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11.5 Suppressing Checks

Checking pragmas give instructions to an implementation on handling language-defined checks. A pragma Suppress gives permission to an implementation to omit certain language-defined checks, while a pragma Unsuppress revokes the permission to omit checks..
A language-defined check (or simply, a “check”) is one of the situations defined by this International Standard that requires a check to be made at run time to determine whether some condition is true. A check fails when the condition being checked is False, causing an exception to be raised. 


The forms of checking pragmas are as follows: 
  pragma Suppress(identifier);
  pragma Unsuppress(identifier);
A checking pragma is allowed only immediately within a declarative_part, immediately within a package_specification, or as a configuration pragma. 

Legality Rules

The identifier shall be the name of a check. 
This paragraph was deleted.

Static Semantics

  A checking pragma applies to the named check in a specific region, and applies to all entities in that region. A checking pragma given in a declarative_part or immediately within a package_specification applies from the place of the pragma to the end of the innermost enclosing declarative region. The region for a checking pragma given as a configuration pragma is the declarative region for the entire compilation unit (or units) to which it applies.
  If a checking pragma applies to a generic_instantiation, then the checking pragma also applies to the entire instance. 
A pragma Suppress gives permission to an implementation to omit the named check (or every check in the case of All_Checks) for any entities to which it applies. If permission has been given to suppress a given check, the check is said to be suppressed
  A pragma Unsuppress revokes the permission to omit the named check (or every check in the case of All_Checks) given by any pragma Suppress that applies at the point of the pragma Unsuppress. The permission is revoked for the region to which the pragma Unsuppress applies. If there is no such permission at the point of a pragma Unsuppress, then the pragma has no effect. A later pragma Suppress can renew the permission.
The following are the language-defined checks: 
The following checks correspond to situations in which the exception Constraint_Error is raised upon failure.

When evaluating a dereference (explicit or implicit), check that the value of the name is not null. When converting to a subtype that excludes null, check that the converted value is not null.

Check that the discriminants of a composite value have the values imposed by a discriminant constraint. Also, when accessing a record component, check that it exists for the current discriminant values.

Check that the second operand is not zero for the operations /, rem and mod.

Check that the bounds of an array value are equal to the corresponding bounds of an index constraint. Also, when accessing a component of an array object, check for each dimension that the given index value belongs to the range defined by the bounds of the array object. Also, when accessing a slice of an array object, check that the given discrete range is compatible with the range defined by the bounds of the array object.

Check that two arrays have matching components, in the case of array subtype conversions, and logical operators for arrays of boolean components.

Check that a scalar value is within the base range of its type, in cases where the implementation chooses to raise an exception instead of returning the correct mathematical result.

Check that a scalar value satisfies a range constraint. Also, for the elaboration of a subtype_indication, check that the constraint (if present) is compatible with the subtype denoted by the subtype_mark. Also, for an aggregate, check that an index or discriminant value belongs to the corresponding subtype. Also, check that when the result of an operation yields an array, the value of each component belongs to the component subtype.

Check that operand tags in a dispatching call are all equal. Check for the correct tag on tagged type conversions, for an assignment_statement, and when returning a tagged limited object from a function. 
The following checks correspond to situations in which the exception Program_Error is raised upon failure.

Check the accessibility level of an entity or view.

For an allocator, check that the master of any tasks to be created by the allocator is not yet completed or some dependents have not yet terminated, and that the finalization of the collection has not started.

When a subprogram or protected entry is called, a task activation is accomplished, or a generic instantiation is elaborated, check that the body of the corresponding unit has already been elaborated.
This paragraph was deleted.
The following check corresponds to situations in which the exception Storage_Error is raised upon failure. 

Check that evaluation of an allocator does not require more space than is available for a storage pool. Check that the space available for a task or subprogram has not been exceeded.
The following check corresponds to all situations in which any predefined exception is raised.

Represents the union of all checks; suppressing All_Checks suppresses all checks other than those associated with assertions. In addition, an implementation is allowed (but not required) to behave as if a pragma Assertion_Policy(Ignore) applies to any region to which pragma Suppress(All_Checks) applies.

Erroneous Execution

If a given check has been suppressed, and the corresponding error situation occurs, the execution of the program is erroneous. 

Implementation Permissions

 An implementation is allowed to place restrictions on checking pragmas, subject only to the requirement that pragma Unsuppress shall allow any check names supported by pragma Suppress. An implementation is allowed to add additional check names, with implementation-defined semantics. When Overflow_Check has been suppressed, an implementation may also suppress an unspecified subset of the Range_Checks. 
   An implementation may support an additional parameter on pragma Unsuppress similar to the one allowed for pragma Suppress (see J.10). The meaning of such a parameter is implementation-defined. 

Implementation Advice

The implementation should minimize the code executed for checks that have been suppressed. 
4  There is no guarantee that a suppressed check is actually removed; hence a pragma Suppress should be used only for efficiency reasons.
5  It is possible to give both a pragma Suppress and Unsuppress for the same check immediately within the same declarative_part. In that case, the last pragma given determines whether or not the check is suppressed. Similarly, it is possible to resuppress a check which has been unsuppressed by giving a pragma Suppress in an inner declarative region. 


 Examples of suppressing and unsuppressing checks: 
pragma Suppress(Index_Check);
pragma Unsuppress(Overflow_Check);

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