3.2.2 Warning Message Control
In addition to error messages, which correspond to illegalities as defined
in the Ada 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
- Assertions (pragma Assert) that are sure to fail
- Unreachable code
- Address clauses with possibly unaligned values, or where an attempt is
made to overlay a smaller variable with a larger one.
- 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 section lists compiler switches that are available
to control the handling of warning messages. It is also possible
to exercise much finer control over what warnings are issued and
suppressed using the GNAT pragma Warnings, See Pragma Warnings.
- -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),
-gnatwl (elaboration warnings),
-gnatw.o (warn on values set by out parameters ignored)
and -gnatwt (tracking of deleted conditional code).
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.
- -gnatw.a
- Activate warnings on failing assertions.
This switch activates warnings for assertions where the compiler can tell at
compile time that the assertion will fail. Note that this warning is given
even if assertions are disabled. The default is that such warnings are
generated.
- -gnatw.A
- Suppress warnings on failing assertions.
This switch suppresses warnings for assertions where the compiler can tell at
compile time that the assertion will fail.
- -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.
- -gnatw.b
- Activate warnings on biased representation.
This switch activates warnings when a size clause, value size clause, component
clause, or component size clause forces the use of biased representation for an
integer type (e.g. representing a range of 10..11 in a single bit by using 0/1
to represent 10/11). The default is that such warnings are generated.
- -gnatw.B
- Suppress warnings on biased representation.
This switch suppresses warnings for representation clauses that force the use
of biased representation.
- -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 option also generates warnings if
one or both tests is optimized away in a membership test for integer
values if the result can be determined at compile time. Range tests on
enumeration types are not included, since it is common for such tests
to include an end point.
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.
- -gnatw.c
- Activate warnings on missing component clauses.
This switch activates warnings for record components where a record
representation clause is present and has component clauses for the
majority, but not all, of the components. A warning is given for each
component for which no component clause is present.
This warning can also be turned on using -gnatwa.
- -gnatw.C
- Suppress warnings on missing component clauses.
This switch suppresses warnings for record components that are
missing a component clause in the situation described above.
- -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.
- -gnatw.e
- Activate every optional warning
This switch activates all optional warnings, including those which
are not activated by
-gnatwa.
- -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. The
default is that these warnings are not generated.
- -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. This warning is also turned on by
the use of -gnatwa.
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. The default is that
such warnings are not given.
This warning can also be turned on using -gnatwa.
- -gnatwK
- Suppress warnings on variables that could be constants.
This switch disables warnings on variables that could be declared constants.
- -gnatwl
- Activate warnings for 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. In dynamic elaboration mode, this switch
generations warnings about the need to add elaboration pragmas. Note however,
that if you blindly follow these warnings, and add Elaborate_All
warnings wherever they are recommended, you basically end up with the
equivalent of the static elaboration model, which may not be what you want for
legacy code for which the static model does not work.
For the static model, the messages generated are labeled "info:" (for
information messages). They are not warnings to add elaboration pragmas,
merely informational messages showing what implicit elaboration pragmas
have been added, for use in analyzing elaboration circularity problems.
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 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.
The default is that these warnings are not given.
- -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.
- -gnatw.o
- Activate warnings on modified but unreferenced out parameters.
This switch activates warnings for variables that are modified by using
them as actuals for a call to a procedure with an out mode formal, where
the resulting assigned value is never read. It is applicable in the case
where there is more than one out mode formal. If there is only one out
mode formal, the warning is issued by default (controlled by -gnatwu).
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.
The default is that these warnings are not given. Note that this warning
is not included in -gnatwa, it must be activated explicitly.
- -gnatw.O
- Disable warnings on modified but unreferenced out parameters.
This switch suppresses warnings for variables that are modified by using
them as actuals for a call to a procedure with an out mode formal, where
the resulting assigned value is never read.
- -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. The default is
that such warnings are not given.
This warning can also be turned on using -gnatwa.
Warnings on ineffective inlining by the gcc back-end can be activated
separately, using the gcc switch -Winline.
- -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.
- -gnatw.p
- Activate warnings on parameter ordering.
This switch activates warnings for cases of suspicious parameter
ordering when the list of arguments are all simple identifiers that
match the names of the formals, but are in a different order. The
warning is suppressed if any use of named parameter notation is used,
so this is the appropriate way to suppress a false positive (and
serves to emphasize that the "misordering" is deliberate). The
default is
that such warnings are not given.
This warning can also be turned on using -gnatwa.
- -gnatw.P
- Suppress warnings on parameter ordering.
This switch suppresses warnings on cases of suspicious parameter
ordering.
- -gnatwq
- Activate warnings on questionable missing parentheses.
This switch activates warnings for cases where parentheses are not used and
the result is potential ambiguity from a readers point of view. For example
(not a > b) when a and b are modular means ((not a) > b) and very likely the
programmer intended (not (a > b)). Similarly (-x mod 5) means (-(x mod 5)) and
quite likely ((-x) mod 5) was intended. In such situations it seems best to
follow the rule of always parenthesizing to make the association clear, and
this warning switch warns if such parentheses are not present. The default
is that these warnings are given.
This warning can also be turned on using -gnatwa.
- -gnatwQ
- Suppress warnings on questionable missing parentheses.
This switch suppresses warnings for cases where the association is not
clear and the use of parentheses is preferred.
- -gnatwr
- Activate warnings on redundant constructs.
This switch activates warnings for redundant constructs. The following
is the current list of constructs regarded as redundant:
- 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.
This warning can also be turned on using -gnatwa.
The default is that warnings for redundant constructs are not given.
- -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.
- -gnatwt
- Activate warnings for tracking of deleted conditional code.
This switch activates warnings for tracking of code in conditionals (IF and
CASE statements) that is detected to be dead code which cannot be executed, and
which is removed by the front end. This warning is off by default, and is not
turned on by -gnatwa, it has to be turned on explicitly. This may be
useful for detecting deactivated code in certified applications.
- -gnatwT
- Suppress warnings for tracking of deleted conditional code.
This switch suppresses warnings for tracking of deleted conditional code.
- -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.
This warning can also be turned on using -gnatwa.
- -gnatwV
- Suppress warnings on unassigned variables.
This switch suppresses warnings for access to variables which
may not be properly initialized.
For variables of a composite type, the warning can also be suppressed in
Ada 2005 by using a default initialization with a box. For example, if
Table is an array of records whose components are only partially uninitialized,
then the following code:
Tab : Table := (others => <>);
will suppress warnings on subsequent statements that access components
of variable Tab.
- -gnatww
- Activate warnings on wrong low bound assumption.
This switch activates warnings for indexing an unconstrained string parameter
with a literal or S'Length. This is a case where the code is assuming that the
low bound is one, which is in general not true (for example when a slice is
passed). The default is that such warnings are generated.
This warning can also be turned on using -gnatwa.
- -gnatwW
- Suppress warnings on wrong low bound assumption.
This switch suppresses warnings for indexing an unconstrained string parameter
with a literal or S'Length. Note that this warning can also be suppressed
in a particular case by adding an
assertion that the lower bound is 1,
as shown in the following example.
procedure K (S : String) is
pragma Assert (S'First = 1);
...
- -gnatw.w
- Activate warnings on unnecessary Warnings Off pragmas
This switch activates warnings for use of
pragma Warnings (Off, entity
where either the pragma is entirely useless (because it suppresses no
warnings), or it could be replaced by pragma Unreferenced or
pragma Unmodified.The default is that these warnings are not given.
Note that this warning is not included in -gnatwa, it must be
activated explicitly.
- -gnatw.W
- Suppress warnings on unnecessary Warnings Off pragmas
This switch suppresses warnings for use of
pragma Warnings (Off, entity.
- -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.
This warning can also be turned on using -gnatwa.
- -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.
- -gnatw.x
- Activate warnings for No_Exception_Propagation mode.
This switch activates warnings for exception usage when pragma Restrictions
(No_Exception_Propagation) is in effect. Warnings are given for implicit or
explicit exception raises which are not covered by a local handler, and for
exception handlers which do not cover a local raise. The default is that these
warnings are not given.
- -gnatw.X
- Disable warnings for No_Exception_Propagation mode.
This switch disables warnings for exception usage when pragma Restrictions
(No_Exception_Propagation) is in effect.
- -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.
This warning can also be turned on using -gnatwa.
- -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.
- -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. Warnings are also
generated for subprogram pointers with different conventions,
and, on VMS only, for data pointers with different conventions.
This warning can also be turned on using -gnatwa.
- -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 or conventions.
- -Wunused
- The warnings controlled by the -gnatw switch are generated by
the front end of the compiler. The GCC back end can provide
additional warnings and they are controlled by the -W switch.
For example, -Wunused activates back end
warnings for entities that are declared but not referenced.
- -Wuninitialized
- Similarly, -Wuninitialized activates
the back end warning for uninitialized variables. This switch must be
used in conjunction with an optimization level greater than zero.
- -Wall
- This switch enables all the above 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 activate them.
The use of this switch also sets the default front end warning mode to
-gnatwa, that is, most front end warnings activated as well.
- -w
- Conversely, this switch suppresses warnings from the GCC back end.
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
-