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spark2014/docs/lrm/source/subprograms.rst
Piotr Trojanek 4fee9a0d12 Reject references to attribute Result in Exceptional_Cases 
Functions with aspect Side_Effects should not reference attribute Result in
consequences of their aspect Exceptional_Cases.

[changelog]
* docs/lrm/source/subprograms.rst (Exceptional_Cases): Reject references to
attribute Result.
2024-12-18 15:50:35 +00:00

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Subprograms
===========
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.
.. index:: subprogram with side effects
A *subprogram with side effects* is either a procedure, a protected entry, or a
function with side effects (see :ref:`Functions With Side Effects`). A
subprogram with side effects may have output parameters, write global
variables, raise exceptions and not terminate.
Rules are imposed in |SPARK| to ensure that the execution of a function call
does not modify any variables declared outside of the function, unless it is a
function with side effects. Outside of this special case, it follows as a
consequence of these rules that the evaluation of any |SPARK| expression is
side-effect free.
.. index:: global item
We also introduce the notion of a *global item*, which is a name that denotes a
global object or a state abstraction (see :ref:`Abstract_State Aspects`). Global items
are presented in Global aspects (see :ref:`Global Aspects`).
.. index:: entire object
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*.
.. container:: heading
Static Semantics
1. The *exit* value of a global item or parameter of a subprogram is its
value immediately following the 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.
.. index:: subprogram output
3. An *output* of a subprogram is a global item or parameter whose final value,
or the final value of any of its reachable parts (see :ref:`Access Types`),
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
:ref:`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 a
statement raising an unexpected exception or to a
``pragma Assert (statically_False)``.]
.. index:: subprogram input
4. An *input* of a subprogram is a global item or parameter whose
initial value (or that of any of its reachable parts -
see :ref:`Access Types`) 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 :ref:`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 :ref:`Depends Aspects`).
5. 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 :ref:`Extensions_Visible Aspects`).
[A state abstraction cannot be fully initialized by initializing
individual constituents unless its refinement is visible.]
.. container:: heading
Legality Rules
6. The declaration of a function without side effects 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 without side effects
for which no explicit declaration is given. A function without side effects
shall have no outputs other than its result.
7. 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 a subprogram with side effects.
.. container:: heading
Verification Rules
8. At the point of a call, all inputs of the callee except for those that have
relaxed initialization (see :ref:`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.
9. If a call propagates an exception, all global outputs of the callee and all
output parameters which either have a *by reference* type or are marked as
aliased shall be fully initialized when the exception is propagated
except for those that have relaxed initialization.
10. A function without side effects shall always return normally.
11. A call to a ghost procedure occurring outside of a ghost context shall
always return normally.
.. index:: precondition, postcondition
Preconditions and Postconditions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. container:: heading
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.]
2. 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.]
3. A subprogram_renaming_declaration shall not declare a primitive
operation of a tagged type.
[Consider
.. code-block:: ada
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.]
.. container:: heading
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.
.. index:: verification condition; on dispatching operation
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.]
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.
.. container:: heading
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 and Depends (but not Contract_Cases) aspects may be specified for
an abstract subprogram.
3. The Global, Depends and Contract_Cases aspects shall not be specified for a
null procedure.
See section :ref:`Contract Cases` for further detail on Contract_Case aspects, section
:ref:`Global Aspects` for further detail on Global aspects and section :ref:`Depends Aspects`
for further detail on Depends aspects.
.. index:: Contract_Cases
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:
.. code-block:: ada
procedure P (...)
with Pre => General_Precondition,
Post => General_Postcondition,
Contract_Cases => (A1 => B1,
A2 => B2,
...
An => Bn);
is short hand for
.. code-block:: ada
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.
.. container:: heading
Syntax
::
contract_case_list ::= (contract_case {, contract_case})
contract_case ::= condition => consequence
| others => consequence
where
``consequence ::=`` *Boolean_*\ ``expression``
.. container:: heading
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``.
.. container:: heading
Static Semantics
3. 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.
.. container:: heading
Dynamic Semantics
4. 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.
5. 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].
.. container:: heading
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.
.. container:: heading
Examples
.. code-block:: ada
-- 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);
.. index:: Global
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 :ref:`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.]
.. container:: heading
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
.. container:: heading
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.
.. container:: heading
Name Resolution Rules
5. 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].
.. container:: heading
Legality Rules
6. 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.
7. A ``global_item`` occurring in a Global aspect specification of a subprogram
shall not denote a formal parameter of the subprogram.
8. 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.]
9. Each ``mode_selector`` shall occur at most once in a single
Global aspect.
10. A function without side effects shall not have a ``mode_selector`` of
Output or In_Out in its Global aspect.
11. A user-defined primitive equality operation on a record type shall have a
Global aspect of ``null``, unless the record type has only limited views
(see :ref:`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.]
12. The ``global_items`` in a single Global aspect specification shall denote
distinct entities. Additionally, if a ``global_item`` is a state
abstraction, none of its constituents shall appear as a ``global_item`` in
the same Global aspect specification.
13. 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.
14. A ``global_item`` occurring with mode Input in the Global aspect
specification of a function annotated with Pure_Function aspect or pragma
shall denote a constant object whose type is not an owning type (see
:ref:`Access Types`).
[This restriction ensures that two calls to the function with the same
parameters return the same value, so that the compiler can safely apply
corresponding optimizations.]
.. container:: heading
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.
.. container:: heading
Verification Rules
15. 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``.
16. 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.
17. Where the refinement of a state abstraction is not visible (see
:ref:`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.]
18. 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 :ref:`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 the output are ignored. Reads of array bounds, discriminants, or
tag of any reachable part of the output are not considered either if they are
constrained by their subtype. Similarly, for purposes of
determining whether an entity is fully initialized as a result of any
successful execution of the call, the mutable discriminants of the output
itself are not 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.]
19. 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.
.. index:: constant with variable inputs; in Global
20. 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**, a constant of (named or anonymous)
access-to-variable type, 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,
.. literalinclude:: ../../../testsuite/gnatprove/tests/RM_Examples/global_and_generics.ads
:language: ada
:linenos:
.. literalinclude:: ../../../testsuite/gnatprove/tests/RM_Examples/global_and_generics.adb
:language: ada
:linenos:
21. The ``mode_selector`` of a ``global_item`` denoting a *constant with
variable inputs* shall be ``Input`` or ``Proof_In``.
.. index:: Constant_After_Elaboration; in Global
22. 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].
.. container:: heading
Examples
.. code-block:: ada
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
.. index:: Depends
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
:ref:`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 :ref:`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 :ref:`Refined_Depends Aspects` for further
details.
See section :ref:`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.
.. container:: heading
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``.
.. container:: heading
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.]
.. container:: heading
Legality Rules
2. 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.
3. 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].
4. 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.
5. 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, or a tagged type (with one exception). 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).
6. 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 subprogram with side effects) the set of formal
parameters of the subprogram of mode **in** of an access-to-variable type.
7. The entity denoted by each ``input`` of a ``dependency_relation`` of a
subprogram shall be a member of the input set of the subprogram.
8. 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.
9. The entity denoted by each ``output`` of a ``dependency_relation`` of a
subprogram shall be a member of the output set of the subprogram.
10. Every member of the output set of a subprogram shall be denoted by exactly
one ``output`` in the ``dependency_relation`` of the subprogram.
11. 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.
12. 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``.
13. 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``.
14. The ``inputs`` in a single ``input_list`` shall denote distinct entities.
15. A ``null_dependency_clause`` shall not have an ``input_list`` of **null**.
.. container:: heading
Static Semantics
16. 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)).]
17. 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)).]
18. 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).
19. 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.
20. 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).]
21. A function without side effects 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.]
22. A subprogram with side effects 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.]
.. container:: heading
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.
.. container:: heading
Verification Rules
23. 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``.
24. Each output of the implementation of the subprogram body is denoted by
an ``output`` in the Depends aspect of the subprogram.
25. Each input of the implementation of a subprogram body is denoted by an
``input`` of the Depends aspect of the subprogram.
26. 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.]
.. container:: heading
Examples
.. code-block:: ada
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.
Global and Depends Aspects of Dispatching Subprograms
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Additional rules apply to the Global and Depends aspects on a dispatching
subprogram, in order to ensure that the effects of dynamically calling an
overriding subprogram are properly captured by the aspects of the statically
denoted callee.
.. container:: heading
Static Semantics
1. A Global 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 thereof; 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.
2. A Depends 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 :ref:`Refined_Depends Aspects`.
.. container:: heading
Legality Rules
3. The Global aspect of an overriding subprogram shall be a valid overriding
of the Global aspect(s) of the overridden inherited subprogram(s).
4. The Depends aspect of an overriding subprogram shall be a valid
overriding of the Depends aspect(s) of the overridden inherited
subprogram(s).
.. index:: Extensions_Visible
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.
.. container:: heading
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.
.. container:: heading
Legality Rules
2. 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).]
3. 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.]
4. 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).]
5. 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.]
6. [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.]
.. container:: heading
Verification Rules
7. [|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 :ref:`Subprogram Declarations`
and then :ref:`Relaxed Initialization`].
8. 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.]
9. [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.]
10. [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.]
.. index:: Subprogram_Variant
termination; of subprogram
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.
.. container:: heading
Syntax
::
subprogram_variant_list ::= structural_subprogram_variant_item | numeric_subprogram_variant_items
numeric_subprogram_variant_items ::= numeric_subprogram_variant_item {, numeric_subprogram_variant_item}
numeric_subprogram_variant_item ::= change_direction => expression
structural_subprogram_variant_item ::= Structural => expression
change_direction ::= Increases | Decreases
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 either both are
structural subprogram variants or both are numeric subprogram variants, the
lengths of the two ``numeric_subprogram_variant_items`` 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 ``expressions``. An
unspecified Subprogram_Variant aspect is compatible with, and only with, another
unspecified Subprogram_Variant aspect (including itself).
.. index:: recursive subprograms
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_Variant 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". For numeric subprogram variants, this
means that the values of the caller's ``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.
.. container:: heading
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.
.. container:: heading
Legality Rules
3. 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.]
4. The expression of a ``numeric_subprogram_variant_item`` shall be either
of a discrete type,
of a subtype of ``Ada.Numerics.Big_Numbers.Big_Integers.Big_Integer``,
of a subtype of ``Ada.Numerics.Big_Numbers.Big_Integers_Ghost.Big_Integer``
or
of a subtype of ``SPARK.Big_Numbers.Big_Integer``.
In the second and third cases the associated ``change_direction`` shall be
Decreases.
5. The expression of a ``structural_subprogram_variant_item`` shall denote a
formal parameter of the subprogram.
6. 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.
7. 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.
.. container:: heading
Dynamic Semantics
8. At the beginning of a subprogram with a specified numeric Subprogram_Variant
aspect,
the ``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)].
For every expression whose type is a subtype of
``Ada.Numerics.Big_Numbers.Big_Integers.Big_Integer``, a check is
performed that it is non-negative.
9. For a direct recursive call (i.e., the calling subprogram is the same
as the callee), if the subprogram variant is numeric,
for every expression in the variant of the call whose type
is a subtype of ``Ada.Numerics.Big_Numbers.Big_Integers.Big_Integer``, a
check is performed that it is non-negative. Then, 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.
.. container:: heading
Verification Rules
10. Statically mutually recursive subprograms shall have compatible variants.
11. 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.
12. For a statically mutually recursive call to a subprogram whose numeric
Subprogram_Variant aspect is specified, a verification condition
is introduced to ensure that the evaluation of the
``expressions`` of the ``subprogram_variant_list`` of the
callee does not a raise any exception.
Then, for every expression in the variant of the called subprogram whose
type is a subtype of ``Ada.Numerics.Big_Numbers.Big_Integers.Big_Integer``,
a check is performed that it is non-negative.
Finally, 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.
13. For a statically mutually recursive call to a subprogram whose structural
Subprogram_Variant aspect is specified, a verification condition is
generated to ensure that the actual parameter corresponding to the
formal parameter denoted by the ``expression`` is a path rooted either
at the formal parameter of the enclosing subprogram denoted by the
expression of its Subprogram_Variant aspect or at a local object
of an anonymous access-to-object type ultimately borrowing or observing a
part of this formal parameter, that this path corresponds to a strict
subcomponent of the structure denoted by the formal parameter of the
enclosing subprogram, and that no deep parts of this structure are
updated before the call. [This ensures that the rule is sufficient to
prove recursion termination on acyclic data structures.]
Exceptional Cases
~~~~~~~~~~~~~~~~~
The aspect Exceptional_Cases may be specified for procedures and functions with
side effects; it can be used to list exceptions that might be propagated by the
subprogram with side effects in the context of its precondition, and associate
them with a specific postcondition. The Exceptional_Cases aspect is specified
with an aspect_specification where the aspect_mark is Exceptional_Cases and the
aspect_definition must follow the grammar of exceptional_case_list given below.
.. container:: heading
Syntax
::
exceptional_case_list ::= ( exceptional_case {, exceptional_case })
exceptional_case ::= exception_choice {'|' exception_choice} => consequence
where ``consequence`` is a boolean expression.
.. container:: heading
Name Resolution Rules
The boolean expression in the consequences should be resolved as regular
postconditions. In particular, references to the Old attribute are allowed to
occur in them.
.. container:: heading
Static Semantics
All prefixes of references to the Old attribute in exceptional cases are
expected to be evaluated at the beginning of the call regardless of whether or
not the particular exception is raised. This allows to introduce constants for
these prefixes at the beginning of the subprogram together with the ones
introduced for the regular postcondition.
.. container:: heading
Dynamic Semantics
Exceptional_Cases aspects are ignored for execution.
.. container:: heading
Legality Rules
1. Parameters of modes *out* or *in out* of the subprogram which are neither
of a *by-reference* type nor marked as aliased shall only occur in
the consequences of an exceptional case:
* directly or indirectly in the prefix of a reference to the Old
attribute, or
* directly as a prefix of the Constrained, First, Last, Length, or Range
attributes.
References to attribute Result shall not occur in the consequences of an
exceptional case.
.. container:: heading
Verification Rules
2. If an exception raised in a subprogram annotated with Exceptional_Cases
is not handled and causes the subprogram body to complete, then a verification
condition is introduced to make sure that the consequence associated to
the exceptional case covering the exception evaluates to True. [Because of
the verification conditions introduced when raising unexpected exceptions,
there is always an exceptional case covering the propagated exception.]
Always_Terminates Aspects
~~~~~~~~~~~~~~~~~~~~~~~~~
The aspect Always_Terminates may be specified for subprograms with
side effects; it can be used to provide a condition under which the subprogram
shall necessarily complete (either return normally or raise an exception). This
aspect may also be specified on packages to provide a default for all
subprograms with side effects declared in the package or in one of its nested
packages. The Always_Terminates aspect is specified with an
aspect_specification where the aspect_mark is Always_Terminates and the
optional aspect_definition is a boolean expression. An Always_Terminates aspect
with no aspect_definition is equivalent to an Always_Terminates aspect with an
aspect_definition of True. [An execution which does not complete can for
example run forever, exit the whole program using GNAT.OS_Lib.OS_Exit, or
transfer the control to another execution in a non-standard way.]
.. container:: heading
Name Resolution Rules
The boolean expression in the aspect_definition should be resolved as a
precondition.
.. container:: heading
Static Semantics
1. If the aspect Always_Terminates is specified for a package, it shall not have
an aspect definition.
2. If the aspect Always_Terminates for a package specification or a subprogram
with side effects P is not otherwise specified and P is declared directly
inside a package (explicitly or implicitly) annotated with an aspect
Always_Terminates then an Always_Terminates aspect of True is implicitly
specified for P.
.. container:: heading
Dynamic Semantics
Always_Terminates aspects are ignored for execution.
.. container:: heading
Legality Rules
3. The Always_Terminates aspect may only be specified for the initial
declaration of a subprogram with side effects (which may be a declaration, a
body or a body stub), or of a package specification.
.. container:: heading
Verification Rules
4. A verification condition is introduced on loops and calls occuring inside
functions without side effects or package elaborations to make sure that
they necessarily complete.
5. A verification condition is introduced on loops and calls occuring inside
subprograms with side effects annotated with an Always_Terminates aspect to
make sure that they necessarily complete in cases where the boolean
condition mentioned in the Always_Terminates aspect evaluates to True on
entry of the subprogram with side effects.
Functions With Side Effects
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The aspect Side_Effects may be specified for functions; it can be used to
indicate that a function should be handled like a procedure with respect to
parameter modes, Global contract, exceptional contract and termination: it may
have output parameters, write global variables, raise exceptions and not
terminate. Such a function is called a *function with side effects*.
Note that a function with side effects is also a volatile function (see section
:ref:`External State`).
.. container:: heading
Static Semantics
1. Side_Effects is a Boolean-valued aspect which may be specified for a
noninstance function or a generic function. If directly specified, the
aspect_definition shall be a static [Boolean] expression. The aspect is
inherited by an inherited primitive function. If the aspect is neither
inherited nor directly specified for a function, then the aspect is False.
.. container:: heading
Legality Rules
2. [Redundant: The declaration of a function with side effects may have a
``parameter_specification`` with a mode of **out** or **in out**. This rule
also applies to a ``subprogram_body`` for a function with side effects for
which no explicit declaration is given.]
3. [Redundant: A function with side effects may have a ``mode_selector`` of
Output or In_Out in its Global aspect.]
4. A call to a function with side effects may only occur as the [right-hand
side] expression of an assignment statement. [Redundant: In particular,
functions with side effects cannot be called inside assertions.]
5. A function with side effects shall not have a Pure_Function aspect or
pragma.
6. A function with side effects shall not be an expression function.
7. A function with side effects shall not be a traversal function (see section
:ref:`Access Types`).
8. A user-defined primitive equality operation on a record type shall not be a
function with side effects, unless the record type has only limited views
(see :ref:`Overloading of Operators`).
[This avoids the case where such a record type is a component of another
composite type, whose predefined equality operation now has side effects
through the primitive equality operation on its component.]
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**.
.. todo: Consider providing a way of proving that all elements of
an array have been updated individually and providing a means to
specify that a composite object is updated but not read by a
subprogram.
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.
.. container:: heading
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 :ref:`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.]
Subprogram Bodies
-----------------
Conformance Rules
~~~~~~~~~~~~~~~~~
No extensions or restrictions.
Inline Expansion of Subprograms
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
No extensions or restrictions.
Subprogram Calls
----------------
No extensions or restrictions.
Parameter Associations
~~~~~~~~~~~~~~~~~~~~~~
No extensions or restrictions.
.. index:: aliasing
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 call to a subprogram
with side effects. Extra verification rules are given that avoid the
possibility of problematic aliasing through actual parameters and global
variables. Except for functions with side effects (see :ref:`Functions With
Side Effects`), a function is not allowed to have side effects and cannot
update an actual parameter or global variable. Therefore, such function calls
cannot introduce problematic aliasing and are excluded from the anti-aliasing
rules given below for calls to subprograms with side effects.
.. container:: heading
Static Semantics
.. index:: interfering object
1. An object is said to be *interfering* if it is unsynchronized (see section
:ref:`Tasks and Synchronization`) or it is synchronized only due to being
*constant after elaboration* (see section :ref:`Object Declarations`).
Two names that potentially overlap (see section :ref:`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.]
.. index:: immutable parameter
2. A formal parameter is said to be *immutable* if it is of mode **in** and
neither of an access-to-variable type nor of an anonymous access-to-constant
type. [Note that access parameters are of mode **in** too.]
Otherwise, the formal parameter is said to be *mutable*.
.. container:: heading
Verification Rules
3. A call to a subprogram with side effects shall only pass two actual
parameters which potentially introduce aliasing via parameter passing when
either
* both of the corresponding formal parameters are either
* immutable; or
* of mode **in** and of an anonymous access-to-constant type; or
* at least one of the corresponding formal parameters is immutable and is of
a by-copy type. [Note that this includes parameters of named
access-to-constant and (named or anonymous) access-to-subprograms
types. Ownership rules prevent other problematic aliasing, see section
:ref:`Access Types`.]
4. If an actual parameter in a call to a subprogram with side effects and a
``global_item`` referenced by the called subprogram potentially introduce
aliasing via parameter passing, then
* the corresponding formal parameter shall be either
* immutable; or
* of mode **in** and of an anonymous access-to-constant type; and
* if the ``global_item``'s mode is Output or In_Out, then the
corresponding formal parameter shall be immutable and of
a by-copy type.
5. A call to a function with side effects shall only pass an actual parameter
which potentially introduces aliasing via parameter passing with an object
referenced from the [left-hand side] name of the enclosing assignment
statement, when the corresponding formal parameter is either
* immutable; or
* of mode **in** and of an anonymous access-to-constant type.
[The rationale for this rule is that, otherwise, the result of the
evaluation of the assignment statement would depend on the order of
evaluation chosen by the compiler, as the object assigned to might depend on
this choice.]
6. A call to a function with side effects shall not reference a ``global_item``
of mode Output or In_Out which potentially introduces aliasing via parameter
passing with an object referenced from the [left-hand side] name of the
enclosing assignment statement.
[The rationale for this rule is the same as for the previous rule.]
7. A call to a function with side effects shall not reference the symbol ``@``
to refer to the target name of the assignment.
[The rationale for this rule is the same as for the previous rule.]
8. 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.
.. container:: heading
Examples
.. literalinclude:: ../../../testsuite/gnatprove/tests/RM_Examples/anti_aliasing.adb
:language: ada
:linenos:
Exception Propagation
~~~~~~~~~~~~~~~~~~~~~
.. container:: heading
Verification Rules
1. A call to a procedure annotated with an aspect Exceptional_Cases
(see :ref:`Exceptional Cases`) introduces
an obligation to prove that potentially raised exceptions are expected as
defined in :ref:`Raise Statements and Raise Expressions`.
Return Statements
-----------------
No extensions or restrictions.
Overloading of Operators
------------------------
.. container:: heading
Legality Rules
1. [A user-defined primitive equality operation on a record type shall have a
Global aspect of ``null``, unless the record type has only limited views;
see :ref:`Global Aspects` for the statement of this rule.]
2. [A user-defined primitive equality operation on a record type shall not be a
volatile function, unless the record type has only limited views; see
:ref:`External State - Variables and Types` for the statement of this rule.]
3. [A user-defined primitive equality operation on a record type shall not be a
function with side effects, unless the record type has only limited views;
see :ref:`Functions With Side Effects` for the statement of this rule.]
4. [A user-defined primitive equality operation on a non-ghost record type
shall not be ghost, unless the record type has only limited views; see
:ref:`Ghost Entities` for the statement of this rule.]
Null Procedures
---------------
No extensions or restrictions.
.. index:: expression function
Expression Functions
--------------------
.. container:: heading
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 :ref:`State Refinement`).
.. index:: ghost code
see: Ghost; ghost code
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 (considering the
association of a ghost formal parameter in a generic instantiation as a
declaration) 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))].
.. container:: heading
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 specifies an aspect of a ghost entity; or
* it is an assertion pragma which encloses a name denoting 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.
.. container:: heading
Legality Rules
5. 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, a generic package, or a generic formal parameter.
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.]
6. 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.
7. 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).]
8. 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.]
9. 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.]
10. 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; or
* within an actual parameter in a generic instantiation when the
corresponding generic formal parameter is ghost.
A ghost attribute like ``Initialized`` shall only be referenced where a
ghost entity would be allowed.
11. 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, unless the predicate is introduced
by aspect Ghost_Predicate; 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).]
12. An **out** or **in out** mode actual parameter in a call to a ghost
subprogram shall be a ghost variable.
13. In a generic declaration:
* the default expression (if any) for a ghost generic formal object [both
of mode **in** and] of access-to-variable type shall be a ghost object
[otherwise writing to a reachable part (see :ref:`Access Types`) of the
ghost formal object would have an effect on a non-ghost variable]; and
* the default subprogram (if any) for a ghost generic formal procedure
shall be a ghost procedure [otherwise a call to the ghost formal
procedure could have effects on non-ghost variables, if the default
non-ghost procedure is writing to non-ghost variables].
14. In a generic instantiation:
* the actual parameter for a ghost generic formal object of mode **in out**
or both of mode **in** and of access-to-variable type, shall be a ghost
object [otherwise writing to a reachable part (see :ref:`Access Types`)
of the ghost formal object would have an effect on a non-ghost variable];
* the actual parameter for a ghost generic formal procedure shall be a
ghost procedure [otherwise a call to the ghost formal procedure could
have effects on non-ghost variables, if the actual non-ghost procedure is
writing to non-ghost variables]; and
* the actual parameter for a ghost generic formal package shall be a ghost
package [otherwise an object or a procedure in the package could lead to
the problems mentions in the two previous cases].
15. 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 outside of
an assertion expression 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.]
16. 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.
17. 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.
18. 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.
19. 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.
20. If the Ghost assertion policy in effect at the point of the declaration
of a ghost entity is Ignore,
and this ghost entity occurs within an assertion expression,
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.
21. 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 :ref:`Tasks and Synchronization`).
A ghost object shall not have a volatile part.
A synchronized state abstraction shall not be a ghost state abstraction
(see :ref:`Abstract_State Aspects`).
22. A user-defined primitive equality operation on a non-ghost record type
shall not be ghost, unless the record type has only limited views (see
:ref:`Overloading of Operators`).
[This avoids the case where such a record type is a component of another
non-ghost composite type, whose predefined non-ghost equality operation now
calls a ghost function through the primitive equality operation on its
component.]
.. container:: heading
Verification Rules
23. A ghost subprogram with side effects shall not have a non-ghost [global]
output.
24. 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.]
25. A global input of a ghost subprogram with side effects shall not be effectively
volatile for reading. [This rule says, in effect, that ghost procedural
subprograms 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 subprogram with side effects.]
26. 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 procedural
subprogram for which that variable or state abstraction is a global output
shall be Check.
.. container:: heading
Examples
.. code-block:: ada
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.
.. index:: Relaxed_Initialization
Initialized
initialization; by proof
Relaxed Initialization
----------------------
|SPARK| defines the Boolean-valued aspect Relaxed_Initialization and the related
Boolean-valued ghost 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, 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.
.. container:: heading
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 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:
.. code-block:: ada
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]
3. 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).
4. For a prefix *X* that denotes an object which has relaxed initialization,
the following attribute is defined:
::
X'Initialized
[It follows as a consequence of the other rules of |SPARK| that if
X'Initialized is True,
then for every reachable part Y of X whose type is not
annotated with the Relaxed_Initialization aspect, Y belongs to its
subtype.] An Initialized attribute reference is never a static expression.
.. container:: heading
Legality Rules
5. 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.
6. No part of a tagged type, or of a tagged object, shall have relaxed
initialization.
7. No part of an effectively volatile type, or of an effectively volatile
object, shall have relaxed initialization.
8. 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.
9. 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.
10. A formal parameter of a dispatching operation shall not have relaxed
initialization; the result of a dispatching function shall not have
relaxed initialization.
11. [Ghost attribute ``Initialized`` shall only be referenced where a
ghost entity would be allowed;
see :ref:`Ghost Entities` for the statement of this rule.]
.. container:: heading
Verification Rules
12. At the point of a read of an elementary 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 part
(see :ref:`Access Types`) of the [source]
expression of an assignment operation and the target of the assignment
does not have relaxed initialization, where X is a reachable part 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.]
13. For any object X, evaluation of
X'Initialized includes the evaluation of any subtype predicate applying
to X. In addition:
* if X has a composite type, evaluation of X'Initialized includes the
evaluation of Y'Initialized for every component Y of X whose type is not
annotated with the Relaxed_Initialization aspect,
* if X has unconstrained discriminants, evaluation of X'Initialized
includes the evaluation of Y'Initialized for every discriminant Y of X,
* if X has an access-to-object type, evaluation of X'Initialized includes
the evaluation X.all'Initialized if X is not null and the designated
type of the type of X is not annotated with the Relaxed_Initialization
aspect,
* if X has an elementary type, its value must have been written either
explicitly or implicitly through default initialization.
Discriminants of out-mode parameters and Output globals of a subprogram are
considered to be initialized at the beginning of the subprogram. Other
reachable parts are not.
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