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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 expression
4.1/2
{AI95-00381-01} restriction_parameter_argument ::= name | expression

Name 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
8.4/1

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.
NOTES
10/2
30  {AI95-00347-01} 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).

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