1.1.5 Classification of Errors
Implementation Requirements
The language definition
classifies errors into several different categories:
Errors that are required to be detected prior to
run time by every Ada implementation;
These errors correspond to any violation
of a rule given in this International Standard, other than those listed
below. In particular, violation of any rule that uses the terms shall,
allowed, permitted, legal, or illegal belongs to this category. Any program
that contains such an error is not a legal Ada program; on the other
hand, the fact that a program is legal does not mean, per se,
that the program is free from other forms of error.
{compile-time
error} {error
(compile-time)} {link-time
error: See post-compilation error} {error
(link-time)} The rules are further classified
as either compile time rules, or post compilation rules, depending on
whether a violation has to be detected at the time a compilation unit
is submitted to the compiler, or may be postponed until the time a compilation
unit is incorporated into a partition of a program.
Ramification: See, for example,
10.1.3,
“
Subunits of Compilation Units”,
for some errors that are detected only after compilation. Implementations
are allowed, but not required, to detect post compilation rules at compile
time when possible.
Errors that are required to be detected at run
time by the execution of an Ada program;
{run-time
error} {error
(run-time)} The corresponding error situations
are associated with the names of the predefined exceptions. Every Ada
compiler is required to generate code that raises the corresponding exception
if such an error situation arises during program execution. [If such
an error situation is certain to arise in every execution of a construct,
then an implementation is allowed (although not required) to report this
fact at compilation time.]
Bounded errors;
The language rules define certain kinds
of errors that need not be detected either prior to or during run time,
but if not detected, the range of possible effects shall be bounded.
{bounded error} The
errors of this category are called
bounded errors.
{Program_Error
(raised by failure of run-time check)} The
possible effects of a given bounded error are specified for each such
error, but in any case one possible effect of a bounded error is the
raising of the exception Program_Error.
Erroneous execution.
{erroneous
execution} In addition to bounded errors,
the language rules define certain kinds of errors as leading to
erroneous
execution. Like bounded errors, the implementation need not detect
such errors either prior to or during run time. Unlike bounded errors,
there is no language-specified bound on the possible effect of erroneous
execution; the effect is in general not predictable.
Ramification: Executions are erroneous,
not programs or parts of programs. Once something erroneous happens,
the execution of the entire program is erroneous from that point on,
and potentially before given possible reorderings permitted by
11.6
and elsewhere. We cannot limit it to just one partition, since partitions
are not required to live in separate address spaces. (But implementations
are encouraged to limit it as much as possible.)
Suppose a program contains a pair of things
that will be executed “in an arbitrary order.” It is possible
that one order will result in something sensible, whereas the other order
will result in erroneous execution. If the implementation happens to
choose the first order, then the execution is not erroneous. This may
seem odd, but it is not harmful.
Saying that something is erroneous is semantically
equivalent to saying that the behavior is unspecified. However, “erroneous”
has a slightly more disapproving flavor.
Implementation Permissions
[
{mode of operation
(nonstandard)} {nonstandard
mode} An implementation may provide
nonstandard
modes of operation. Typically these modes would be selected by a
pragma or
by a command line switch when the compiler is invoked. When operating
in a nonstandard mode, the implementation may reject
compilation_units
that do not conform to additional requirements associated with the mode,
such as an excessive number of warnings or violation of coding style
guidelines. Similarly, in a nonstandard mode, the implementation may
apply special optimizations or alternative algorithms that are only meaningful
for programs that satisfy certain criteria specified by the implementation.
{mode of operation (standard)}
{standard mode}
In any case, an implementation shall support a
standard
mode that conforms to the requirements of this International Standard;
in particular, in the standard mode, all legal
compilation_units
shall be accepted.]
Discussion: These permissions are designed
to authorize explicitly the support for alternative modes. Of course,
nothing we say can prevent them anyway, but this (redundant) paragraph
is designed to indicate that such alternative modes are in some sense
“approved” and even encouraged where they serve the specialized
needs of a given user community, so long as the standard mode, designed
to foster maximum portability, is always available.
Implementation Advice
{Program_Error (raised
by failure of run-time check)} If an implementation
detects a bounded error or erroneous execution, it should raise Program_Error.
Implementation Advice: If a bounded error
or erroneous execution is detected, Program_Error should be raised.
Wording Changes from Ada 83
Some situations that are erroneous in Ada 83
are no longer errors at all. For example, depending on the parameter
passing mechanism when unspecified is possibly non-portable, but not
erroneous.
Other situations that are erroneous in Ada 83
are changed to be bounded errors. In particular, evaluating an uninitialized
scalar variable is a bounded error. {
Program_Error (raised by failure
of run-time check)}
The possible results are
to raise Program_Error (as always), or to produce a machine-representable
value (which might not be in the subtype of the variable). {
Constraint_Error
(raised by failure of run-time check)}
Violating
a Range_Check or Overflow_Check raises Constraint_Error, even if the
value came from an uninitialized variable. This means that optimizers
can no longer “assume” that all variables are initialized
within their subtype's range. Violating a check that is suppressed remains
erroneous.
The “incorrect order dependences”
category of errors is removed. All such situations are simply considered
potential non-portabilities. This category was removed due to the difficulty
of defining what it means for two executions to have a “different
effect.” For example, if a function with a side-effect is called
twice in a single expression, it is not in principle possible for the
compiler to decide whether the correctness of the resulting program depends
on the order of execution of the two function calls. A compile time warning
might be appropriate, but raising of Program_Error at run time would
not be.
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