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3.2.4 Validity Checking

The Ada 95 Reference Manual has specific requirements for checking for invalid values. In particular, RM 13.9.1 requires that the evaluation of invalid values (for example from unchecked conversions), not result in erroneous execution. In GNAT, the result of such an evaluation in normal default mode is to either use the value unmodified, or to raise Constraint_Error in those cases where use of the unmodified value would cause erroneous execution. The cases where unmodified values might lead to erroneous execution are case statements (where a wild jump might result from an invalid value), and subscripts on the left hand side (where memory corruption could occur as a result of an invalid value).

The -gnatVx switch allows more control over the validity checking mode. The x argument is a string of letters that indicate validity checks that are performed or not performed in addition to the default checks described above.

All validity checks. All validity checks are turned on. That is, -gnatVa is equivalent to gnatVcdfimorst.
Validity checks for copies. The right hand side of assignments, and the initializing values of object declarations are validity checked.
Default (RM) validity checks. Some validity checks are done by default following normal Ada semantics (RM 13.9.1 (9-11)). A check is done in case statements that the expression is within the range of the subtype. If it is not, Constraint_Error is raised. For assignments to array components, a check is done that the expression used as index is within the range. If it is not, Constraint_Error is raised. Both these validity checks may be turned off using switch -gnatVD. They are turned on by default. If -gnatVD is specified, a subsequent switch -gnatVd will leave the checks turned on. Switch -gnatVD should be used only if you are sure that all such expressions have valid values. If you use this switch and invalid values are present, then the program is erroneous, and wild jumps or memory overwriting may occur.
Validity checks for floating-point values. In the absence of this switch, validity checking occurs only for discrete values. If -gnatVf is specified, then validity checking also applies for floating-point values, and NaN's and infinities are considered invalid, as well as out of range values for constrained types. Note that this means that standard IEEE infinity mode is not allowed. The exact contexts in which floating-point values are checked depends on the setting of other options. For example, -gnatVif or -gnatVfi (the order does not matter) specifies that floating-point parameters of mode in should be validity checked.
Validity checks for in mode parameters Arguments for parameters of mode in are validity checked in function and procedure calls at the point of call.
Validity checks for in out mode parameters. Arguments for parameters of mode in out are validity checked in procedure calls at the point of call. The 'm' here stands for modify, since this concerns parameters that can be modified by the call. Note that there is no specific option to test out parameters, but any reference within the subprogram will be tested in the usual manner, and if an invalid value is copied back, any reference to it will be subject to validity checking.
No validity checks. This switch turns off all validity checking, including the default checking for case statements and left hand side subscripts. Note that the use of the switch -gnatp suppresses all run-time checks, including validity checks, and thus implies -gnatVn. When this switch is used, it cancels any other -gnatV previously issued.
Validity checks for operator and attribute operands. Arguments for predefined operators and attributes are validity checked. This includes all operators in package Standard, the shift operators defined as intrinsic in package Interfaces and operands for attributes such as Pos. Checks are also made on individual component values for composite comparisons.
Validity checks for parameters. This controls the treatment of parameters within a subprogram (as opposed to -gnatVi and -gnatVm which control validity testing of parameters on a call. If either of these call options is used, then normally an assumption is made within a subprogram that the input arguments have been validity checking at the point of call, and do not need checking again within a subprogram). If -gnatVp is set, then this assumption is not made, and parameters are not assumed to be valid, so their validity will be checked (or rechecked) within the subprogram.
Validity checks for function returns. The expression in return statements in functions is validity checked.
Validity checks for subscripts. All subscripts expressions are checked for validity, whether they appear on the right side or left side (in default mode only left side subscripts are validity checked).
Validity checks for tests. Expressions used as conditions in if, while or exit statements are checked, as well as guard expressions in entry calls.

The -gnatV switch may be followed by a string of letters to turn on a series of validity checking options. For example, -gnatVcr specifies that in addition to the default validity checking, copies and function return expressions are to be validity checked. In order to make it easier to specify the desired combination of effects, the upper case letters CDFIMORST may be used to turn off the corresponding lower case option. Thus -gnatVaM turns on all validity checking options except for checking of in out procedure arguments.

The specification of additional validity checking generates extra code (and in the case of -gnatVa the code expansion can be substantial. However, these additional checks can be very useful in detecting uninitialized variables, incorrect use of unchecked conversion, and other errors leading to invalid values. The use of pragma Initialize_Scalars is useful in conjunction with the extra validity checking, since this ensures that wherever possible uninitialized variables have invalid values.

See also the pragma Validity_Checks which allows modification of the validity checking mode at the program source level, and also allows for temporary disabling of validity checks.