13.12 Pragma Restrictions
1
[A pragma Restrictions
expresses the user's intent to abide by certain restrictions. This may
facilitate the construction of simpler run-time environments.]
Syntax
2
The form of
a pragma Restrictions is as follows:
3
pragma Restrictions(
restriction{,
restriction});
4/2
{
AI95-00381-01}
restriction ::= restriction_identifier
|
restriction_parameter_identifier =>
restriction_parameter_argument expression4.1/2
{
AI95-00381-01}
restriction_parameter_argument ::= name | expressionName Resolution Rules
5
{expected type (restriction
parameter expression) [partial]} Unless
otherwise specified for a particular restriction, the
expression
is expected to be of any integer type.
Legality Rules
6
Unless otherwise specified for a particular restriction,
the expression shall be static, and its value
shall be nonnegative.
Static Semantics
7/2
{
AI95-00394-01}
The set of
restrictions restrictions
is implementation defined.
7.a/2
Implementation defined: The set of restrictions restrictions
allowed in a pragma Restrictions.
Post-Compilation Rules
8
{configuration pragma
(Restrictions) [partial]} {pragma,
configuration (Restrictions) [partial]} A
pragma Restrictions is a configuration pragma;
unless otherwise specified for a particular restriction, a partition
shall obey the restriction if a
pragma Restrictions
applies to any compilation unit included in the partition.
8.1/1
{
8652/0042}
{
AI95-00130-01}
For the purpose of checking whether a partition
contains constructs that violate any restriction (unless specified otherwise
for a particular restriction):8.2/1
8.3/1
- {8652/0042}
{AI95-00130-01}
If an object of a type is declared or allocated
and not explicitly initialized, then all expressions appearing in the
definition for the type and any of its ancestors are presumed to be used;
8.4/1
- {8652/0042}
{AI95-00130-01}
A default_expression
for a formal parameter or a generic formal object is considered to be
used if and only if the corresponding actual parameter is not provided
in a given call or instantiation.
Implementation Permissions
9
An implementation may place limitations on the values
of the expression that are supported, and
limitations on the supported combinations of restrictions. The consequences
of violating such limitations are implementation defined.
9.a
Implementation defined: The consequences
of violating limitations on Restrictions pragmas.
9.b
Ramification: Such limitations may be
enforced at compile time or at run time. Alternatively, the implementation
is allowed to declare violations of the restrictions to be erroneous,
and not enforce them at all.
9.1/1
{
8652/0042}
{
AI95-00130-01}
An implementation is permitted to omit restriction
checks for code that is recognized at compile time to be unreachable
and for which no code is generated.9.2/1
{
8652/0043}
{
AI95-00190-01}
Whenever enforcement of a restriction is not required
prior to execution, an implementation may nevertheless enforce the restriction
prior to execution of a partition to which the restriction applies, provided
that every execution of the partition would violate the restriction.
10/2
30 {
AI-00347-01}
Restrictions intended to facilitate the construction of efficient tasking
run-time systems are defined in
D.7.
Restrictions
intended for use when constructing high integrity systems Safety-
and security-related restrictions are defined in
H.4.
11
31 An implementation has to enforce the
restrictions in cases where enforcement is required, even if it chooses
not to take advantage of the restrictions in terms of efficiency.
11.a
Discussion: It is not the intent that
an implementation will support a different run-time system for every
possible combination of restrictions. An implementation might support
only two run-time systems, and document a set of restrictions that is
sufficient to allow use of the more efficient and safe one.
Extensions to Ada 83
11.b
{
extensions to Ada 83}
Pragma
Restrictions is new to Ada 95.
Wording Changes from Ada 95
11.c/2
{
8652/0042}
{
AI95-00130-01}
Corrigendum: Corrected the wording so that
restrictions are checked inside of generic instantiations and in default
expressions. Since not making these checks would violate the purpose
of restrictions, we are not documenting this as an incompatibility.11.d/2
{
8652/0043}
{
AI95-00190-01}
Corrigendum: Added a permission that restrictions
can be enforced at compile-time. While this is technically incompatible,
documenting it as such would be unnecessarily alarming - there should
not be any programs depending on the runtime failure of restrictions.11.e/2
{
AI95-00381-01}
The syntax of a restriction_parameter_argument
has been defined to better support restriction No_Dependence (see 13.12.1).
Sponsored by Ada-Europe