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1.1.3 Conformity of an Implementation with the Standard
Implementation Requirements
1
{conformance
(of an implementation with the Standard)} A
conforming implementation shall:
1.a
Discussion: {implementation}
The implementation is the software and hardware
that implements the language. This includes compiler, linker, operating
system, hardware, etc.
1.b
We first define what it means
to ``conform'' in general -- basically, the implementation has to properly
implement the normative rules given throughout the standard. Then we
define what it means to conform to a Specialized Needs Annex -- the implementation
must support the core features plus the features of that Annex. Finally,
we define what it means to ``conform to the Standard'' -- this requires
support for the core language, and allows partial (but not conflicting)
support for the Specialized Needs Annexes.
2
- Translate and correctly execute legal
programs written in Ada, provided that they are not so large as to exceed
the capacity of the implementation;
3
- Identify all programs or program units
that are so large as to exceed the capacity of the implementation (or
raise an appropriate exception at run time);
3.a
Implementation defined: Capacity
limitations of the implementation.
4
- Identify all programs or program units
that contain errors whose detection is required by this International
Standard;
4.a
Discussion: Note that
we no longer use the term ``rejection'' of programs or program units.
We require that programs or program units with errors or that exceed
some capacity limit be ``identified.'' The way in which errors or capacity
problems are reported is not specified.
4.b
An implementation is allowed
to use standard error-recovery techniques. We do not disallow such techniques
from being used across compilation_unit
or compilation boundaries.
4.c
See also the Implementation
Requirements of 10.2, which disallow the execution
of illegal partitions.
5
- Supply all language-defined library
units required by this International Standard;
5.a
Implementation Note: An
implementation cannot add to or modify the visible part of a language-defined
library unit, except where such permission is explicitly granted, unless
such modifications are semantically neutral with respect to the client
compilation units of the library unit. An implementation defines the
contents of the private part and body of language-defined library units.
5.b
An implementation can add with_clauses
and use_clauses, since these modifications
are semantically neutral to clients. (The implementation might need with_clauses
in order to implement the private part, for example.) Similarly, an implementation
can add a private part even in cases where a private part is not shown
in the standard. Explicit declarations can be provided implicitly or
by renaming, provided the changes are semantically neutral.
5.c
{italics (implementation-defined)}
Wherever in the standard the text of a language-defined
library unit contains an italicized phrase starting with ``implementation-defined'',
the implementation's version will replace that phrase with some implementation-defined
text that is syntactically legal at that place, and follows any other
applicable rules.
5.d
Note that modifications are
permitted, even if there are other tools in the environment that can
detect the changes (such as a program library browser), so long as the
modifications make no difference with respect to the static or dynamic
semantics of the resulting programs, as defined by the standard.
6
- Contain no variations except those
explicitly permitted by this International Standard, or those that are
impossible or impractical to avoid given the implementation's execution
environment;
6.a
Implementation defined: Variations
from the standard that are impractical to avoid given the implementation's
execution environment.
6.b
Reason: The ``impossible
or impractical'' wording comes from AI-325. It takes some judgement and
common sense to interpret this. Restricting compilation units to less
than 4 lines is probably unreasonable, whereas restricting them to less
than 4 billion lines is probably reasonable (at least given today's technology).
We do not know exactly where to draw the line, so we have to make the
rule vague.
7
- Specify all such variations in the
manner prescribed by this International Standard.
8
{external
effect (of the execution of an Ada program)} {effect
(external)} The
external effect
of the execution of an Ada program is defined in terms of its interactions
with its external environment.
{external interaction}
The following are defined as
external interactions:
9
- Any interaction with an external file
(see A.7);
10
- The execution of certain code_statements
(see 13.8); which code_statements
cause external interactions is implementation defined.
10.a
Implementation defined: Which
code_statements cause external interactions.
11
- Any call on an imported subprogram
(see Annex B), including any parameters passed
to it;
12
- Any result returned or exception propagated
from a main subprogram (see 10.2) or an exported
subprogram (see Annex B) to an external caller;
12.a
Discussion: By ``result
returned'' we mean to include function results and values returned in
[in] out parameters.
12.a.1/1
{8652/0094}
{AI95-00119}
The lack of a result from a program that does not terminate is also
included here.
13
- [Any read or update of an atomic or
volatile object (see C.6);]
14
- The values of imported and exported
objects (see Annex B) at the time of any other
interaction with the external environment.
14.a
To be honest: Also other
uses of imported and exported entities, as defined by the implementation,
if the implementation supports such pragmas.
15
A conforming implementation of this International
Standard shall produce for the execution of a given Ada program a set
of interactions with the external environment whose order and timing
are consistent with the definitions and requirements of this International
Standard for the semantics of the given program.
15.a
Ramification: There is
no need to produce any of the ``internal effects'' defined for the semantics
of the program -- all of these can be optimized away -- so long as an
appropriate sequence of external interactions is produced.
15.b
Discussion: See also
11.6 which specifies various liberties associated
with optimizations in the presence of language-defined checks, that could
change the external effects that might be produced. These alternative
external effects are still consistent with the standard, since 11.6
is part of the standard.
15.c
Note also that we only require
``an appropriate sequence of external interactions'' rather than
``the same sequence...'' An optimizer may cause a different sequence
of external interactions to be produced than would be produced without
the optimizer, so long as the new sequence still satisfies the requirements
of the standard. For example, optimization might affect the relative
rate of progress of two concurrent tasks, thereby altering the order
in which two external interactions occur.
15.d
Note that RM83 explicitly mentions
the case of an ``exact effect'' of a program, but since so few programs
have their effects defined that exactly, we don't even mention this ``special''
case. In particular, almost any program that uses floating point or tasking
has to have some level of inexactness in the specification of its effects.
And if one includes aspects of the timing of the external interactions
in the external effect of the program (as is appropriate for a real-time
language), no ``exact effect'' can be specified. For example, if two
external interactions initiated by a single task are separated by a ``delay
1.0;'' then the language rules imply that the two external interactions
have to be separated in time by at least one second, as defined by the
clock associated with the delay_relative_statement.
This in turn implies that the time at which an external interaction occurs
is part of the characterization of the external interaction, at least
in some cases, again making the specification of the required ``exact
effect'' impractical.
16
An implementation that conforms to this Standard
shall support each capability required by the core language as specified.
In addition, an implementation that conforms to this Standard may conform
to one or more Specialized Needs Annexes (or to none). Conformance to
a Specialized Needs Annex means that each capability required by the
Annex is provided as specified.
16.a
Discussion: The last
sentence defines what it means to say that an implementation conforms
to a Specialized Needs Annex, namely, only by supporting all capabilities
required by the Annex.
17
An implementation conforming to this International
Standard may provide additional attributes, library units, and pragmas.
However, it shall not provide any attribute, library unit, or pragma
having the same name as an attribute, library unit, or pragma (respectively)
specified in a Specialized Needs Annex unless the provided construct
is either as specified in the Specialized Needs Annex or is more limited
in capability than that required by the Annex. A program that attempts
to use an unsupported capability of an Annex shall either be identified
by the implementation before run time or shall raise an exception at
run time.
17.a
Discussion: The last
sentence of the preceding paragraph defines what an implementation is
allowed to do when it does not "conform" to a Specialized Needs
Annex. In particular, the sentence forbids implementations from providing
a construct with the same name as a corresponding construct in a Specialized
Needs Annex but with a different syntax (e.g., an extended syntax) or
quite different semantics. The phrase concerning "more limited in
capability" is intended to give permission to provide a partial
implementation, such as not implementing a subprogram in a package or
having a restriction not permitted by an implementation that conforms
to the Annex. For example, a partial implementation of the package Ada.Decimal
might have Decimal.Max_Decimal_Digits as 15 (rather than the required
18). This allows a partial implementation to grow to a fully conforming
implementation.
17.b
A restricted implementation
might be restricted by not providing some subprograms specified in one
of the packages defined by an Annex. In this case, a program that tries
to use the missing subprogram will usually fail to compile. Alternatively,
the implementation might declare the subprogram as abstract, so it cannot
be called. {Program_Error (raised by failure of run-time check)}
Alternatively, a subprogram body might be implemented
just to raise Program_Error. The advantage of this approach is that a
program to be run under a fully conforming Annex implementation can be
checked syntactically and semantically under an implementation that only
partially supports the Annex. Finally, an implementation might provide
a package declaration without the corresponding body, so that programs
can be compiled, but partitions cannot be built and executed.
17.c
To ensure against wrong answers
being delivered by a partial implementation, implementers are required
to raise an exception when a program attempts to use an unsupported capability
and this can be detected only at run time. For example, a partial implementation
of Ada.Decimal might require the length of the Currency string to be
1, and hence, an exception would be raised if a subprogram were called
in the package Edited_Output with a length greater than 1.
Documentation Requirements
18
{implementation defined}
{unspecified} {specified
(not!)} {implementation-dependent:
See unspecified} {documentation
(required of an implementation)} Certain
aspects of the semantics are defined to be either
implementation defined
or
unspecified. In such cases, the set of possible effects is
specified, and the implementation may choose any effect in the set. Implementations
shall document their behavior in implementation-defined situations, but
documentation is not required for unspecified situations. The implementation-defined
characteristics are summarized in
Annex M.
18.a
Discussion: We used to
use the term ``implementation dependent'' instead of ``unspecified''.
However, that sounded too much like ``implementation defined''. Furthermore,
the term ``unspecified'' is used in the ANSI C and POSIX standards for
this purpose, so that is another advantage. We also use ``not specified''
and ``not specified by the language'' as synonyms for ``unspecified.''
The documentation requirement is the only difference between implementation
defined and unspecified.
18.b
Note that the ``set of possible
effects'' can be ``all imaginable effects'', as is the case with erroneous
execution.
19
The implementation may choose to document implementation-defined
behavior either by documenting what happens in general, or by providing
some mechanism for the user to determine what happens in a particular
case.
19.a
Discussion: For example,
if the standard says that library unit elaboration order is implementation
defined, the implementation might describe (in its user's manual) the
algorithm it uses to determine the elaboration order. On the other hand,
the implementation might provide a command that produces a description
of the elaboration order for a partition upon request from the user.
It is also acceptable to provide cross references to existing documentation
(for example, a hardware manual), where appropriate.
19.b
Note that dependence of a program
on implementation-defined or unspecified functionality is not defined
to be an error; it might cause the program to be less portable, however.
Implementation Advice
20
{Program_Error (raised by
failure of run-time check)} If an implementation
detects the use of an unsupported Specialized Needs Annex feature at
run time, it should raise Program_Error if feasible.
20.a
Reason: The reason we
don't require Program_Error is that there are situations where
other exceptions might make sense. For example, if the Real Time Systems
Annex requires that the range of System.Priority include at least 30
values, an implementation could conform to the Standard (but not to the
Annex) if it supported only 12 values. Since the rules of the language
require Constraint_Error to be raised for out-of-range values, we cannot
require Program_Error to be raised instead.
21
If an implementation wishes to provide implementation-defined
extensions to the functionality of a language-defined library unit, it
should normally do so by adding children to the library unit.
21.a
Implementation Note: If
an implementation has support code (``run-time system code'') that is
needed for the execution of user-defined code, it can put that support
code in child packages of System. Otherwise, it has to use some trick
to avoid polluting the user's namespace. It is important that such tricks
not be available to user-defined code (not in the standard mode, at least)
-- that would defeat the purpose.
22
2 The above requirements
imply that an implementation conforming to this Standard may support
some of the capabilities required by a Specialized Needs Annex without
supporting all required capabilities.
22.a
Discussion: A conforming
implementation can partially support a Specialized Needs Annex. Such
an implementation does not conform to the Annex, but it does conform
to the Standard.
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