_Out;

    • otherwise the global_item denotes both an input and an output, and has a mode_selector of In_Out.

[For purposes of determining whether an output of a subprogram shall have a mode_selector of Output or In_Out, reads of array bounds, discriminants, or tags of any part of the output are ignored. Similarly, for purposes of determining whether an entity is fully initialized as a result of any successful execution of the call, only nondiscriminant parts are considered. This implies that given an output of a discriminated type that is not known to be constrained (“known to be constrained” is defined in Ada RM 3.3), the discriminants of the output might or might not be updated by the call.]

  1. An entity that is denoted by a global_item which is referenced by a subprogram but is neither an input nor an output but is only referenced directly, or indirectly in assertion expressions has a mode_selector of Proof_In.

  1. A global_item shall not denote a constant object other than a formal parameter [of an enclosing subprogram] of mode in, a generic formal object of mode in, or a constant with variable inputs.

    If a global_item denotes a generic formal object of mode in, then the corresponding global_item in an instance of the generic unit may denote a constant which has no variable inputs. [This can occur if the corresponding actual parameter is an expression which has no variable inputs]. Outside of the instance, such a global_item is ignored. For example,

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package Global_And_Generics is

   generic
      X : Integer;
   package G is
      procedure P (Y : in out Integer) with
        Global  => X,
        Depends => (Y =>+ X);
   end G;

   procedure Q (Z : in out Integer) with
     Global  => null,
     Depends => (Z =>+ null);

end Global_And_Generics;
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package body Global_And_Generics is

   package body G is
      procedure P (Y : in out Integer) is
      begin
         Y := Integer'Max (X, Y);
      end P;
   end G;

   package I is new G
     (X => 123); -- actual parameter lacks variable inputs

   -- Q's Global and Depends aspects don't mention I.X even though
   -- Q calls I.P which does reference I.X as a global.
   -- As seen from outside of I, I.P's references to I.X in its
   -- Global and Depends aspect specifications are ignored.
   procedure Q (Z : in out Integer) is
   begin
      I.P (Y => Z);
   end Q;

end Global_And_Generics;
  1. The mode_selector of a global_item denoting a constant with variable inputs shall be Input or Proof_In.

  1. The mode_selector of a global_item denoting a variable marked as a constant after elaboration shall be Input or Proof_In [, to ensure that such variables are only updated directly by package elaboration code]. A subprogram or entry having such a global_item shall not be called during library unit elaboration[, to ensure only the final (“constant”) value of the object is referenced].

Examples

with Global => null; -- Indicates that the subprogram does not reference
                     -- any global items.
with Global => V;    -- Indicates that V is an input of the subprogram.
with Global => (X, Y, Z);  -- X, Y and Z are inputs of the subprogram.
with Global => (Input    => V); -- Indicates that V is an input of the subprogram.
with Global => (Input    => (X, Y, Z)); -- X, Y and Z are inputs of the subprogram.
with Global => (Output   => (A, B, C)); -- A, B and C are outputs of
                                        -- the subprogram.
with Global => (In_Out   => (D, E, F)); -- D, E and F are both inputs and
                                        -- outputs of the subprogram
with Global => (Proof_In => (G, H));    -- G and H are only used in
                                        -- assertion expressions within
                                        -- the subprogram
with Global => (Input    => (X, Y, Z),
                Output   => (A, B, C),
                In_Out   => (P, Q, R),
                Proof_In => (T, U));
                -- A global aspect with all types of global specification

6.1.5. Depends Aspects

A Depends aspect defines a dependency relation for a subprogram which may be given in the aspect_specification of the subprogram. A dependency relation is a sort of formal specification which specifies a simple relationship between inputs and outputs of the subprogram. It may be used with or without a postcondition.

The Depends aspect shall only be specified for the initial declaration of a subprogram (which may be a declaration, a body or a body stub), of a protected entry, or of a task unit.

Unlike a postcondition, the Depends aspect must be complete in the sense that every input and output of the subprogram must appear in it. A postcondition need only specify properties of particular interest.

Like a postcondition, the dependency relation may be omitted from a subprogram declaration when it defaults to the conservative relation that each output depends on every input of the subprogram. A particular SPARK tool may synthesize a more accurate approximation from the subprogram implementation if it is present (see Synthesis of SPARK Aspects).

For accurate information flow analysis the Depends aspect should be present on every subprogram.

A Depends aspect for a subprogram specifies for each output every input on which it depends. The meaning of X depends on Y in this context is that the input value(s) of Y may affect:

  • the exit value of X; and

  • the intermediate values of X if it is an external state (see section External State), or if the subprogram is a nonreturning procedure [, possibly the notional nonreturning procedure corresponding to a task body].

This is written X => Y. As in UML, the entity at the tail of the arrow depends on the entity at the head of the arrow.

If an output does not depend on any input this is indicated using a null, e.g., X => null. An output may be self-dependent but not dependent on any other input. The shorthand notation denoting self-dependence is useful here, X =>+ null.

Note that a Refined_Depends aspect may be applied to a subprogram body when using state abstraction; see section Refined_Depends Aspects for further details.

See section Global Aspects regarding how the rules given in this section apply to protected operations and to task bodies.

The Depends aspect is introduced by an aspect_specification where the aspect_mark is Depends and the aspect_definition must follow the grammar of dependency_relation given below.

Syntax

dependency_relation    ::= null
                         | (dependency_clause {, dependency_clause})
dependency_clause      ::= output_list =>[+] input_list
                         | null_dependency_clause
null_dependency_clause ::= null => input_list
output_list            ::= output
                         | (output {, output})
input_list             ::= input
                         | (input {, input})
                         | null
input                  ::= name
output                 ::= name | function_result

where

function_result is a function Result attribute_reference.

Name Resolution Rules

  1. An input or output of a dependency_relation shall denote only an entire object or a state abstraction. [This is a name resolution rule because an input or output can unambiguously denote a state abstraction even if a function having the same fully qualified name is also present.]

Legality Rules

  1. The Depends aspect shall only be specified for the initial declaration of a subprogram (which may be a declaration, a body or a body stub), of a protected entry, or of a task unit.

  2. An input or output of a dependency_relation shall not denote a state abstraction whose refinement is visible [a state abstraction cannot be named within its enclosing package’s body other than in its refinement].

  3. The explicit input set of a subprogram is the set of formal parameters of the subprogram of mode in and in out along with the entities denoted by global_items of the Global aspect of the subprogram with a mode_selector of Input and In_Out.

  4. The input set of a subprogram is the explicit input set of the subprogram augmented with those formal parameters of mode out and those global_items with a mode_selector of Output having discriminants, array bounds, or a tag which can be read and whose values are not implied by the subtype of the parameter. More specifically, it includes formal parameters of mode out and global_items with a mode_selector of Output which are of an unconstrained array subtype, an unconstrained discriminated subtype, a tagged type (with one exception), or a type having a subcomponent of an unconstrained discriminated subtype. The exception mentioned in the previous sentence is in the case where the formal parameter is of a specific tagged type and the applicable Extensions_Visible aspect is False. In that case, the tag of the parameter cannot be read and so the fact that the parameter is tagged does not cause it to included in the subprogram’s input_set, although it may be included for some other reason (e.g., if the parameter is of an unconstrained discriminated subtype).

  5. The output set of a subprogram is the set of formal parameters of the subprogram of mode in out and out along with the entities denoted by global_items of the Global aspect of the subprogram with a mode_selector of In_Out and Output and (for a function) the function_result or (for a procedure or entry) the set of formal parameters of the subprogram of mode in of an access-to-variable type.

  6. The entity denoted by each input of a dependency_relation of a subprogram shall be a member of the input set of the subprogram.

  7. Every member of the explicit input set of a subprogram shall be denoted by at least one input of the dependency_relation of the subprogram.

  8. The entity denoted by each output of a dependency_relation of a subprogram shall be a member of the output set of the subprogram.

  9. Every member of the output set of a subprogram shall be denoted by exactly one output in the dependency_relation of the subprogram.

  10. For the purposes of determining the legality of a Result attribute_reference, a dependency_relation is considered to be a postcondition of the function to which the enclosing aspect_specification applies.

  11. In a dependency_relation there can be at most one dependency_clause which is a null_dependency_clause and if it exists it shall be the last dependency_clause in the dependency_relation.

  12. An entity denoted by an input which is in an input_list of a null_dependency_clause shall not be denoted by an input in another input_list of the same dependency_relation.

  13. The inputs in a single input_list shall denote distinct entities.

  14. A null_dependency_clause shall not have an input_list of null.

Static Semantics

  1. A dependency_clause with a “+” symbol in the syntax output_list =>+ input_list means that each output in the output_list has a self-dependency, that is, it is dependent on itself. [The text (A, B, C) =>+ Z is shorthand for (A => (A, Z), B => (B, Z), C => (C, Z)).]

  2. A dependency_clause of the form A =>+ A has the same meaning as A => A. [The reason for this rule is to allow the short hand: ((A, B) =>+ (A, C)) which is equivalent to (A => (A, C), B => (A, B, C)).]

  3. A dependency_clause with a null input_list means that the final value of the entity denoted by each output in the output_list does not depend on any member of the input set of the subprogram (other than itself, if the output_list =>+ null self-dependency syntax is used).

  4. The inputs in the input_list of a null_dependency_clause may be read by the subprogram but play no role in determining the values of any outputs of the subprogram.

  5. A Depends aspect of a subprogram with a null dependency_relation indicates that the subprogram has no inputs or outputs. [From an information flow analysis viewpoint it is a null operation (a no-op).]

  6. A function without an explicit Depends aspect specification has the default dependency_relation that its result is dependent on all of its inputs. [Generally an explicit Depends aspect is not required for a function declaration.]

  7. A procedure without an explicit Depends aspect specification has a default dependency_relation that each member of its output set is dependent on every member of its input set. [This conservative approximation may be improved by analyzing the body of the subprogram if it is present.]

Dynamic Semantics

There are no dynamic semantics associated with a Depends aspect as it is used purely for static analysis purposes and is not executed.

Verification Rules

  1. Each entity denoted by an output given in the Depends aspect of a subprogram shall be an output in the implementation of the subprogram body and the output shall depend on all, but only, the entities denoted by the inputs given in the input_list associated with the output.

  2. Each output of the implementation of the subprogram body is denoted by an output in the Depends aspect of the subprogram.

  3. Each input of the implementation of a subprogram body is denoted by an input of the Depends aspect of the subprogram.

  4. If not all parts of an output are updated, then the updated entity is dependent on itself as the parts that are not updated have their current value preserved.

    [In the case of a parameter of a tagged type (specific or class-wide), see the definition of “fully initialized” for a clarification of what the phrase “all parts” means in the preceding sentence.]

Examples

procedure P (X, Y, Z in : Integer; Result : out Boolean)
  with Depends => (Result => (X, Y, Z));
-- The exit value of Result depends on the entry values of X, Y and Z

procedure Q (X, Y, Z in : Integer; A, B, C, D, E : out Integer)
  with Depends => ((A, B) => (X, Y),
                   C      => (X, Z),
                   D      => Y,
                   E      => null);
-- The exit values of A and B depend on the entry values of X and Y.
-- The exit value of C depends on the entry values of X and Z.
-- The exit value of D depends on the entry value of Y.
-- The exit value of E does not depend on any input value.

procedure R (X, Y, Z : in Integer; A, B, C, D : in out Integer)
  with Depends => ((A, B) =>+ (A, X, Y),
                   C      =>+ Z,
                   D      =>+ null);
-- The "+" sign attached to the arrow indicates self-dependency, that is
-- the exit value of A depends on the entry value of A as well as the
-- entry values of X and Y.
-- Similarly, the exit value of B depends on the entry value of B
-- as well as the entry values of A, X and Y.
-- The exit value of C depends on the entry value of C and Z.
-- The exit value of D depends only on the entry value of D.

procedure S
  with Global  => (Input  => (X, Y, Z),
                   In_Out => (A, B, C, D)),
       Depends => ((A, B) =>+ (A, X, Y, Z),
                   C      =>+ Y,
                   D      =>+ null);
-- Here globals are used rather than parameters and global items may appear
-- in the Depends aspect as well as formal parameters.

function F (X, Y : Integer) return Integer
  with Global  => G,
       Depends => (F'Result => (G, X),
                   null     => Y);
-- Depends aspects on functions are only needed for special cases like here where the
-- parameter Y has no discernible effect on the result of the function.

6.1.6. Class-Wide Global and Depends Aspects

The Global’Class and Depends’Class aspects may be specified for a dispatching subprogram just as the Global and Depends aspects may be specified for any subprogram (dispatching or not). [The syntax, static semantics, and legality rules are all the same, except that the Depends’Class aspect of a subprogram is checked for consistency with the Global’Class aspect of the subprogram rather than with the Global aspect.]

Verification Rules

When analyzing a dispatching call, the Global and Depends aspects of the statically denoted callee play no role; the corresponding class-wide aspects are used instead.

[No relationship between the Global’Class/Depends’Class aspects of a subprogram and the subprogram’s implementation is explicitly verified. This is instead accomplished implicitly by checking the consistency of the subprogram’s implementation with its Global/Depends aspects (as described in preceding sections) and then checking (as described in this section) the consistency of the Global/Depends aspects with the Global’Class/Depends’Class aspects.]

Static Semantics

A Global or Global’Class aspect specification G2 is said to be a valid overriding of another such specification, G1, if the following conditions are met:

  • each Input-mode item of G2 is an Input-mode or an In_Out-mode item of G1 or a direct or indirect constituent thereof; and

  • each In_Out-mode item of G2 is an In_Out-mode item of G1 or a direct or indirect constituent thereof; and

  • each Output-mode item of G2 is an Output-mode or In_Out-mode item of G1 or a direct or indirect constituent therof; and

  • each Output-mode item of G1 which is not a state abstraction whose refinement is visible at the point of G2 is an Output-mode item of G2; and

  • for each Output-mode item of G1 which is a state abstraction whose refinement is visible at the point of G2, each direct or indirect constituent thereof is an Output-mode item of G2.

A Depends or Depends’Class aspect specification D2 is said to be a valid overriding of another such specification, D1, if the set of dependencies of D2 is a subset of the dependencies of D1 or, in the case where D1 mentions a state abstraction whose refinement is visible at the point of D2, if D2 is derivable from such a subset as described in Refined_Depends Aspects.

Legality Rules

The Global aspect of a subprogram shall be a valid overriding of the Global’Class aspect of the subprogram. The Global’Class aspect of an an overriding subprogram shall be a valid overriding of the Global’Class aspect(s) of the overridden inherited subprogram(s).

The Depends aspect of a subprogram shall be a valid overriding of the Depends’Class aspect of the subprogram. The Depends’Class aspect of an an overriding subprogram shall be a valid overriding of the Depends’Class aspect(s) of the overridden inherited subprogram(s).

6.1.7. Extensions_Visible Aspects

The Extensions_Visible aspect provides a mechanism for ensuring that “hidden” components of a formal parameter of a specific tagged type are unreferenced. For example, if a formal parameter of a specific tagged type T is converted to a class-wide type and then used as a controlling operand in a dispatching call, then the (dynamic) callee might reference components of the parameter which are declared in some extension of T. Such a use of the formal parameter could be forbidden via an Extensions_Visible aspect specification as described below. The aspect also plays a corresponding role in the analysis of callers of the subprogram.

Static Semantics

  1. Extensions_Visible is a Boolean-valued aspect which may be specified for a noninstance subprogram or a generic subprogram. If directly specified, the aspect_definition shall be a static [Boolean] expression. The aspect is inherited by an inherited primitive subprogram. If the aspect is neither inherited nor directly specified for a subprogram, then the aspect is False, except in the case of the predefined equality operator of a type extension. In that case, the aspect value is that of the primitive [(possibly user-defined)] equality operator for the parent type.

Legality Rules

  1. If the Extensions_Visible aspect is False for a subprogram, then certain restrictions are imposed on the use of any parameter of the subprogram which is of a specific tagged type (or of a private type whose full view is a specific tagged type). Such a parameter shall not be converted (implicitly or explicitly) to a class-wide type. Such a parameter shall not be passed as an actual parameter in a call to a subprogram whose Extensions_Visible aspect is True. These restrictions also apply to any parenthesized expression, qualified expression, or type conversion whose operand is subject to these restrictions, to any Old or Loop_Entry attribute_reference whose prefix is subject to these restrictions, to any delta aggregate whose expression is subject to these restrictions, and to any conditional expression having at least one dependent_expression which is subject to these restrictions. [A subcomponent of a parameter is not itself a parameter and is therefore not subject to these restrictions. A parameter whose type is class-wide is not subject to these restrictions. An Old or Loop_Entry attribute_reference does not itself violate these restrictions (despite the fact that (in the tagged case) each of these attributes yields a result having the same underlying dynamic tag as their prefix).]

  2. A subprogram whose Extensions_Visible aspect is True shall not override an inherited primitive operation of a tagged type whose Extensions_Visible aspect is False. [The reverse is allowed.]

  3. If a nonnull type extension inherits a procedure having both a False Extensions_Visible aspect and one or more controlling out-mode parameters, then the inherited procedure requires overriding. [This is because the inherited procedure would not initialize the noninherited component(s).]

  4. The Extensions_Visible aspect shall not be specified for a subprogram which has no parameters of either a specific tagged type or a private type unless the subprogram is declared in an instance of a generic unit and the corresponding subprogram in the generic unit satisfies this rule. [Such an aspect specification, if allowed, would be ineffective.]

  5. [These rules ensure that the value of the underlying tag (at run time) of the actual parameter of a call to a subprogram whose Extensions_Visible aspect is False will have no effect on the behavior of that call. In particular, if the actual parameter has any additional components which are not components of the type of the formal parameter, then these components are unreferenced by the execution of the call.]

Verification Rules

  1. [SPARK typically requires that an actual parameter corresponding to an in mode or in out mode formal parameter in a call shall be fully initialized before the call; similarly, the callee is typically responsible for fully initializing any out-mode formal parameters before returning. For details (including interactions with relaxed initialization), see the verification rule about full initialization of subprogram inputs and outputs (which include parameters) in Subprogram Declarations and then Relaxed Initialization].

  2. In the case of a formal parameter of a specific tagged type T (or of a private type whose full view is a specific tagged type), the set of components which shall be initialized in order to meet these requirements depends on the Extensions_Visible aspect of the callee. If the aspect is False, then that set of components is the [statically known] set of nondiscriminant components of T. If the aspect is True, then this set is the set of nondiscriminant components of the specific type associated with the tag of the corresponding actual parameter. [In general, this is not statically known. This set will always include the nondiscriminant components of T, but it may also include additional components.]

  3. [To put it another way, if the applicable Extensions_Visible aspect is True, then the initialization requirements (for both the caller and the callee) for a parameter of a specific tagged type T are the same as if the formal parameter’s type were T’Class. If the aspect is False, then components declared in proper descendants of T need not be initialized. In the case of an out mode parameter, such initialization by the callee is not only not required, it is effectively forbidden because such an out-mode parameter could not be fully initialized without some form of dispatching (e.g., a class-wide assignment or a dispatching call in which an out-mode parameter is a controlling operand). Such a dispatching assignment will always fully initialize its controlling out-mode parameters, regardless of the Extensions_Visible aspect of the callee. An assignment statement whose target is of a class-wide type T’Class is treated, for purposes of formal verification, like a call to a procedure with two parameters of type T’Class, one of mode out and one of mode in.]

  4. [In the case of an actual parameter of a call to a subprogram whose Extensions_Visible aspect is False where the corresponding formal parameter is of a specific tagged type T, these rules imply that formal verification can safely assume that any components of the actual parameter which are not components of T will be neither read nor written by the call.]

6.1.8. Subprogram_Variant Aspects

The aspect Subprogram_Variant may be specified for subprograms; it can be used to ensure termination of recursive subprograms in a way that is similar to how pragma Loop_Variant can be used to ensure termination of loops.

Syntax

subprogram_variant_list ::= subprogram_variant_item {, subprogram_variant_item}
subprogram_variant_item ::= change_direction => discrete_expression
change_direction        ::= Increases | Decreases

where discrete_expression is an expression of a discrete type.

The aspect_definition for a Subprogram_Variant aspect_specification shall be a subprogram_variant_list. The Subprogram_Variant aspect of an inherited subprogram for a derived type is always unspecified.

Two Subprogram_Variant aspects are said to be compatible if the lengths of the two subprogram_variant_item_list are equal and corresponding pairs of the elements of the two lists agree with respect to both change_direction and the type of their respective discrete_expressions. An unspecified Subprogram_Variant aspect is compatible with, and only with, another unspecified Subprogram_Variant aspect (including itself).

Two subprograms are said to be statically mutually recursive, if they are mutually recursive taking into account only direct calls (that is, ignoring dispatching calls and calls through access-to-subprogram values). For example, if subprogram Aa calls Bb (that is, Aa statically contains a direct call to Bb), Bb calls Cc, Cc calls Dd, and Dd calls Aa, then any 2 of those 4 subprograms (e.g., Bb and Dd) are statically mutually recursive. The case of a direct recursive call is just a special case of a statically mutually recursive call; thus, it is possible [and not unusual] for a subprogram to be statically mutually recursive with itself and with no other subprogram.

In some cases (described in more detail below) involving a call where the calling subprogram and the called subprogram have compatible (specified) Subprogram_Variable aspects, a runtime check (or a verification condition corresponding to such a runtime check) may be be introduced to ensure that the “variant of the call progresses”. This means that the values of the caller’s discrete_expressions (which were saved upon entry to the caller, as will be described below) are compared in textual order with those of the callee (which are evaluated only as needed as part of the check) until either a pair of unequal values is encountered or until all pairs have been compared. In either case, a check is performed that the last pair of values to be compared satisfies the following condition: if the change_direction for the associated subprogram_variant_item is Increases (respectively, Decreases) then the expression value obtained for the call is greater (respectively, less) than the value that was saved upon entry to the caller.

Static Semantics

  1. [Aspect Subprogram_Variant can be used to demonstrate that execution of any of a set of statically mutually recursive subprogram(s) will not result in unbounded recursion. This is accomplished by specifying expressions that will increase or decrease at each (mutually) recursive call.]

  2. Subprogram_Variant is an assertion aspect [and may be used in an Assertion_Policy pragma]. Subprogram_Variant is an assertion (as defined in Ada RM 11.4.2(1.1/3)); any Subprogram_Variant runtime checking associated with a call (see below) is governed by the Subprogram_Variant assertion policy that is in effect at the point of the call.

Legality Rules

  1. A Subprogram_Variant aspect may be specified for the same subprograms that a Pre aspect may be specified for. [This implies, for example, that the Subprogram_Variant aspect cannot be specified for an abstract subprogram.]

  2. The expression of a subprogram_variant_item shall be of any discrete type.

  3. The Subprogram_Variant assertion policy in effect at the point of a direct recursive call (i.e., a call where the calling subprogram is the same as the callee) and at the point where the subprogram is declared shall agree.

  4. For purposes of the rules given in this section (including static semantics, dynamic semantics, legality rules, and verification rules), a call to an inherited subprogram associated with a derived type is treated as if the call were replaced with the equivalent call to the corresponding primitive subprogram of the parent or progenitor type described in the “Dynamic Semantics” section of Ada RM 3.4. This notional transformation is applied repeatedly in the case of multiple levels of subprogram inheritance.

Dynamic Semantics

  1. At the beginning of a subprogram with a specified Subprogram_Variant aspect, the discrete_expressions are evaluated in textual order and their values are each saved in a constant that is implicitly declared at the beginning of the subprogram body[, in the same way as for an unconditionally evaluated Old attribute reference (see Ada RM 6.1.1)].

  2. For a direct recursive call (i.e., the calling subprogram is the same as the callee), a check is made that the variant of the call progresses (as described above). If the check fails, Assertion_Error is raised. [No runtime check is performed in the case of a direct call from one subprogram to a different subprogram, even if the two subprograms are statically mutually recursive. No runtime check is performed for a dispatching call or a call through an access-to-subprogram value.] No runtime check is performed if the Subprogram_Variant assertion policy in effect at the point of the call is not Check.

Verification Rules

  1. Statically mutually recursive subprograms shall have compatible variants.

  2. A statically mutually recursive call (that is, a direct call where the caller and the callee are statically mutually recursive) where the Subprogram_Variant aspects of the two subprograms are specified shall not occur with a precondition expression, within a subtype predicate expression, within a type invariant expression, within a Default_Initial_Condition expression, within a discrete_expression of a Subprogram_Variant aspect specification, or as part of the default initialization of a type. Such a call shall also not occur inside the elaboration of a package unless the package is located within a subprogram and not within a declare block.

  3. For a statically mutually recursive call to a subprogram whose Subprogram_Variant aspect is specified, a verification condition is introduced to ensure that the evaluation of the discrete_expressions of the subprogram_variant_list of the callee does not a raise any exception. Additionally, a verification condition is generated to ensure that the variant of the call progresses. This verification condition is already implicitly generated in the case where the caller and the callee are the same (a direct recursive call) as a consequence of the runtime check taking place in that case. It is also generated in the case of other mutually recursive calls, although no checks are introduced at runtime due to compiler implementation constraints.

6.2. Formal Parameter Modes

In flow analysis, particularly information flow analysis, the update of a component of composite object is treated as updating the whole of the composite object with the component set to its new value and the remaining components of the composite object with their value preserved.

This means that if a formal parameter of a subprogram is a composite type and only individual components, but not all, are updated, then the mode of the formal parameter should be in out.

In general, it is not possible to statically determine whether all elements of an array have been updated by a subprogram if individual array elements are updated. The mode of a formal parameter of an array with such updates should be in out.

A formal parameter with a mode of out is treated as not having an entry value (apart from any discriminant or attributes of properties of the formal parameter). Hence, a subprogram cannot read a value of a formal parameter of mode out until the subprogram has updated it.

Verification Rules

  1. A subprogram formal parameter of a composite type which is updated but not fully initialized by the subprogram shall have a mode of in out, unless it has relaxed initialization (see section Relaxed Initialization).

  2. A subprogram formal parameter of mode out shall not be read by the subprogram until it has been updated by the subprogram. The use of a discriminant or an attribute related to a property, not its value, of the formal parameter is not considered to be a read of the formal parameter. [Examples of attributes that may be used are A’First, A’Last and A’Length; examples of attributes that are dependent on the value of the formal parameter and shall not be used are X’Old and X’Loop_Entry.]

6.3. Subprogram Bodies

6.3.1. Conformance Rules

No extensions or restrictions.

6.3.2. Inline Expansion of Subprograms

No extensions or restrictions.

6.4. Subprogram Calls

No extensions or restrictions.

6.4.1. Parameter Associations

No extensions or restrictions.

6.4.2. Anti-Aliasing

An alias is a name which refers to the same object as another name. The presence of aliasing is inconsistent with the underlying flow analysis and proof models used by the tools which assume that different names represent different entities. In general, it is not possible or is difficult to deduce that two names refer to the same object and problems arise when one of the names is used to update the object (although object renaming declarations are not problematic in SPARK).

A common place for aliasing to be introduced is through the actual parameters and between actual parameters and global variables in a procedure call. Extra verification rules are given that avoid the possibility of aliasing through actual parameters and global variables. A function is not allowed to have side-effects and cannot update an actual parameter or global variable. Therefore, function calls cannot introduce aliasing and are excluded from the anti-aliasing rules given below for procedure calls.

Static Semantics

  1. An object is said to be interfering if it is unsynchronized (see section Tasks and Synchronization) or it is synchronized only due to being constant after elaboration (see section Object Declarations).

    Two names that potentially overlap (see section Access Types) and which each denotes an interfering object are said to potentially introduce aliasing via parameter passing. [This definition has the effect of exempting most synchronized objects from the anti-aliasing rules given below; aliasing of most synchronized objects via parameter passing is allowed.]

  1. A formal parameter is said to be immutable in the following cases:

    • it is an anonymous access-to-constant parameter; or

    • it is of mode in and not of an access-to-object type.

    Otherwise, the formal parameter is said to be mutable.

Verification Rules

  1. A procedure call shall not pass two actual parameters which potentially introduce aliasing via parameter passing unless either

    • both of the corresponding formal parameters are immutable; or

    • at least one of the corresponding formal parameters is immutable and is of a by-copy type that is not an access type.

  2. If an actual parameter in a procedure call and a global_item referenced by the called procedure potentially introduce aliasing via parameter passing, then

    • the corresponding formal parameter shall be immutable; and

    • if the global_item’s mode is Output or In_Out, then the corresponding formal parameter shall be of a by-copy type that is not an access type.

  3. Where one of these rules prohibits the occurrence of an object V or any of its subcomponents as an actual parameter, the following constructs are also prohibited in this context:

    • A type conversion whose operand is a prohibited construct;

    • A call to an instance of Unchecked_Conversion whose operand is a prohibited construct;

    • A qualified expression whose operand is a prohibited construct;

    • A prohibited construct enclosed in parentheses.

Examples

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procedure Anti_Aliasing is
   type Rec is record
      X : Integer;
      Y : Integer;
   end record;

   type Arr is array (1 .. 10) of Integer;

   Local_1, Local_2 : Integer := 0;

   Rec_1 : Rec := (0, 0);

   Arr_1 : Arr := (others => 0);

   procedure One_In_One_Out (X : in Integer; Y : in out Integer)
   is
   begin
      Y := X + Y;
   end One_In_One_Out;

   procedure Two_In_Out (X, Y : in out Integer) with Global => null
   is
      Temp : Integer;
   begin
      Temp := Y;
      Y := X + Y;
      X := Temp;
   end Two_In_Out;

   procedure With_In_Global (I : in out Integer)
     with Global => Local_1
   is
   begin
      I := I + Local_1;
   end With_In_Global;

begin
   -- This is ok because parameters are by copy and there
   -- is only one out parameter
   One_In_One_Out (Local_1, Local_1);

   -- This is ok the variables do not overlap even though
   -- they are part of the same record.
   Two_In_Out (Rec_1.X, Rec_1.Y);

   -- This is ok the variables do not overlap they
   -- can statically determined to be distinct elements
   Two_In_Out (Arr_1 (1), Arr_1 (2));

   -- This is not ok because Global and formal in out parameter overlap
   With_In_Global (Local_1);

end Anti_Aliasing;

6.5. Return Statements

No extensions or restrictions.

6.5.1. Nonreturning Procedures

Verification Rules

  1. A call to a nonreturning procedure introduces an obligation to prove that the statement will not be executed, much like the verification condition associated with

    pragma Assert (False);

    [In other words, the verification conditions introduced for a call to a nonreturning procedure are the same as those introduced for a runtime check which fails unconditionally. See also section Exceptions, where a similar verification rule is imposed on raise_statements.]

6.6. Overloading of Operators

Legality Rules

  1. [A user-defined equality operation on a record type shall have a Global aspect of null; see Global Aspects for the statement of this rule.]

Verification Rules

  1. A user-defined equality operation on a record type shall always terminate.

6.7. Null Procedures

No extensions or restrictions.

6.8. Expression Functions

Legality Rules

  1. Contract_Cases, Global and Depends aspects may be applied to an expression function as for any other function declaration if it does not have a separate declaration. If it has a separate declaration then the aspects are applied to that. It may have refined aspects applied (see State Refinement).

6.9. Ghost Entities

Ghost entities are intended for use in discharging verification conditions and in making it easier to express assertions about a program. The essential property of ghost entities is that they have no effect on the dynamic behavior of a valid SPARK program. More specifically, if one were to take a valid SPARK program and remove all ghost entity declarations from it and all “innermost” statements, declarations, and pragmas which refer to those declarations (replacing removed statements with null statements when syntactically required), then the resulting program might no longer be a valid SPARK program (e.g., it might no longer be possible to discharge all of the program’s verification conditions) but its dynamic semantics (when viewed as an Ada program) should be unaffected by this transformation. [This transformation might affect the performance characteristics of the program (e.g., due to no longer evaluating provably true assertions), but that is not what we are talking about here. In rare cases, it might be necessary to make a small additional change after the removals (e.g., adding an Elaborate_Body pragma) in order to avoid producing a library package that no longer needs a body (see Ada RM 7.2(4))].

Static Semantics

  1. SPARK defines the Boolean-valued representation aspect Ghost. Ghost is an aspect of all entities (e.g., subprograms, types, objects). An entity whose Ghost aspect is True is said to be a ghost entity; terms such as “ghost function” or “ghost variable” are defined analogously (e.g., a function whose Ghost aspect is True is said to be a ghost function). In addition, a subcomponent of a ghost object is a ghost object.

    Ghost is an assertion aspect. [This means that Ghost can be named in an Assertion_Policy pragma.]

  2. The Ghost aspect of an entity declared inside of a ghost entity (e.g., within the body of a ghost subprogram) is defined to be True. The Ghost aspect of an entity implicitly declared as part of the explicit declaration of a ghost entity (e.g., an implicitly declared subprogram associated with the declaration of a ghost type) is defined to be True. The Ghost aspect of a child of a ghost library unit is defined to be True.

  3. A statement or pragma is said to be a “ghost statement” if

    • it occurs within a ghost subprogram or package; or

    • it is a call to a ghost procedure; or

    • it is an assignment statement whose target is a ghost variable; or

    • it is a pragma which encloses a name denoting a ghost entity or which specifies an aspect of a ghost entity.

  4. If the Ghost assertion policy in effect at the point of a ghost statement or the declaration of a ghost entity is Ignore, then the elaboration of that construct (at run time) has no effect, other Ada or SPARK rules notwithstanding. Similarly, the elaboration of the completion of a ghost entity has no effect if the Ghost assertion policy in effect at the point of the entity’s initial declaration is Ignore. [A Ghost assertion policy of Ignore can be used to ensure that a compiler generates no code for ghost constructs.] Such a declaration is said to be a disabled ghost declaration; terms such as “disabled ghost type” and “disabled ghost subprogram” are defined analogously.

Legality Rules

  1. The Ghost aspect may only be specified [explicitly] for the declaration of a subprogram, a generic subprogram, a type (including a partial view thereof), an object (or list of objects, in the case of an aspect_specification for an object_declaration having more than one defining_identifier), a package, or a generic package. The Ghost aspect may be specified via either an aspect_specification or via a pragma. The representation pragma Ghost takes a single argument, a name denoting one or more entities whose Ghost aspect is then specified to be True. [In particular, SPARK does not currently include any form of ghost components of non-ghost record types, or ghost parameters of non-ghost subprograms. SPARK does define ghost state abstractions, but these are described elsewhere.]

  2. A Ghost aspect value of False shall not be explicitly specified except in a confirming aspect specification. [For example, a non-ghost declaration cannot occur within a ghost subprogram.]

    The value specified for the Ghost assertion policy in an Assertion_Policy pragma shall be either Check or Ignore. [In other words, implementation-defined assertion policy values are not permitted.] The Ghost assertion policy in effect at any point of a SPARK program shall be either Check or Ignore.

  3. A ghost type or object shall not be effectively volatile. A ghost object shall not be imported or exported. [In other words, no ghost objects for which reading or writing would constitute an external effect (see Ada RM 1.1.3).]

  4. A ghost primitive subprogram of a non-ghost type extension shall not override an inherited non-ghost primitive subprogram. A non-ghost primitive subprogram of a type extension shall not override an inherited ghost primitive subprogram. [A ghost subprogram may be a primitive subprogram of a non-ghost tagged type. A ghost type extension may have a non-ghost parent type or progenitor; primitive subprograms of such a type may override inherited (ghost or non-ghost) subprograms.]

  5. A Ghost pragma which applies to a declaration occuring in the visible part of a package shall not occur in the private part of that package. [This rule is to ensure that the ghostliness of a visible entity can be determined without having to look into the private part of the enclosing package.]

  6. A ghost entity shall only be referenced:

    • from within an assertion expression; or

    • from within an aspect specification [(i.e., either an aspect_specification or an aspect-specifying pragma)]; or

    • within the declaration or completion of a ghost entity (e.g., from within the body of a ghost subprogram); or

    • within a ghost statement; or

    • within a with_clause or use_clause; or

    • within a renaming_declaration which either renames a ghost entity or occurs within a ghost subprogram or package.

  7. A ghost entity shall not be referenced within an aspect specification [(including an aspect-specifying pragma)] which specifies an aspect of a non-ghost entity except in the following cases:

    • the reference occurs within an assertion expression which is not a predicate expression; or

    • the specified aspect is either Global, Depends, Refined_Global, Refined_Depends, Initializes, or Refined_State. [For example, the Global aspect of a non-ghost subprogram might refer to a ghost variable.]

[Predicate expressions are excluded because predicates participate in membership tests; no Assertion_Policy pragma has any effect on this participation. In the case of a Static_Predicate expression, there are also other reasons (e.g., case statements).]

  1. An out or in out mode actual parameter in a call to a ghost subprogram shall be a ghost variable.

  2. If the Ghost assertion policy in effect at the point of the declaration of a ghost entity is Ignore, then the Ghost assertion policy in effect at the point of any reference to that entity shall be Ignore. If the Ghost assertion policy in effect at the point of the declaration of a ghost variable is Check, then the Ghost assertion policy in effect at the point of any assignment to a part of that variable shall be Check. [This includes both assignment statements and passing a ghost variable as an out or in out mode actual parameter.]

  3. An Assertion_Policy pragma specifying a Ghost assertion policy shall not occur within a ghost subprogram or package. If a ghost entity has a completion then the Ghost assertion policies in effect at the declaration and at the completion of the entity shall be the same. [This rule applies to subprograms, packages, types, and deferred constants.]

    The Ghost assertion policies in effect at the point of the declaration of an entity and at the point of an aspect specification which applies to that entity shall be the same.

  4. The Ghost assertion policies in effect at the declaration of a state abstraction and at the declaration of each constituent of that abstraction shall be the same.

  5. The Ghost assertion policies in effect at the declaration of a primitive subprogram of a ghost tagged type and at the declaration of the ghost tagged type shall be the same.

  6. If a tagged type is not a disabled ghost type, and if a primitive operation of the tagged type overrides an inherited operation, then the corresponding operation of the ancestor type shall be a disabled ghost subprogram if and only if the overriding subprogram is a disabled ghost subprogram.

  7. If the Ghost assertion policy in effect at the point of an a reference to a Ghost entity which occurs within an assertion expression is Ignore, then the assertion policy which governs the assertion expression (e.g., Pre for a precondition expression, Assert for the argument of an Assert pragma) shall [also] be Ignore.

  8. A ghost type shall not have a task or protected part. A ghost object shall not be of a type which yields synchronized objects (see section Tasks and Synchronization). A ghost object shall not have a volatile part. A synchronized state abstraction shall not be a ghost state abstraction (see Abstract_State Aspects).

Verification Rules

  1. A ghost procedure shall not have a non-ghost [global] output.

  2. An output of a non-ghost subprogram other than a state abstraction or a ghost global shall not depend on a ghost input. [It is intended that this follows as a consequence of other rules. Although a non-ghost state abstraction output which depends on a ghost input may have a non-ghost constituent, other rules prevent such a non-ghost constituent from depending on the ghost input.]

  3. A global input of a ghost procedure shall not be effectively volatile for reading. [This rule says, in effect, that ghost procedures are subject to the same restrictions as non-ghost nonvolatile functions with respect to reading volatile objects.] A name occurring within a ghost statement shall not denote an object that is effectively volatile for reading. [In other words, a ghost statement is subject to effectively the same restrictions as a ghost procedure.]

  4. If the Ghost assertion policy in effect at the point of the declaration of a ghost variable or ghost state abstraction is Check, then the Ghost assertion policy in effect at the point of any call to a procedure for which that variable or state abstraction is a global output shall be Check.

Examples

function A_Ghost_Expr_Function (Lo, Hi : Natural) return Natural is
  (if Lo > Integer'Last - Hi then Lo else ((Lo + Hi) / 2))
  with Pre        => Lo <= Hi,
       Post       => A_Ghost_Expr_Function'Result in Lo .. Hi,
       Ghost;

function A_Ghost_Function (Lo, Hi : Natural) return Natural
  with Pre        => Lo <= Hi,
       Post       => A_Ghost_Function'Result in Lo .. Hi,
       Ghost;
-- The body of the function is declared elsewhere.

function A_Nonexecutable_Ghost_Function (Lo, Hi : Natural) return Natural
  with Pre        => Lo <= Hi,
       Post       => A_Nonexecutable_Ghost_Function'Result in Lo .. Hi,
       Ghost,
       Import;
-- The body of the function is not declared elsewhere.

6.10. Relaxed Initialization

SPARK defines the Boolean-valued aspect Relaxed_Initialization and the related Boolean-valued attribute, Initialized.

Without the Relaxed_Initialization aspect, the rules that statically prevent reading an uninitialized scalar object are defined with “whole object” granularity. For example, all inputs of a subprogram are required to be fully initialized at the point of a call to the subprogram and all outputs of a subprogram are required to be fully initialized at the point of a return from the subprogram. The Relaxed_Initialization aspect, together with the Initialized attribute, provides a mechanism for safely (i.e., without introducing the possibility of improperly reading an uninitialized scalar) referencing partially initialized Inputs and Outputs.

The Relaxed_Initialization aspect may be specified for a type, for a standalone object, for a state abstraction, or (at least in effect - see below for details) for a parameter or function result of a subprogram or entry. The prefix of an Initialized attribute reference shall denote an object.

Static Semantics

  1. An object is said to have relaxed initialization if and only if

    • its Relaxed_Initialization aspect is True; or

    • the Relaxed_Initialization aspect of its type is True; or

    • it is a subcomponent of an object that has relaxed initialization; or

    • it is the return object of a function call and the Relaxed_Initialization aspect of the function’s result is True; or

    • it is the return object of a call to a predefined concatenation operator and at least one of the operands is a name denoting an object having relaxed initialization; or

    • it is the result object of an aggregate having a least one component whose value is that of an object that has relaxed initialization; or

    • it is the result of evaluating a value conversion whose operand has relaxed initialization; or

    • it is the associated object of an expression (e.g., a view conversion, a qualified expression, or a conditional expression) which has at least one operative constituent (see Ada RM 4.4) which is not the expression itself and whose associated object has relaxed initialization.

    A state abstraction or a type has relaxed initialization if its Relaxed_Initialization aspect is True. An expression has relaxed initialization if its evaluation yields an object that has relaxed initialization.

  2. A Relaxed_Initialization aspect specification for a formal parameter of a callable entity or for a function’s result is expressed syntactically as an aspect_specification of the declaration of the enclosing callable entity. [This is expressed this way because Ada does not provide syntax for specifying aspects for subprogram/entry parameters, or for the result of a function.] In the following example, the parameter X1 and the result of F are specified as having relaxed initialization; the parameters X2 and X3 are not:

    function F (X1 : T1; X2 : T2; X3 : T3) return T4
      with Relaxed_Initialization => (X1 => True, F'Result);
    

More precisely, the Relaxed_Initialization aspect for a subprogram or entry (or a generic subprogram) is specified by an aspect_specification where the aspect_mark is Relaxed_Initialization and the aspect_definition follows the following grammar for profile_aspect_spec:

profile_aspect_spec ::= ( profile_spec_item {, profile_spec_item} )
profile_spec_item   ::= parameter_name [=> aspect_definition]
                      | function_name'Result [=> aspect_definition]
  1. Relaxed_Initialization aspect specifications are inherited by a derived type (if the aspect is specified for the ancestor type) and by an inherited subprogram (if the aspect is specified for the corresponding primitive subprogram of the ancestor type).

  2. For a prefix X that denotes an object, the following attribute is defined:

    X'Initialized
    

    X’Initialized is True if and only if every scalar reachable element of X has been initialized. [It typicallly follows as a consequence of this definition and the other rules of SPARK that if X’Initialized is True, then for every reachable element Y of X (scalar or not), Y belongs to its subtype. There are pathological counterexamples, such as a componentless record declared with “Dynamic_Predicate => False”.] An Initialized attribute reference is never a static expression.

Legality Rules

  1. The following rules apply to the profile_aspect_spec of a Relaxed_Initialization aspect specification for a subprogram, a generic subprogram, or an entry.

    • Each parameter_name shall name a parameter of the given callable entity and no parameter shall be named more than once. It is not required that every parameter be named.

    • Each aspect_definition within a profile_aspect_spec shall be as for a Boolean aspect.

    • The form of profile_spec_item that includes a Result attribute reference shall only be provided if the given callable entity is a function or generic function; in that case, the prefix of the attribute reference shall denote that function or generic function. Such a Result attribute reference is allowed, other language restrictions on the use of Result attribute references notwithstanding (i.e., despite the fact that such a Result attribute reference does not occur within a postcondition expression).

    • A parameter or function result named in the aspect_specification shall not be of an elementary type. [It is a bounded error to pass an uninitialized scalar parameter as input for an input parameter or as output for an output parameter or function result, so there is no benefit of marking such a parameter or result as having relaxed initialization. An object of access type is always initialized.]

    • A Boolean value of True is implicitly specified if no aspect_definition is provided, as per Ada RM 13.1.1’s rules for Boolean-valued aspects. A Boolean value of False is implicitly specified if a given parameter (or, in the case of a function or generic function, the result) is not mentioned in any profile_spec_item.

  2. A constituent of a state abstraction shall have relaxed initialization if and only if the state abstraction has relaxed initialization.

  3. No part of a tagged type, or of a tagged object, shall have relaxed initialization.

  4. No part of an effectively volatile type, or of an effectively volatile object, shall have relaxed initialization.

  5. No part of an Unchecked_Union type shall have relaxed initialization. No part of the type of the prefix of an Initialized attribute reference shall be of an Unchecked_Union type.

  6. A Relaxed_Initialization aspect specification which applies to a declaration occuring in the visible part of a package [(e.g., the declaration of a private type or of a deferred constant)] shall not occur in the private part of that package.

  7. A formal parameter of a dispatching operation shall not have relaxed initialization; the result of a dispatching function shall not have relaxed initialization.

Verification Rules

  1. At the point of a read of a scalar object X that has relaxed initialization, a verification condition is introduced to ensure that X is initialized. This includes the case where X is a subcomponent of a composite object that is passed as an argument in a call to a predefined relational operator (e.g., “=” or “<”). Such a verification condition is also introduced in the case where X is a reachable element of the [source] expression of an assignment operation and the target of the assignment does not have relaxed initialization, where X is a reachable element of an actual parameter in a call where the corresponding formal parameter is of mode in or in out and does not have relaxed initialization, upon a call whose precondition implies X’Initialized, and upon return from a call whose postcondition implies X’Initialized.

    [For updates to X that do not involve calls, this check that X is initialized is implemented via flow analysis and no additional annotations are required. Preconditions and postconditions that mention X’Initialized may also be used to communicate information about the initialization status of X across subprogram boundaries.

    These rules statically prevent any of the bounded-error or erroneous execution scenarios associated with reading an uninitialized scalar object described in Ada RM 13.9.1. It may provide useful intuition to think of a subprogram as having (roughly speaking) an implicit precondition of X’Initialized for each of its inputs X that does not have relaxed initialization and an implicit postcondition of Y’Initialized for each of its outputs Y that does not have relaxed initialization; this imprecise description ignores things like volatile objects and state abstractions. For a particular call, this notional precondition is also in effect for a given formal parameter if the corresponding actual parameter does not have relaxed initialization (even if the formal parameter does).

    The verification conditions described here are not needed if X does not have relaxed initialization because the more conservative whole-object-granularity rules that govern that case will ensure that X is initialized whenever it is read.]

  2. For any object X, evaluation of X’Initialized includes the evaluation of Y’Initialized for every scalar reachable element Y of X (excluding “hidden” components of tagged objects - see Type Invariants). Evaluation of X’Initialized for a scalar object X is considered to be a read of X if and only if X does not have relaxed initialization. If X has relaxed initialization, then an evaluation of X’Initialized is instead treated like an evaluation of X’Valid [, which is not a read of X]. If X does not have relaxed initialization, then this implies that evaluation of X’Initialized introduces the same initialization requirements as would be introduced for any other read of X; as a result of meeting these requirements, X’Initialized will always return True for such an object.