3.2.2 Warning Message Control
In addition to error messages, which correspond to illegalities as defined
in the Ada 95 Reference Manual, the compiler detects two kinds of warning
situations.
First, the compiler considers some constructs suspicious and generates a
warning message to alert you to a possible error. Second, if the
compiler detects a situation that is sure to raise an exception at
run time, it generates a warning message. The following shows an example
of warning messages:
e.adb:4:24: warning: creation of object may raise Storage_Error
e.adb:10:17: warning: static value out of range
e.adb:10:17: warning: "Constraint_Error" will be raised at run time
GNAT considers a large number of situations as appropriate
for the generation of warning messages. As always, warnings are not
definite indications of errors. For example, if you do an out-of-range
assignment with the deliberate intention of raising a
Constraint_Error exception, then the warning that may be
issued does not indicate an error. Some of the situations for which GNAT
issues warnings (at least some of the time) are given in the following
list. This list is not complete, and new warnings are often added to
subsequent versions of GNAT. The list is intended to give a general idea
of the kinds of warnings that are generated.
- Possible infinitely recursive calls
- Out-of-range values being assigned
- Possible order of elaboration problems
- Unreachable code
- Fixed-point type declarations with a null range
- Direct_IO or Sequential_IO instantiated with a type that has access values
- Variables that are never assigned a value
- Variables that are referenced before being initialized
- Task entries with no corresponding
accept statement
- Duplicate accepts for the same task entry in a
select
- Objects that take too much storage
- Unchecked conversion between types of differing sizes
- Missing
return statement along some execution path in a function
- Incorrect (unrecognized) pragmas
- Incorrect external names
- Allocation from empty storage pool
- Potentially blocking operation in protected type
- Suspicious parenthesization of expressions
- Mismatching bounds in an aggregate
- Attempt to return local value by reference
- Premature instantiation of a generic body
- Attempt to pack aliased components
- Out of bounds array subscripts
- Wrong length on string assignment
- Violations of style rules if style checking is enabled
- Unused
with clauses
Bit_Order usage that does not have any effect
Standard.Duration used to resolve universal fixed expression
- Dereference of possibly null value
- Declaration that is likely to cause storage error
- Internal GNAT unit
with'ed by application unit
- Values known to be out of range at compile time
- Unreferenced labels and variables
- Address overlays that could clobber memory
- Unexpected initialization when address clause present
- Bad alignment for address clause
- Useless type conversions
- Redundant assignment statements and other redundant constructs
- Useless exception handlers
- Accidental hiding of name by child unit
- Access before elaboration detected at compile time
- A range in a
for loop that is known to be null or might be null
The following switches are available to control the handling of
warning messages:
- -gnatwa
- Activate all optional errors.
This switch activates most optional warning messages, see remaining list
in this section for details on optional warning messages that can be
individually controlled. The warnings that are not turned on by this
switch are
-gnatwd (implicit dereferencing),
-gnatwh (hiding),
and -gnatwl (elaboration warnings).
All other optional warnings are turned on.
- -gnatwA
- Suppress all optional errors.
This switch suppresses all optional warning messages, see remaining list
in this section for details on optional warning messages that can be
individually controlled.
- -gnatwb
- Activate warnings on bad fixed values.
This switch activates warnings for static fixed-point expressions whose
value is not an exact multiple of Small. Such values are implementation
dependent, since an implementation is free to choose either of the multiples
that surround the value. GNAT always chooses the closer one, but this is not
required behavior, and it is better to specify a value that is an exact
multiple, ensuring predictable execution. The default is that such warnings
are not generated.
- -gnatwB
- Suppress warnings on bad fixed values.
This switch suppresses warnings for static fixed-point expressions whose
value is not an exact multiple of Small.
- -gnatwc
- Activate warnings on conditionals.
This switch activates warnings for conditional expressions used in
tests that are known to be True or False at compile time. The default
is that such warnings are not generated.
Note that this warning does
not get issued for the use of boolean variables or constants whose
values are known at compile time, since this is a standard technique
for conditional compilation in Ada, and this would generate too many
“false positive” warnings.
This warning option also activates a special test for comparisons using
the operators “>=” and“ <=”.
If the compiler can tell that only the equality condition is possible,
then it will warn that the “>” or “<” part of the test
is useless and that the operator could be replaced by “=”.
An example would be comparing a Natural variable <= 0.
This warning can also be turned on using -gnatwa.
- -gnatwC
- Suppress warnings on conditionals.
This switch suppresses warnings for conditional expressions used in
tests that are known to be True or False at compile time.
- -gnatwd
- Activate warnings on implicit dereferencing.
If this switch is set, then the use of a prefix of an access type
in an indexed component, slice, or selected component without an
explicit
.all will generate a warning. With this warning
enabled, access checks occur only at points where an explicit
.all appears in the source code (assuming no warnings are
generated as a result of this switch). The default is that such
warnings are not generated.
Note that -gnatwa does not affect the setting of
this warning option.
- -gnatwD
- Suppress warnings on implicit dereferencing.
This switch suppresses warnings for implicit dereferences in
indexed components, slices, and selected components.
- -gnatwe
- Treat warnings as errors.
This switch causes warning messages to be treated as errors.
The warning string still appears, but the warning messages are counted
as errors, and prevent the generation of an object file.
- -gnatwf
- Activate warnings on unreferenced formals.
This switch causes a warning to be generated if a formal parameter
is not referenced in the body of the subprogram. This warning can
also be turned on using -gnatwa or -gnatwu.
- -gnatwF
- Suppress warnings on unreferenced formals.
This switch suppresses warnings for unreferenced formal
parameters. Note that the
combination -gnatwu followed by -gnatwF has the
effect of warning on unreferenced entities other than subprogram
formals.
- -gnatwg
- Activate warnings on unrecognized pragmas.
This switch causes a warning to be generated if an unrecognized
pragma is encountered. Apart from issuing this warning, the
pragma is ignored and has no effect. This warning can
also be turned on using -gnatwa. The default
is that such warnings are issued (satisfying the Ada Reference
Manual requirement that such warnings appear).
- -gnatwG
- Suppress warnings on unrecognized pragmas.
This switch suppresses warnings for unrecognized pragmas.
- -gnatwh
- Activate warnings on hiding.
This switch activates warnings on hiding declarations.
A declaration is considered hiding
if it is for a non-overloadable entity, and it declares an entity with the
same name as some other entity that is directly or use-visible. The default
is that such warnings are not generated.
Note that -gnatwa does not affect the setting of this warning option.
- -gnatwH
- Suppress warnings on hiding.
This switch suppresses warnings on hiding declarations.
- -gnatwi
- Activate warnings on implementation units.
This switch activates warnings for a
with of an internal GNAT
implementation unit, defined as any unit from the Ada,
Interfaces, GNAT,
or System
hierarchies that is not
documented in either the Ada Reference Manual or the GNAT
Programmer's Reference Manual. Such units are intended only
for internal implementation purposes and should not be with'ed
by user programs. The default is that such warnings are generated
This warning can also be turned on using -gnatwa.
- -gnatwI
- Disable warnings on implementation units.
This switch disables warnings for a
with of an internal GNAT
implementation unit.
- -gnatwj
- Activate warnings on obsolescent features (Annex J).
If this warning option is activated, then warnings are generated for
calls to subprograms marked with
pragma Obsolescent and
for use of features in Annex J of the Ada Reference Manual. In the
case of Annex J, not all features are flagged. In particular use
of the renamed packages (like Text_IO) and use of package
ASCII are not flagged, since these are very common and
would generate many annoying positive warnings. The default is that
such warnings are not generated.
In addition to the above cases, warnings are also generated for
GNAT features that have been provided in past versions but which
have been superseded (typically by features in the new Ada standard).
For example, pragma Ravenscar will be flagged since its
function is replaced by pragma Profile(Ravenscar).
Note that this warning option functions differently from the
restriction No_Obsolescent_Features in two respects.
First, the restriction applies only to annex J features.
Second, the restriction does flag uses of package ASCII.
- -gnatwJ
- Suppress warnings on obsolescent features (Annex J).
This switch disables warnings on use of obsolescent features.
- -gnatwk
- Activate warnings on variables that could be constants.
This switch activates warnings for variables that are initialized but
never modified, and then could be declared constants.
- -gnatwK
- Suppress warnings on variables that could be constants.
This switch disables warnings on variables that could be declared constants.
- -gnatwl
- Activate warnings for missing elaboration pragmas.
This switch activates warnings on missing
Elaborate_All and Elaborate pragmas.
See the section in this guide on elaboration checking for details on
when such pragmas should be used. Warnings are also generated if you
are using the static mode of elaboration, and a pragma Elaborate
is encountered. The default is that such warnings
are not generated.
This warning is not automatically turned on by the use of -gnatwa.
- -gnatwL
- Suppress warnings for missing elaboration pragmas.
This switch suppresses warnings on missing Elaborate and Elaborate_All pragmas.
See the section in this guide on elaboration checking for details on
when such pragmas should be used.
- -gnatwm
- Activate warnings on modified but unreferenced variables.
This switch activates warnings for variables that are assigned (using
an initialization value or with one or more assignment statements) but
whose value is never read. The warning is suppressed for volatile
variables and also for variables that are renamings of other variables
or for which an address clause is given.
This warning can also be turned on using -gnatwa.
- -gnatwM
- Disable warnings on modified but unreferenced variables.
This switch disables warnings for variables that are assigned or
initialized, but never read.
- -gnatwn
- Set normal warnings mode.
This switch sets normal warning mode, in which enabled warnings are
issued and treated as warnings rather than errors. This is the default
mode. the switch -gnatwn can be used to cancel the effect of
an explicit -gnatws or
-gnatwe. It also cancels the effect of the
implicit -gnatwe that is activated by the
use of -gnatg.
- -gnatwo
- Activate warnings on address clause overlays.
This switch activates warnings for possibly unintended initialization
effects of defining address clauses that cause one variable to overlap
another. The default is that such warnings are generated.
This warning can also be turned on using -gnatwa.
- -gnatwO
- Suppress warnings on address clause overlays.
This switch suppresses warnings on possibly unintended initialization
effects of defining address clauses that cause one variable to overlap
another.
- -gnatwp
- Activate warnings on ineffective pragma Inlines.
This switch activates warnings for failure of front end inlining
(activated by -gnatN) to inline a particular call. There are
many reasons for not being able to inline a call, including most
commonly that the call is too complex to inline.
This warning can also be turned on using -gnatwa.
- -gnatwP
- Suppress warnings on ineffective pragma Inlines.
This switch suppresses warnings on ineffective pragma Inlines. If the
inlining mechanism cannot inline a call, it will simply ignore the
request silently.
- -gnatwr
- Activate warnings on redundant constructs.
This switch activates warnings for redundant constructs. The following
is the current list of constructs regarded as redundant:
This warning can also be turned on using -gnatwa.
- Assignment of an item to itself.
- Type conversion that converts an expression to its own type.
- Use of the attribute
Base where typ'Base is the same
as typ.
- Use of pragma
Pack when all components are placed by a record
representation clause.
- Exception handler containing only a reraise statement (raise with no
operand) which has no effect.
- Use of the operator abs on an operand that is known at compile time
to be non-negative
- Comparison of boolean expressions to an explicit True value.
- -gnatwR
- Suppress warnings on redundant constructs.
This switch suppresses warnings for redundant constructs.
- -gnatws
- Suppress all warnings.
This switch completely suppresses the
output of all warning messages from the GNAT front end.
Note that it does not suppress warnings from the gcc back end.
To suppress these back end warnings as well, use the switch -w
in addition to -gnatws.
- -gnatwu
- Activate warnings on unused entities.
This switch activates warnings to be generated for entities that
are declared but not referenced, and for units that are
with'ed
and not
referenced. In the case of packages, a warning is also generated if
no entities in the package are referenced. This means that if the package
is referenced but the only references are in use
clauses or renames
declarations, a warning is still generated. A warning is also generated
for a generic package that is with'ed but never instantiated.
In the case where a package or subprogram body is compiled, and there
is a with on the corresponding spec
that is only referenced in the body,
a warning is also generated, noting that the
with can be moved to the body. The default is that
such warnings are not generated.
This switch also activates warnings on unreferenced formals
(it includes the effect of -gnatwf).
This warning can also be turned on using -gnatwa.
- -gnatwU
- Suppress warnings on unused entities.
This switch suppresses warnings for unused entities and packages.
It also turns off warnings on unreferenced formals (and thus includes
the effect of -gnatwF).
- -gnatwv
- Activate warnings on unassigned variables.
This switch activates warnings for access to variables which
may not be properly initialized. The default is that
such warnings are generated.
- -gnatwV
- Suppress warnings on unassigned variables.
This switch suppresses warnings for access to variables which
may not be properly initialized.
- -gnatwy
- Activate warnings for Ada 2005 compatibility issues.
For the most part Ada 2005 is upwards compatible with Ada 95,
but there are some exceptions (for example the fact that
interface is now a reserved word in Ada 2005). This
switch activates several warnings to help in identifying
and correcting such incompatibilities. The default is that
these warnings are generated. Note that at one point Ada 2005
was called Ada 0Y, hence the choice of character.
- -gnatwY
- Disable warnings for Ada 2005 compatibility issues.
This switch suppresses several warnings intended to help in identifying
incompatibilities between Ada 95 and Ada 2005.
- -gnatwx
- Activate warnings on Export/Import pragmas.
This switch activates warnings on Export/Import pragmas when
the compiler detects a possible conflict between the Ada and
foreign language calling sequences. For example, the use of
default parameters in a convention C procedure is dubious
because the C compiler cannot supply the proper default, so
a warning is issued. The default is that such warnings are
generated.
- -gnatwX
- Suppress warnings on Export/Import pragmas.
This switch suppresses warnings on Export/Import pragmas.
The sense of this is that you are telling the compiler that
you know what you are doing in writing the pragma, and it
should not complain at you.
- -gnatwz
- Activate warnings on unchecked conversions.
This switch activates warnings for unchecked conversions
where the types are known at compile time to have different
sizes. The default
is that such warnings are generated.
- -gnatwZ
- Suppress warnings on unchecked conversions.
This switch suppresses warnings for unchecked conversions
where the types are known at compile time to have different
sizes.
- -Wuninitialized
- The warnings controlled by the -gnatw switch are generated by the
front end of the compiler. In some cases, the gcc back end
can provide additional warnings. One such useful warning is provided by
-Wuninitialized. This must be used in
conjunction with turning on optimization mode. This causes the flow
analysis circuits of the back end optimizer to output additional
warnings about uninitialized variables.
- -w
- This switch suppresses warnings from the gcc back end. The
code generator detects a number of warning situations that are missed
by the GNAT front end, and this switch can be used to suppress them.
The use of this switch also sets the default front end warning mode to
-gnatws, that is, front end warnings suppressed as well.
A string of warning parameters can be used in the same parameter. For example:
-gnatwaLe
will turn on all optional warnings except for elaboration pragma warnings,
and also specify that warnings should be treated as errors.
When no switch -gnatw is used, this is equivalent to:
- -gnatwC
- -gnatwD
- -gnatwF
- -gnatwg
- -gnatwH
- -gnatwi
- -gnatwJ
- -gnatwK
- -gnatwL
- -gnatwM
- -gnatwn
- -gnatwo
- -gnatwP
- -gnatwR
- -gnatwU
- -gnatwv
- -gnatwz
- -gnatwx
-