11.5 Suppressing Checks
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.
Glossary entry: To
suppress a check is to assert that the check cannot fail, and to request
that the compiler optimize by disabling the check. The compiler is not
required to honor this request. Suppressing checks that can fail can
cause program to behave in arbitrary ways.
(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
A check fails
when the condition being
checked is False, causing an exception to be raised.
Discussion: All such checks are defined
under “Dynamic Semantics” in clauses and subclauses throughout
Glossary entry: A
check is a test made during execution to determine whether a language
rule has been violated.
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
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.
This means that a Suppress pragma that occurs in a scope enclosing the
declaration of a generic unit but not also enclosing the declaration
of a given instance of that generic unit will not apply to constructs
within the given instance.
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 check is suppressed even
if the implementation chooses not to actually generate better code.
allows the implementation to raise Program_Error, for example, if the
erroneousness is detected.
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
then the pragma
has no effect. A later pragma
Suppress can renew the permission.
The following are the
[The following checks correspond to situations in
which the exception Constraint_Error is raised upon failure of a language-defined check
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
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. Also, for the attributes Value, Wide_Value, and Wide_Wide_Value, check
that the given string has the appropriate syntax and value for the base
subtype of the prefix
of the attribute_reference.
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 of a language-defined check
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.
Other language-defined checks that raise Program_Error:
that subtypes with predicates are not used to index an array in a generic
unit; that the maximum number of chunks is greater than zero; that the
default value of an out parameter is convertible; that there is no misuse
of functions in a generic with a class-wide actual type; that there are
not colliding External_Tag values; that there is no misuse of operations
of unchecked union types.
[The following check corresponds to situations in which the exception
Storage_Error is raised upon failure of a language-defined
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.
We considered splitting this
out into three categories: Pool_Check (for allocator
Stack_Check (for stack usage), and Heap_Check (for implicit use of the
heap — use of the heap other than through an allocator
Storage_Check would then represent the union of these three. However,
there seems to be no compelling reason to do this, given that it is not
feasible to split Storage_Error.
[The following check corresponds to situations
in which the exception Tasking_Error is raised upon failure of a language-defined
Check that all tasks activated successfully. Check
that a called task has not yet terminated.
[The following checks correspond to situations
in which the exception Assertion_Error is raised upon failure of a language-defined
check.] For a language-defined unit U associated
with one of these checks in the list below, the check refers to performance
of checks associated with the Pre, Static_Predicate, and Dynamic_Predicate
aspects associated with any entity declared in a descendant of U,
or in an instance of a generic unit which is, or is declared in, a descendant
of U. Each check is associated with one or more units:
Characters, Wide_Characters, and Wide_Wide_Characters.
Sequential_IO, Direct_IO, Text_IO, Wide_Text_IO,
Wide_Wide_Text_IO, Storage_IO, Streams.Stream_IO, and Directories.
Any unit U is a descendant of itself.
One could use pragma Assertion_Policy to
eliminate such checks, but that would require recompiling the language
defined packages (the assertion policy that determines whether the assertion
checks are made is that used to compile the unit). In addition, we do
not want to specify the behavior of language-defined operations if the
precondition fails; that is different than the usual behavior of Assertion_Policy.
By using Suppress for this purpose, we make it clear that a failed check
that is suppressed means erroneous execution.
To be honest:
The preceding rule about an instance of a generic
where the generic is declared in U really extends recursively
to handle the case of a generic package G1 which declares another
generic package G1.G2, which declares another generic package
G1.G2.G3, and so on. So if G1 is declared in some predefined
unit U then, for purposes of defining these checks, Some_Instance_Of_G1.G2
is also considered to be declared in U.
[The following check corresponds to all situations in which any predefined
exception is raised upon failure of a check
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)
All_Checks may include includes
both language-defined and
implementation-defined checks. It does not include, however, explicit raises of predefined exceptions
(including those mandated for language constructs), nor those propagated
from language-defined subprograms.
To be honest:
There are additional checks defined in various Specialized Needs Annexes
that are not listed here. Nevertheless, they are included in All_Checks
and named in a Suppress pragma on implementations that support the relevant
annex. Look up “check, language-defined” in the index to
find the complete list.
We don't want to say that assertions are suppressed, because we don't
want the potential failure of an assertion to cause erroneous execution
(see below). Thus they are excluded from the suppression part of the
above rule and then handled with an implicit Ignore policy.
If a given check has been suppressed, and the corresponding
error situation occurs, the execution of the program is erroneous. Similarly, if a precondition check has been suppressed and the evaluation
of the precondition would have raised an exception, execution is erroneous.
It's unclear that a precondition expression that
executes raise some_exception is an "error
situation"; the precondition never actually evaluates to False in
that case. Thus, we spell out that case. We only allow suppressing preconditions
associated with language-defined units; other preconditions follow the
rules of the appropriate Assertion_Policy.
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.
has been suppressed, an implementation may also suppress an unspecified
subset of the Range_Checks.
Implementation defined: Implementation-defined
Discussion: For Overflow_Check, the intention
is that the implementation will suppress any Range_Checks that are implemented
in the same manner as Overflow_Checks (unless they are free).
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.
meaning of second parameter of pragma
The implementation should minimize the code executed
for checks that have been suppressed.
Implementation Advice: Code executed
for checks that have been suppressed should be minimized.
Implementation Note: However, if a given
check comes for free (for example, the hardware automatically performs
the check in parallel with doing useful work) or nearly free (for example,
the check is a tiny portion of an expensive run-time system call), the
implementation should not bother to suppress the check. Similarly, if
the implementation detects the failure at compile time and provides a
warning message, there is no need to actually suppress the check.
is no guarantee that a suppressed check is actually removed; hence a
should be used only for efficiency reasons.
It is possible to give both a pragma
Suppress and Unsuppress for the same check immediately within the same
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
in an inner declarative region.
Examples of suppressing and unsuppressing checks:
Extensions to Ada 83
Additional check names are added. We allow implementations
to define their own checks.
Wording Changes from Ada 83
We define the checks in a distributed manner.
Therefore, the long list of what checks apply to what is merely a NOTE.
We have removed the detailed rules about what
is allowed in a pragma
Suppress, and allow implementations to invent their own. The RM83 rules
weren't quite right, and such a change is necessary anyway in the presence
of implementation-defined checks.
We make it clear that the difference between
a Range_Check and an Overflow_Check is fuzzy. This was true in Ada 83,
given RM83-11.6, but it was not clear. We considered removing Overflow_Check
from the language or making it obsolescent, just as we did for Numeric_Error.
However, we kept it for upward compatibility, and because it may be useful
on machines where range checking costs more than overflow checking, but
overflow checking still costs something. Different compilers will suppress
different checks when asked to suppress Overflow_Check — the nonuniformity
in this case is not harmful, and removing it would have a serious impact
Extensions to Ada 95
Wording Changes from Ada 95
The description of Access_Check was corrected by the Corrigendum to include
the discriminant case. This change was then replaced by the more general
notion of checking conversions to subtypes that exclude null in Ada 2005.
The On parameter of pragma Suppress was moved to Annex J (see J.10
This feature's effect is inherently nonportable, depending on the implementation's
model of computation. Compiler surveys demonstrated this, showing that
implementations vary widely in the interpretation of these parameters,
even on the same target. While this is relatively harmless for Suppress
(which is never required to do anything), it would be a significant problem
for Unsuppress (we want the checks to be made for all implementations).
By moving it, we avoid needing to define the meaning of Unsuppress with
an On parameter.
The order of the Program_Error checks was corrected to be alphabetical.
Wording Changes from Ada 2005
The effect of a checking pragma no longer applies inside an inlined subprogram
body. While this could change the behavior of a program that depends
on a check being suppressed in an inlined body, such a program is erroneous
and thus no behavior can be depended upon anyway. It's also likely to
be very rare. We make this change so that inlining has no effect on the
meaning of the subprogram body (since inlining is never required requiring
this is necessary in order to be able to reason about the body), and
so that assertion policies and suppress work the same way for inlining.
Extensions to Ada 2012
Program_Error_Check and Tasking_Check are new;
all core language-defined checks are now covered with check names.
Wording Changes from Ada 2012
Correction: Range_Check is defined to include
checks associated with the Value and related attributes.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe