Exceptions

Asserts

When we want to indicate a condition in the code that must always be valid, we can use the pragma Assert. As the name implies, when we use this pragma, we're asserting some truth about the source-code. (We can also use the procedural form, as we'll see later.)

Important

Another method to assert the truth about the source-code is to use pre and post-conditions.

A simple assert has this form:

procedure Show_Pragma_Assert is I : constant Integer := 10; pragma Assert (I = 10); begin null; end Show_Pragma_Assert;

In this example, we're asserting that the value of I is always 10. We could also display a message if the assertion is false:

procedure Show_Pragma_Assert is I : constant Integer := 11; pragma Assert (I = 10, "I is not 10"); begin null; end Show_Pragma_Assert;

Similarly, we can use the procedural form of Assert. For example, the code above can implemented as follows:

with Ada.Assertions; use Ada.Assertions; procedure Show_Procedure_Assert is I : constant Integer := 11; begin Assert (I = 10, "I is not 10"); end Show_Procedure_Assert;

Note that a call to Assert is simply translated to a check — and the Assertion_Error exception from the Ada.Assertions package being raised in the case that the check fails. For example, the code above roughly corresponds to this:

with Ada.Assertions; use Ada.Assertions; procedure Show_Assertion_Error is I : constant Integer := 11; begin if I /= 10 then raise Assertion_Error with "I is not 10"; end if; end Show_Assertion_Error;

In the Ada Reference Manual

Assertion policies

We can activate and deactivate assertions based on assertion policies. We can do that by using the pragma Assertion_Policy. As an argument to this pragma, we indicate whether a specific policy must be checked or ignored.

For example, we can deactivate assertion checks by specifying Assert => Ignore. Similarly, we can activate assertion checks by specifying Assert => Check. Let's see a code example:

procedure Show_Pragma_Assertion_Policy is I : constant Integer := 11; pragma Assertion_Policy (Assert => Ignore); begin pragma Assert (I = 10); end Show_Pragma_Assertion_Policy;

Here, we're specifying that asserts shall be ignored. Therefore, the call to the pragma Assert doesn't raise an exception. If we replace Ignore with Check in the call to Assertion_Policy, the assert will raise the Assertion_Error exception.

The following table presents all policies that we can set:

Policy

Descripton

Assert

Check assertions

Static_Predicate

Check static predicates

Dynamic_Predicate

Check dynamic predicates

Pre

Check pre-conditions

Pre'Class

Check pre-condition of classes of tagged types

Post

Check post-conditions

Post'Class

Check post-condition of classes of tagged types

Type_Invariant

Check type invariants

Type_Invariant'Class

Check type invariant of classes of tagged types

In the GNAT toolchain

Compilers are free to include policies that go beyond the ones listed above. For example, GNAT includes the following policies — called assertion kinds in this context:

  • Assertions

  • Assert_And_Cut

  • Assume

  • Contract_Cases

  • Debug

  • Ghost

  • Initial_Condition

  • Invariant

  • Invariant'Class

  • Loop_Invariant

  • Loop_Variant

  • Postcondition

  • Precondition

  • Predicate

  • Refined_Post

  • Statement_Assertions

  • Subprogram_Variant

Also, in addtion to Check and Ignore, GNAT allows you to set a policy to Disable and Suppressible.

You can read more about them in the GNAT Reference Manual.

You can specify multiple policies in a single call to Assertion_Policy. For example, you can activate all policies by writing:

procedure Show_Multiple_Assertion_Policies is pragma Assertion_Policy (Assert => Check, Static_Predicate => Check, Dynamic_Predicate => Check, Pre => Check, Pre'Class => Check, Post => Check, Post'Class => Check, Type_Invariant => Check, Type_Invariant'Class => Check); begin null; end Show_Multiple_Assertion_Policies;

In the GNAT toolchain

With GNAT, policies can be specified in multiple ways. In addition to calls to Assertion_Policy, you can use configuration pragmas files. You can use these files to specify all pragmas that are relevant to your application, including Assertion_Policy. In addition, you can manage the granularity for those pragmas. For example, you can use a global configuration pragmas file for your complete application, or even different files for each source-code file you have.

Also, by default, all policies listed in the table above are deactivated, i.e. they're all set to Ignore. You can use the command-line switch -gnata to activate them.

Note that the Assert procedure raises an exception independently of the assertion policy (Assertion_Policy (Assert => Ignore)). For example:

with Ada.Text_IO; use Ada.Text_IO; with Ada.Assertions; use Ada.Assertions; procedure Show_Assert_Procedure_Policy is pragma Assertion_Policy (Assert => Ignore); I : constant Integer := 1; begin Put_Line ("------ Pragma Assert -----"); pragma Assert (I = 0); Put_Line ("---- Procedure Assert ----"); Assert (I = 0); Put_Line ("Finished."); end Show_Assert_Procedure_Policy;

Here, the pragma Assert is ignored due to the assertion policy. However, the call to Assert is not ignored.

In the Ada Reference Manual

Checks and exceptions

This table shows all language-defined checks and the associated exceptions:

Check

Exception

Access_Check

Constraint_Error

Discriminant_Check

Constraint_Error

Division_Check

Constraint_Error

Index_Check

Constraint_Error

Length_Check

Constraint_Error

Overflow_Check

Constraint_Error

Range_Check

Constraint_Error

Tag_Check

Constraint_Error

Accessibility_Check

Program_Error

Allocation_Check

Program_Error

Elaboration_Check

Program_Error

Storage_Check

Storage_Error

In addition, we can use All_Checks to refer to all those checks above at once.

Let's discuss each check and see code examples where those checks are performed. Note that all examples are erroneous, so please avoid reusing them elsewhere.

Access Check

As you know, an object of an access type might be null. It would be an error to dereference this object, as it doesn't indicate a valid position in memory. Therefore, the access check verifies that an access object is not null when dereferencing it. For example:

procedure Show_Access_Check is type Access_Integer is access all Integer; AI : Access_Integer; begin AI.all := 10; end Show_Access_Check;

Here, the value of AI is null by default, so we cannot dereference it.

The access check also performs this verification when assigning to a subtype that excludes null (not null access). For example:

procedure Show_Access_Check is type Access_Integer is access all Integer; type Non_Null_Access_Integer is not null access all Integer; AI : Access_Integer; NNAI : Non_Null_Access_Integer := new Integer; begin NNAI := Non_Null_Access_Integer (AI); end Show_Access_Check;

Here, the value of AI is null (by default), so we cannot assign it to NNAI because its type excludes null.

Note that, if we remove the := new Integer assignment from the declaration of NNAI, the null exclusion fails in the declaration itself (because the default value of the access type is null).

Discriminant Check

As we've seen earlier, a variant record is a record with discriminants that allows for changing its structure. In operations such as an assignment, it's important to ensure that the discriminants of the objects match — i.e. to ensure that the structure of the objects matches. The discriminant check verifies whether this is the case. For example:

procedure Show_Discriminant_Check is type Rec (Valid : Boolean) is record case Valid is when True => Counter : Integer; when False => null; end case; end record; R : Rec (Valid => False); begin R := (Valid => True, Counter => 10); end Show_Discriminant_Check;

Here, R's discriminant (Valid) is False, so we cannot assign an object whose Valid discriminant is True.

Also, when accessing a component, the discriminant check ensures that this component exists for the current discriminant value:

procedure Show_Discriminant_Check is type Rec (Valid : Boolean) is record case Valid is when True => Counter : Integer; when False => null; end case; end record; R : Rec (Valid => False); I : Integer; begin I := R.Counter; end Show_Discriminant_Check;

Here, R's discriminant (Valid) is False, so we cannot access the Counter component, for it only exists when the Valid discriminant is True.

Division Check

The division check verifies that we're not trying to divide a value by zero when using the /, rem and mod operators. For example:

package Ops is function Div_Op (A, B : Integer) return Integer is (A / B); function Rem_Op (A, B : Integer) return Integer is (A rem B); function Mod_Op (A, B : Integer) return Integer is (A mod B); end Ops;
with Ops; use Ops; procedure Show_Division_Check is I : Integer; begin I := Div_Op (10, 0); I := Rem_Op (10, 0); I := Mod_Op (10, 0); end Show_Division_Check;

All three calls in the Show_Division_Check procedure — to the Div_Op, Rem_Op and Mod_Op functions — can raise an exception because we're using 0 as the second argument, which makes the division check in those functions fail.

Index Check

We use indices to access components of an array. An index check verifies that the index we're using to access a specific component is within the array's bounds. For example:

procedure Show_Index_Check is type Integer_Array is array (Positive range <>) of Integer; function Value_Of (A : Integer_Array; I : Integer) return Integer is type Half_Integer_Array is new Integer_Array (A'First .. A'First + A'Length / 2); A_2 : Half_Integer_Array := (others => 0); begin return A_2 (I); end Value_Of; Arr_1 : Integer_Array (1 .. 10) := (others => 1); begin Arr_1 (10) := Value_Of (Arr_1, 10); end Show_Index_Check;

The range of A_2 — which is passed as an argument to the Value_Of function — is 1 to 6. However, in that function call, we're trying to access position 10, which is outside A_2 's bounds.

Length Check

In array assignments, both arrays must have the same length. To ensure that this is the case, a length check is performed. For example:

procedure Show_Length_Check is type Integer_Array is array (Positive range <>) of Integer; procedure Assign (To : out Integer_Array; From : Integer_Array) is begin To := From; end Assign; Arr_1 : Integer_Array (1 .. 10); Arr_2 : Integer_Array (1 .. 9) := (others => 1); begin Assign (Arr_1, Arr_2); end Show_Length_Check;

Here, the length of Arr_1 is 10, while the length of Arr_2 is 9, so we cannot assign Arr_2 (From parameter) to Arr_1 (To parameter) in the Assign procedure.

Overflow Check

Operations on scalar objects might lead to overflow, which, if not checked, lead to wrong information being computed and stored. Therefore, an overflow check verifies that the value of a scalar object is within the base range of its type. For example:

procedure Show_Overflow_Check is A, B : Integer; begin A := Integer'Last; B := 1; A := A + B; end Show_Overflow_Check;

In this example, A already has the last possible value of the Integer'Base range, so increasing it by one causes an overflow error.

Range Check

The range check verifies that a scalar value is within a specific range — for instance, the range of a subtype. Let's see an example:

procedure Show_Range_Check is subtype Int_1_10 is Integer range 1 .. 10; I : Int_1_10; begin I := 11; end Show_Range_Check;

In this example, we're trying to assign 11 to the variable I of the Int_1_10 subtype, which has a range from 1 to 10. Since 11 is outside that range, the range check fails.

Tag Check

The tag check ensures that the tag of a tagged object matches the expected tag in a dispatching operation. For example:

package P is type T is tagged null record; type T1 is new T with null record; type T2 is new T with null record; end P;
with Ada.Text_IO; use Ada.Text_IO; with Ada.Tags; with P; use P; procedure Show_Tag_Check is A1 : T'Class := T1'(null record); A2 : T'Class := T2'(null record); begin Put_Line ("A1'Tag: " & Ada.Tags.Expanded_Name (A1'Tag)); Put_Line ("A2'Tag: " & Ada.Tags.Expanded_Name (A2'Tag)); A2 := A1; end Show_Tag_Check;

Here, A1 and A2 have different tags:

  • A1'Tag = T1'Tag, while

  • A2'Tag = T2'Tag.

Since the tags don't match, the tag check fails in the assignment of A1 to A2.

Accessibility Check

The accessibility check verifies that the accessibility level of an entity matches the expected level.

Todo

Add link to "Accessibility levels" section once it's available.

Let's look at an example that mixes access types and anonymous access types. Here, we use an anonymous access type in the declaration of A1 and a named access type in the declaration of A2:

package P is type T is tagged null record; type T_Class is access all T'Class; end P;
with P; use P; procedure Show_Accessibility_Check is A1 : access T'Class := new T; A2 : T_Class; begin A2 := T_Class (A1); end Show_Accessibility_Check;

The anonymous type (access T'Class), which is used in the declaration of A1, doesn't have the same accessibility level as the T_Class type. Therefore, the accessibility check fails during the T_Class (A1) conversion.

We can see the accessibility check failing in this example as well:

with P; use P; procedure Show_Accessibility_Check is A : access T'Class := new T; procedure P (A : T_Class) is null; begin P (T_Class (A)); end Show_Accessibility_Check;

Again, the check fails in the T_Class (A) conversion and raises a Program_Error exception.

Allocation Check

The allocation check ensures, when a task is about to be created, that its master has not been completed or the finalization has not been started.

This is an example adapted from AI-00280:

with Ada.Finalization; with Ada.Unchecked_Deallocation; package P is type T1 is new Ada.Finalization.Controlled with null record; procedure Finalize (X : in out T1); type T2 is new Ada.Finalization.Controlled with null record; procedure Finalize (X : in out T2); X1 : T1; type T2_Ref is access T2; procedure Free is new Ada.Unchecked_Deallocation (T2, T2_Ref); end P;
with Ada.Text_IO; use Ada.Text_IO; package body P is procedure Finalize (X : in out T1) is X2 : T2_Ref := new T2; begin Put_Line ("Finalizing T1..."); Free (X2); end Finalize; procedure Finalize (X : in out T2) is begin Put_Line ("Finalizing T2..."); end Finalize; end P;
with P; use P; procedure Show_Allocation_Check is X2 : T2_Ref := new T2; begin Free (X2); end Show_Allocation_Check;

Here, in the finalization of the X1 object of T1 type, we're trying to create an object of T2 type. This is forbidden and, therefore, the allocation check raises a Program_Error exception.

Elaboration Check

The elaboration check verifies that subprograms — or protected entries, or task activations — have been elaborated before being called.

This is an example adapted from AI-00064:

function P return Integer;
function P return Integer is begin return 1; end P;
with P; procedure Show_Elaboration_Check is function F return Integer; type Pointer_To_Func is access function return Integer; X : constant Pointer_To_Func := P'Access; Y : constant Integer := F; Z : constant Pointer_To_Func := X; -- Renaming-as-body function F return Integer renames Z.all; begin null; end Show_Elaboration_Check;

This is a curious example: first, we declare a function F and assign the value returned by this function to constant Y in its declaration. Then, we declare F as a renamed function, thereby providing a body to F — this is called renaming-as-body. Consequently, the compiler doesn't complain that a body is missing for function F. (If you comment out the function renaming, you'll see that the compiler can then detect the missing body.) Therefore, at runtime, the elaboration check fails because the body of the first declaration of the F function is actually missing.

Storage Check

The storage check ensures that the storage pool has enough space when allocating memory. Let's revisit an example that we discussed earlier:

package Custom_Types is type UInt_7 is range 0 .. 127; type UInt_7_Reserved_Access is access UInt_7 with Storage_Size => 8; end Custom_Types;
with Ada.Text_IO; use Ada.Text_IO; with Custom_Types; use Custom_Types; procedure Show_Storage_Check is RAV1, RAV2 : UInt_7_Reserved_Access; begin Put_Line ("Allocating RAV1..."); RAV1 := new UInt_7; Put_Line ("Allocating RAV2..."); RAV2 := new UInt_7; New_Line; end Show_Storage_Check;

On each allocation (new UInt_7), a storage check is performed. Because there isn't enough reserved storage space before the second allocation, the checks fails and raises a Storage_Error exception.

In the Ada Reference Manual

Ada.Exceptions package

Note

Parts of this section were originally published as Gem #142 : Exception-ally

The standard Ada run-time library provides the package Ada.Exceptions. This package provides a number of services to help analyze exceptions.

Each exception is associated with a (short) message that can be set by the code that raises the exception, as in the following code:

raise Constraint_Error with "some message";

Historically

Since Ada 2005, we can use the raise Constraint_Error with "some message" syntax. In Ada 95, you had to call the Raise_Exception procedure:

Ada.Exceptions.Raise_Exception         --  Ada 95
  (Constraint_Error'Identity, "some message");

In Ada 83, there was no way to do it at all.

The new syntax is now very convenient, and developers should be encouraged to provide as much information as possible along with the exception.

In the GNAT toolchain

The length of the message is limited to 200 characters by default in GNAT, and messages longer than that will be truncated.

In the Ada Reference Manual

Retrieving exception information

Exceptions also embed information set by the run-time itself that can be retrieved by calling the Exception_Information function. The function Exception_Information also displays the Exception_Message.

For example:

exception
    when E : others =>
        Put_Line (Ada.Exceptions.Exception_Information (E));

In the GNAT toolchain

In the case of GNAT, the information provided by an exception might include the source location where the exception was raised and a nonsymbolic traceback.

You can also retrieve this information individually. Here, you can use:

  • the Exception_Name functions — and its derivatives Wide_Exception_Name and Wide_Wide_Exception_Name — to retrieve the name of an exception.

  • the Exception_Message function to retrieve the message associated with an exception.

Let's see a complete example:

with Ada.Text_IO; use Ada.Text_IO; with Ada.Exceptions; use Ada.Exceptions; procedure Show_Exception_Info is Custom_Exception : exception; procedure Nested is begin raise Custom_Exception with "We got a problem"; end Nested; begin Nested; exception when E : others => Put_Line ("Exception info: " & Exception_Information (E)); Put_Line ("Exception name: " & Exception_Name (E)); Put_Line ("Exception msg: " & Exception_Message (E)); end Show_Exception_Info;

Collecting exceptions

Save_Occurrence

You can save an exception occurrence using the Save_Occurrence procedure. (Note that a Save_Occurrence function exists as well.)

For example, the following application collects exceptions into a list and displays them after running the Test_Exceptions procedure:

with Ada.Exceptions; use Ada.Exceptions; package Exception_Tests is Custom_Exception : exception; type Exception_Occurrences is array (Positive range <>) of Exception_Occurrence; procedure Test_Exceptions (Occurrences : in out Exception_Occurrences; Last_Occurence : out Integer); end Exception_Tests;
package body Exception_Tests is procedure Save_To_List (E : Exception_Occurrence; Occurrences : in out Exception_Occurrences; Last_Occurence : in out Integer) is L : Integer renames Last_Occurence; O : Exception_Occurrences renames Occurrences; begin L := L + 1; if L > O'Last then raise Constraint_Error with "Cannot save occurrence"; end if; Save_Occurrence (Target => O (L), Source => E); end Save_To_List; procedure Test_Exceptions (Occurrences : in out Exception_Occurrences; Last_Occurence : out Integer) is procedure Nested_1 is begin raise Custom_Exception with "We got a problem"; exception when E : others => Save_To_List (E, Occurrences, Last_Occurence); end Nested_1; procedure Nested_2 is begin raise Constraint_Error with "Constraint is not correct"; exception when E : others => Save_To_List (E, Occurrences, Last_Occurence); end Nested_2; begin Last_Occurence := 0; Nested_1; Nested_2; end Test_Exceptions; end Exception_Tests;
with Ada.Text_IO; use Ada.Text_IO; with Ada.Exceptions; use Ada.Exceptions; with Exception_Tests; use Exception_Tests; procedure Show_Exception_Info is L : Integer; O : Exception_Occurrences (1 .. 10); begin Test_Exceptions (O, L); for I in O 'First .. L loop Put_Line (Exception_Information (O (I))); end loop; end Show_Exception_Info;

In the Save_To_List procedure of the Exception_Tests package, we call the Save_Occurrence procedure to store the exception occurence to the Occurrences array. In the Show_Exception_Info, we display all the exception occurrences that we collected.

Read and Write attributes

Similarly, we can use files to read and write exception occurences. To do that, we can simply use the 'Read and 'Write attributes.

with Ada.Text_IO; with Ada.Streams.Stream_IO; use Ada.Streams.Stream_IO; with Ada.Exceptions; use Ada.Exceptions; procedure Exception_Occurrence_Stream is Custom_Exception : exception; S : Stream_Access; procedure Nested_1 is begin raise Custom_Exception with "We got a problem"; exception when E : others => Exception_Occurrence'Write (S, E); end Nested_1; procedure Nested_2 is begin raise Constraint_Error with "Constraint is not correct"; exception when E : others => Exception_Occurrence'Write (S, E); end Nested_2; F : File_Type; File_Name : constant String := "exceptions_file.bin"; begin Create (F, Out_File, File_Name); S := Stream (F); Nested_1; Nested_2; Close (F); Read_Exceptions : declare E : Exception_Occurrence; begin Open (F, In_File, File_Name); S := Stream (F); while not End_Of_File (F) loop Exception_Occurrence'Read (S, E); Ada.Text_IO.Put_Line (Exception_Information (E)); end loop; Close (F); end Read_Exceptions; end Exception_Occurrence_Stream;

In this example, we store the exceptions raised in the application in the exceptions_file.bin file. In the exception part of procedures Nested_1 and Nested_2, we call Exception_Occurrence'Write to store an exception occurence in the file. In the Read_Exceptions block, we read the exceptions from the the file by calling Exception_Occurrence'Read.

Debugging exceptions in the GNAT toolchain

Here is a typical exception handler that catches all unexpected exceptions in the application:

with Ada.Exceptions; with Ada.Text_IO; use Ada.Text_IO; procedure Main is procedure Nested is begin raise Constraint_Error with "some message"; end Nested; begin Nested; exception when E : others => Put_Line (Ada.Exceptions.Exception_Information (E)); end Main;

The output we get when running the application is not very informative. To get more information, we need to rerun the program in the debugger. To make the session more interesting though, we should add debug information in the executable, which means using the -g switch in the gnatmake command.

The session would look like the following (omitting some of the output from the debugger):

> rm *.o      # Cleanup previous compilation
> gnatmake -g main.adb
> gdb ./main
(gdb)  catch exception
(gdb)  run
Catchpoint 1, CONSTRAINT_ERROR at 0x0000000000402860 in main.nested () at main.adb:8
8               raise Constraint_Error with "some message";

(gdb) bt
#0  <__gnat_debug_raise_exception> (e=0x62ec40 <constraint_error>) at s-excdeb.adb:43
#1  0x000000000040426f in ada.exceptions.complete_occurrence (x=x@entry=0x637050)
at a-except.adb:934
#2  0x000000000040427b in ada.exceptions.complete_and_propagate_occurrence (
x=x@entry=0x637050) at a-except.adb:943
#3  0x00000000004042d0 in <__gnat_raise_exception> (e=0x62ec40 <constraint_error>,
message=...) at a-except.adb:982
#4  0x0000000000402860 in main.nested ()
#5  0x000000000040287c in main ()

And we now know exactly where the exception was raised. But in fact, we could have this information directly when running the application. For this, we need to bind the application with the switch -E, which tells the binder to store exception tracebacks in exception occurrences. Let's recompile and rerun the application.

> rm *.o   # Cleanup previous compilation
> gnatmake -g main.adb -bargs -E
> ./main

Exception name: CONSTRAINT_ERROR
Message: some message
Call stack traceback locations:
0x10b7e24d1 0x10b7e24ee 0x10b7e2472

The traceback, as is, is not very useful. We now need to use another tool that is bundled with GNAT, called addr2line. Here is an example of its use:

> addr2line -e main --functions --demangle 0x10b7e24d1 0x10b7e24ee 0x10b7e2472
/path/main.adb:8
_ada_main
/path/main.adb:12
main
/path/b~main.adb:240

This time we do have a symbolic backtrace, which shows information similar to what we got in the debugger.

For users on OSX machines, addr2line does not exist. On these machines, however, an equivalent solution exists. You need to link your application with an additional switch, and then use the tool atos, as in:

> rm *.o
> gnatmake -g main.adb -bargs -E -largs -Wl,-no_pie
> ./main

Exception name: CONSTRAINT_ERROR
Message: some message
Call stack traceback locations:
0x1000014d1 0x1000014ee 0x100001472
> atos -o main 0x1000014d1 0x1000014ee 0x100001472
main__nested.2550 (in main) (main.adb:8)
_ada_main (in main) (main.adb:12)
main (in main) + 90

We will now discuss a relatively new switch of the compiler, namely -gnateE. When used, this switch will generate extra information in exception messages.

Let's amend our test program to:

with Ada.Exceptions; with Ada.Text_IO; use Ada.Text_IO; procedure Main is procedure Nested (Index : Integer) is type T_Array is array (1 .. 2) of Integer; T : constant T_Array := (10, 20); begin Put_Line (T (Index)'Img); end Nested; begin Nested (3); exception when E : others => Put_Line (Ada.Exceptions.Exception_Information (E)); end Main;

When running the application, we see that the exception information (traceback) is the same as before, but this time the exception message is set automatically by the compiler. So we know we got a Constraint_Error because an incorrect index was used at the named source location (main.adb, line 10). But the significant addition is the second line of the message, which indicates exactly the cause of the error. Here, we wanted to get the element at index 3, in an array whose range of valid indexes is from 1 to 2. (No need for a debugger in this case.)

The column information on the first line of the exception message is also very useful when dealing with null pointers. For instance, a line such as:

A := Rec1.Rec2.Rec3.Rec4.all;

where each of the Rec is itself a pointer, might raise Constraint_Error with a message "access check failed". This indicates for sure that one of the pointers is null, and by using the column information it is generally easy to find out which one it is.

Exception renaming

We can rename exceptions by using the an exception renaming declaration in this form Renamed_Exception : exception renames Existing_Exception;. For example:

procedure Show_Exception_Renaming is CE : exception renames Constraint_Error; begin raise CE; end Show_Exception_Renaming;

Exception renaming creates a new view of the original exception. If we rename an exception from package A in package B, that exception will become visible in package B. For example:

package Internal_Exceptions is Int_E : exception; end Internal_Exceptions;
with Internal_Exceptions; package Test_Constraints is Ext_E : exception renames Internal_Exceptions.Int_E; end Test_Constraints;
with Ada.Text_IO; use Ada.Text_IO; with Ada.Exceptions; use Ada.Exceptions; with Test_Constraints; use Test_Constraints; procedure Show_Exception_Renaming_View is begin raise Ext_E; exception when E : others => Put_Line (Ada.Exceptions.Exception_Information (E)); end Show_Exception_Renaming_View;

Here, we're renaming the Int_E exception in the Test_Constraints package. The Int_E exception isn't directly visible in the Show_Exception_Renaming procedure because we're not withing the Internal_Exceptions package. However, it is indirectly visible in that procedure via the renaming (Ext_E) in the Test_Constraints package.

In the Ada Reference Manual

Out and Uninitialized

Note

This section was originally written by Robert Dewar and published as Gem #150: Out and Uninitialized

Perhaps surprisingly, the Ada standard indicates cases where objects passed to out and in out parameters might not be updated when a procedure terminates due to an exception. Let's take an example:

with Ada.Text_IO; use Ada.Text_IO; procedure Show_Out_Uninitialized is procedure Local (A : in out Integer; Error : Boolean) is begin A := 1; if Error then raise Program_Error; end if; end Local; B : Integer := 0; begin Local (B, Error => True); exception when Program_Error => Put_Line ("Value for B is" & Integer'Image (B)); -- "0" end Show_Out_Uninitialized;

This program outputs a value of 0 for B, whereas the code indicates that A is assigned before raising the exception, and so the reader might expect B to also be updated.

The catch, though, is that a compiler must by default pass objects of elementary types (scalars and access types) by copy and might choose to do so for other types (records, for example), including when passing out and in out parameters. So what happens is that while the formal parameter A is properly initialized, the exception is raised before the new value of A has been copied back into B (the copy will only happen on a normal return).

In the GNAT toolchain

In general, any code that reads the actual object passed to an out or in out parameter after an exception is suspect and should be avoided. GNAT has useful warnings here, so that if we simplify the above code to:

with Ada.Text_IO; use Ada.Text_IO; procedure Show_Out_Uninitialized_Warnings is procedure Local (A : in out Integer) is begin A := 1; raise Program_Error; end Local; B : Integer := 0; begin Local (B); exception when others => Put_Line ("Value for B is" & Integer'Image (B)); end Show_Out_Uninitialized_Warnings;

We now get a compilation warning that the pass-by-copy formal may have no effect.

Of course, GNAT is not able to point out all such errors (see first example above), which in general would require full flow analysis.

The behavior is different when using parameter types that the standard mandates be passed by reference, such as tagged types for instance. So the following code will work as expected, updating the actual parameter despite the exception:

with Ada.Text_IO; use Ada.Text_IO; procedure Show_Out_Initialized_Rec is type Rec is tagged record Field : Integer; end record; procedure Local (A : in out Rec) is begin A.Field := 1; raise Program_Error; end Local; V : Rec; begin V.Field := 0; Local (V); exception when others => Put_Line ("Value of Field is" & V.Field'Img); -- "1" end Show_Out_Initialized_Rec;

In the GNAT toolchain

It's worth mentioning that GNAT provides a pragma called Export_Procedure that forces reference semantics on out parameters. Use of this pragma would ensure updates of the actual parameter prior to abnormal completion of the procedure. However, this pragma only applies to library-level procedures, so the examples above have to be rewritten to avoid the use of a nested procedure, and really this pragma is intended mainly for use in interfacing with foreign code. The code below shows an example that ensures that B is set to 1 after the call to Local:

package Exported_Procedures is procedure Local (A : in out Integer; Error : Boolean); pragma Export_Procedure (Local, Mechanism => (A => Reference)); end Exported_Procedures;
package body Exported_Procedures is procedure Local (A : in out Integer; Error : Boolean) is begin A := 1; if Error then raise Program_Error; end if; end Local; end Exported_Procedures;
with Ada.Text_IO; use Ada.Text_IO; with Exported_Procedures; use Exported_Procedures; procedure Show_Out_Reference is B : Integer := 0; begin Local (B, Error => True); exception when Program_Error => Put_Line ("Value for B is" & Integer'Image (B)); -- "1" end Show_Out_Reference;

In the case of direct assignments to global variables, the behavior in the presence of exceptions is somewhat different. For predefined exceptions, most notably Constraint_Error, the optimization permissions allow some flexibility in whether a global variable is or is not updated when an exception occurs (see Ada RM 11.6). For instance, the following code makes an incorrect assumption:

X := 0;     -- about to try addition
Y := Y + 1; -- see if addition raises exception
X := 1      -- addition succeeded

A program is not justified in assuming that X = 0 if the addition raises an exception (assuming X is a global here). So any such assumptions in a program are incorrect code which should be fixed.

In the Ada Reference Manual

Suppressing checks

pragma Suppress

Note

This section was originally written by Gary Dismukes and published as Gem #63: The Effect of Pragma Suppress.

One of Ada's key strengths has always been its strong typing. The language imposes stringent checking of type and subtype properties to help prevent accidental violations of the type system that are a common source of program bugs in other less-strict languages such as C. This is done using a combination of compile-time restrictions (legality rules), that prohibit mixing values of different types, together with run-time checks to catch violations of various dynamic properties. Examples are checking values against subtype constraints and preventing dereferences of null access values.

At the same time, Ada does provide certain "loophole" features, such as Unchecked_Conversion, that allow selective bypassing of the normal safety features, which is sometimes necessary when interfacing with hardware or code written in other languages.

Ada also permits explicit suppression of the run-time checks that are there to ensure that various properties of objects are not violated. This suppression can be done using pragma Suppress, as well as by using a compile-time switch on most implementations — in the case of GNAT, with the -gnatp switch.

In addition to allowing all checks to be suppressed, pragma Suppress supports suppression of specific forms of check, such as Index_Check for array indexing, Range_Check for scalar bounds checking, and Access_Check for dereferencing of access values. (See section 11.5 of the Ada Reference Manual for further details.)

Here's a simple example of suppressing index checks within a specific subprogram:

procedure Main is
   procedure Sort_Array (A : in out Some_Array) is
      pragma Suppress (Index_Check);  -- eliminate check overhead
   begin
     ...
   end Sort_Array;
end Main;

Unlike a feature such as Unchecked_Conversion, however, the purpose of check suppression is not to enable programs to subvert the type system, though many programmers seem to have that misconception.

What's important to understand about pragma Suppress is that it only gives permission to the implementation to remove checks, but doesn't require such elimination. The intention of Suppress is not to allow bypassing of Ada semantics, but rather to improve efficiency, and the Ada Reference Manual has a clear statement to that effect in the note in RM-11.5, paragraph 29:

There is no guarantee that a suppressed check is actually removed; hence a pragma Suppress should be used only for efficiency reasons.

There is associated Implementation Advice that recommends that implementations should minimize the code executed for checks that have been suppressed, but it's still the responsibility of the programmer to ensure that the correct functioning of the program doesn't depend on checks not being performed.

There are various reasons why a compiler might choose not to remove a check. On some hardware, certain checks may be essentially free, such as null pointer checks or arithmetic overflow, and it might be impractical or add extra cost to suppress the check. Another example where it wouldn't make sense to remove checks is for an operation implemented by a call to a run-time routine, where the check might be only a small part of a more expensive operation done out of line.

Furthermore, in many cases GNAT can determine at compile time that a given run-time check is guaranteed to be violated. In such situations, it gives a warning that an exception will be raised, and generates code specifically to raise the exception. Here's an example:

X : Integer range 1..10 := ...;

..

if A > B then
   X := X + 1;
  ..
end if;

For the assignment incrementing X, the compiler will normally generate machine code equivalent to:

Temp := X + 1;
if Temp > 10 then
   raise Constraint_Error;
end if;
X := Temp;

If range checks are suppressed, then the compiler can just generate the increment and assignment. However, if the compiler is able to somehow prove that X = 10 at this point, it will issue a warning, and replace the entire assignment with simply:

raise Constraint_Error;

even though checks are suppressed. This is appropriate, because

  1. we don't care about the efficiency of buggy code, and

  2. there is no "extra" cost to the check, because if we reach that point, the code will unconditionally fail.

One other important thing to note about checks and pragma Suppress is this statement in the Ada RM (RM-11.5, paragraph 26):

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

In Ada, erroneous execution is a bad situation to be in, because it means that the execution of your program could have arbitrary nasty effects, such as unintended overwriting of memory. Note also that a program whose "correct" execution somehow depends on a given check being suppressed might work as the programmer expects, but could still fail when compiled with a different compiler, or for a different target, or even with a newer version of the same compiler. Other changes such as switching on optimization or making a change to a totally unrelated part of the code could also cause the code to start failing.

So it's definitely not wise to write code that relies on checks being removed. In fact, it really only makes sense to suppress checks once there's good reason to believe that the checks can't fail, as a result of testing or other analysis. Otherwise, you're removing an important safety feature of Ada that's intended to help catch bugs.

pragma Unsuppress

We can use pragma Unsuppress to reverse the effect of a pragma Suppress. While pragma Suppress gives permission to the compiler to remove a specific check, pragma Unsuppress revokes that permission.

Let's see an example:

procedure Show_Index_Check is type Integer_Array is array (Positive range <>) of Integer; pragma Suppress (Index_Check); -- from now on, the compiler may -- eliminate index checks... function Unchecked_Value_Of (A : Integer_Array; I : Integer) return Integer is type Half_Integer_Array is new Integer_Array (A'First .. A'First + A'Length / 2); A_2 : Half_Integer_Array := (others => 0); begin return A_2 (I); end Unchecked_Value_Of; pragma Unsuppress (Index_Check); -- from now on, index checks are -- typically performed... function Value_Of (A : Integer_Array; I : Integer) return Integer is type Half_Integer_Array is new Integer_Array (A'First .. A'First + A'Length / 2); A_2 : Half_Integer_Array := (others => 0); begin return A_2 (I); end Value_Of; Arr_1 : Integer_Array (1 .. 10) := (others => 1); begin Arr_1 (10) := Unchecked_Value_Of (Arr_1, 10); Arr_1 (10) := Value_Of (Arr_1, 10); end Show_Index_Check;

In this example, we first use a pragma Suppress (Index_Check), so the compiler is allowed to remove the index check from the Unchecked_Value_Of function. (Therefore, depending on the compiler, the call to the Unchecked_Value_Of function may complete without raising an exception.) Of course, in this specific example, suppressing the index check masks a severe issue.

In contrast, an index check is performed in the Value_Of function because of the pragma Unsuppress. As a result, the index checks fails in the call to this function, which raises a Constraint_Error exception.

In the Ada Reference Manual