18. Compatibility and Porting Guide

This chapter presents some guidelines for developing portable Ada code, describes the compatibility issues that may arise between GNAT and other Ada compilation systems (including those for Ada 83), and shows how GNAT can expedite porting applications developed in other Ada environments.

18.1. Writing Portable Fixed-Point Declarations

The Ada Reference Manual gives an implementation freedom to choose bounds that are narrower by Small from the given bounds. For example, if we write

type F1 is delta 1.0 range -128.0 .. +128.0;

then the implementation is allowed to choose -128.0 .. +127.0 if it likes, but is not required to do so.

This leads to possible portability problems, so let’s have a closer look at this, and figure out how to avoid these problems.

First, why does this freedom exist, and why would an implementation take advantage of it? To answer this, take a closer look at the type declaration for F1 above. If the compiler uses the given bounds, it would need 9 bits to hold the largest positive value (and typically that means 16 bits on all machines). But if the implementation chooses the +127.0 bound then it can fit values of the type in 8 bits.

Why not make the user write +127.0 if that’s what is wanted? The rationale is that if you are thinking of fixed point as a kind of ‘poor man’s floating-point’, then you don’t want to be thinking about the scaled integers that are used in its representation. Let’s take another example:

type F2 is delta 2.0**(-15) range -1.0 .. +1.0;

Looking at this declaration, it seems casually as though it should fit in 16 bits, but again that extra positive value +1.0 has the scaled integer equivalent of 2**15 which is one too big for signed 16 bits. The implementation can treat this as:

type F2 is delta 2.0**(-15) range -1.0 .. +1.0-(2.0**(-15));

and the Ada language design team felt that this was too annoying to require. We don’t need to debate this decision at this point, since it is well established (the rule about narrowing the ranges dates to Ada 83).

But the important point is that an implementation is not required to do this narrowing, so we have a potential portability problem. We could imagine three types of implementation:

  1. those that narrow the range automatically if they can figure out that the narrower range will allow storage in a smaller machine unit,
  2. those that will narrow only if forced to by a 'Size clause, and
  3. those that will never narrow.

Now if we are language theoreticians, we can imagine a fourth approach: to narrow all the time, e.g. to treat

type F3 is delta 1.0 range -10.0 .. +23.0;

as though it had been written:

type F3 is delta 1.0 range -9.0 .. +22.0;

But although technically allowed, such a behavior would be hostile and silly, and no real compiler would do this. All real compilers will fall into one of the categories (a), (b) or (c) above.

So, how do you get the compiler to do what you want? The answer is give the actual bounds you want, and then use a 'Small clause and a 'Size clause to absolutely pin down what the compiler does. E.g., for F2 above, we will write:

My_Small : constant := 2.0**(-15);
My_First : constant := -1.0;
My_Last  : constant := +1.0 - My_Small;

type F2 is delta My_Small range My_First .. My_Last;

and then add

for F2'Small use my_Small;
for F2'Size  use 16;

In practice all compilers will do the same thing here and will give you what you want, so the above declarations are fully portable. If you really want to play language lawyer and guard against ludicrous behavior by the compiler you could add

Test1 : constant := 1 / Boolean'Pos (F2'First = My_First);
Test2 : constant := 1 / Boolean'Pos (F2'Last  = My_Last);

One or other or both are allowed to be illegal if the compiler is behaving in a silly manner, but at least the silly compiler will not get away with silently messing with your (very clear) intentions.

If you follow this scheme you will be guaranteed that your fixed-point types will be portable.

18.2. Compatibility with Ada 83

Ada 95 and the subsequent revisions Ada 2005 and Ada 2012 are highly upwards compatible with Ada 83. In particular, the design intention was that the difficulties associated with moving from Ada 83 to later versions of the standard should be no greater than those that occur when moving from one Ada 83 system to another.

However, there are a number of points at which there are minor incompatibilities. The Ada 95 Annotated Reference Manual contains full details of these issues as they relate to Ada 95, and should be consulted for a complete treatment. In practice the following subsections treat the most likely issues to be encountered.

18.2.2. More deterministic semantics

  • Conversions

    Conversions from real types to integer types round away from 0. In Ada 83 the conversion Integer(2.5) could deliver either 2 or 3 as its value. This implementation freedom was intended to support unbiased rounding in statistical applications, but in practice it interfered with portability. In Ada 95 the conversion semantics are unambiguous, and rounding away from 0 is required. Numeric code may be affected by this change in semantics. Note, though, that this issue is no worse than already existed in Ada 83 when porting code from one vendor to another.

  • Tasking

    The Real-Time Annex introduces a set of policies that define the behavior of features that were implementation dependent in Ada 83, such as the order in which open select branches are executed.

18.2.3. Changed semantics

The worst kind of incompatibility is one where a program that is legal in Ada 83 is also legal in Ada 95 but can have an effect in Ada 95 that was not possible in Ada 83. Fortunately this is extremely rare, but the one situation that you should be alert to is the change in the predefined type Character from 7-bit ASCII to 8-bit Latin-1.

  • Range of type ``Character``

    The range of Standard.Character is now the full 256 characters of Latin-1, whereas in most Ada 83 implementations it was restricted to 128 characters. Although some of the effects of this change will be manifest in compile-time rejection of legal Ada 83 programs it is possible for a working Ada 83 program to have a different effect in Ada 95, one that was not permitted in Ada 83. As an example, the expression Character'Pos(Character'Last) returned 127 in Ada 83 and now delivers 255 as its value. In general, you should look at the logic of any character-processing Ada 83 program and see whether it needs to be adapted to work correctly with Latin-1. Note that the predefined Ada 95 API has a character handling package that may be relevant if code needs to be adapted to account for the additional Latin-1 elements. The desirable fix is to modify the program to accommodate the full character set, but in some cases it may be convenient to define a subtype or derived type of Character that covers only the restricted range.

18.2.4. Other language compatibility issues

  • -gnat83 switch

    All implementations of GNAT provide a switch that causes GNAT to operate in Ada 83 mode. In this mode, some but not all compatibility problems of the type described above are handled automatically. For example, the new reserved words introduced in Ada 95 and Ada 2005 are treated simply as identifiers as in Ada 83. However, in practice, it is usually advisable to make the necessary modifications to the program to remove the need for using this switch. See the Compiling Different Versions of Ada section in the GNAT User’s Guide.

  • Support for removed Ada 83 pragmas and attributes

    A number of pragmas and attributes from Ada 83 were removed from Ada 95, generally because they were replaced by other mechanisms. Ada 95 and Ada 2005 compilers are allowed, but not required, to implement these missing elements. In contrast with some other compilers, GNAT implements all such pragmas and attributes, eliminating this compatibility concern. These include pragma Interface and the floating point type attributes (Emax, Mantissa, etc.), among other items.

18.3. Compatibility between Ada 95 and Ada 2005

Although Ada 2005 was designed to be upwards compatible with Ada 95, there are a number of incompatibilities. Several are enumerated below; for a complete description please see the Annotated Ada 2005 Reference Manual, or section 9.1.1 in Rationale for Ada 2005.

  • New reserved words.

    The words interface, overriding and synchronized are reserved in Ada 2005. A pre-Ada 2005 program that uses any of these as an identifier will be illegal.

  • New declarations in predefined packages.

    A number of packages in the predefined environment contain new declarations: Ada.Exceptions, Ada.Real_Time, Ada.Strings, Ada.Strings.Fixed, Ada.Strings.Bounded, Ada.Strings.Unbounded, Ada.Strings.Wide_Fixed, Ada.Strings.Wide_Bounded, Ada.Strings.Wide_Unbounded, Ada.Tags, Ada.Text_IO, and Interfaces.C. If an Ada 95 program does a with and use of any of these packages, the new declarations may cause name clashes.

  • Access parameters.

    A nondispatching subprogram with an access parameter cannot be renamed as a dispatching operation. This was permitted in Ada 95.

  • Access types, discriminants, and constraints.

    Rule changes in this area have led to some incompatibilities; for example, constrained subtypes of some access types are not permitted in Ada 2005.

  • Aggregates for limited types.

    The allowance of aggregates for limited types in Ada 2005 raises the possibility of ambiguities in legal Ada 95 programs, since additional types now need to be considered in expression resolution.

  • Fixed-point multiplication and division.

    Certain expressions involving ‘*’ or ‘/’ for a fixed-point type, which were legal in Ada 95 and invoked the predefined versions of these operations, are now ambiguous. The ambiguity may be resolved either by applying a type conversion to the expression, or by explicitly invoking the operation from package Standard.

  • Return-by-reference types.

    The Ada 95 return-by-reference mechanism has been removed. Instead, the user can declare a function returning a value from an anonymous access type.

18.4. Implementation-dependent characteristics

Although the Ada language defines the semantics of each construct as precisely as practical, in some situations (for example for reasons of efficiency, or where the effect is heavily dependent on the host or target platform) the implementation is allowed some freedom. In porting Ada 83 code to GNAT, you need to be aware of whether / how the existing code exercised such implementation dependencies. Such characteristics fall into several categories, and GNAT offers specific support in assisting the transition from certain Ada 83 compilers.

18.4.1. Implementation-defined pragmas

Ada compilers are allowed to supplement the language-defined pragmas, and these are a potential source of non-portability. All GNAT-defined pragmas are described in Implementation Defined Pragmas, and these include several that are specifically intended to correspond to other vendors’ Ada 83 pragmas. For migrating from VADS, the pragma Use_VADS_Size may be useful. For compatibility with HP Ada 83, GNAT supplies the pragmas Extend_System, Ident, Inline_Generic, Interface_Name, Passive, Suppress_All, and Volatile. Other relevant pragmas include External and Link_With. Some vendor-specific Ada 83 pragmas (Share_Generic, Subtitle, and Title) are recognized, thus avoiding compiler rejection of units that contain such pragmas; they are not relevant in a GNAT context and hence are not otherwise implemented.

18.4.2. Implementation-defined attributes

Analogous to pragmas, the set of attributes may be extended by an implementation. All GNAT-defined attributes are described in Implementation Defined Attributes, and these include several that are specifically intended to correspond to other vendors’ Ada 83 attributes. For migrating from VADS, the attribute VADS_Size may be useful. For compatibility with HP Ada 83, GNAT supplies the attributes Bit, Machine_Size and Type_Class.

18.4.3. Libraries

Vendors may supply libraries to supplement the standard Ada API. If Ada 83 code uses vendor-specific libraries then there are several ways to manage this in Ada 95 and later versions of the standard:

  • If the source code for the libraries (specs and bodies) are available, then the libraries can be migrated in the same way as the application.
  • If the source code for the specs but not the bodies are available, then you can reimplement the bodies.
  • Some features introduced by Ada 95 obviate the need for library support. For example most Ada 83 vendors supplied a package for unsigned integers. The Ada 95 modular type feature is the preferred way to handle this need, so instead of migrating or reimplementing the unsigned integer package it may be preferable to retrofit the application using modular types.

18.4.4. Elaboration order

The implementation can choose any elaboration order consistent with the unit dependency relationship. This freedom means that some orders can result in Program_Error being raised due to an ‘Access Before Elaboration’: an attempt to invoke a subprogram before its body has been elaborated, or to instantiate a generic before the generic body has been elaborated. By default GNAT attempts to choose a safe order (one that will not encounter access before elaboration problems) by implicitly inserting Elaborate or Elaborate_All pragmas where needed. However, this can lead to the creation of elaboration circularities and a resulting rejection of the program by gnatbind. This issue is thoroughly described in the Elaboration Order Handling in GNAT appendix in the GNAT User’s Guide. In brief, there are several ways to deal with this situation:

  • Modify the program to eliminate the circularities, e.g., by moving elaboration-time code into explicitly-invoked procedures
  • Constrain the elaboration order by including explicit Elaborate_Body or Elaborate pragmas, and then inhibit the generation of implicit Elaborate_All pragmas either globally (as an effect of the -gnatE switch) or locally (by selectively suppressing elaboration checks via pragma Suppress(Elaboration_Check) when it is safe to do so).

18.4.5. Target-specific aspects

Low-level applications need to deal with machine addresses, data representations, interfacing with assembler code, and similar issues. If such an Ada 83 application is being ported to different target hardware (for example where the byte endianness has changed) then you will need to carefully examine the program logic; the porting effort will heavily depend on the robustness of the original design. Moreover, Ada 95 (and thus Ada 2005 and Ada 2012) are sometimes incompatible with typical Ada 83 compiler practices regarding implicit packing, the meaning of the Size attribute, and the size of access values. GNAT’s approach to these issues is described in Representation Clauses.

18.5. Compatibility with Other Ada Systems

If programs avoid the use of implementation dependent and implementation defined features, as documented in the Ada Reference Manual, there should be a high degree of portability between GNAT and other Ada systems. The following are specific items which have proved troublesome in moving Ada 95 programs from GNAT to other Ada 95 compilers, but do not affect porting code to GNAT. (As of January 2007, GNAT is the only compiler available for Ada 2005; the following issues may or may not arise for Ada 2005 programs when other compilers appear.)

  • Ada 83 Pragmas and Attributes

    Ada 95 compilers are allowed, but not required, to implement the missing Ada 83 pragmas and attributes that are no longer defined in Ada 95. GNAT implements all such pragmas and attributes, eliminating this as a compatibility concern, but some other Ada 95 compilers reject these pragmas and attributes.

  • Specialized Needs Annexes

    GNAT implements the full set of special needs annexes. At the current time, it is the only Ada 95 compiler to do so. This means that programs making use of these features may not be portable to other Ada 95 compilation systems.

  • Representation Clauses

    Some other Ada 95 compilers implement only the minimal set of representation clauses required by the Ada 95 reference manual. GNAT goes far beyond this minimal set, as described in the next section.

18.6. Representation Clauses

The Ada 83 reference manual was quite vague in describing both the minimal required implementation of representation clauses, and also their precise effects. Ada 95 (and thus also Ada 2005) are much more explicit, but the minimal set of capabilities required is still quite limited.

GNAT implements the full required set of capabilities in Ada 95 and Ada 2005, but also goes much further, and in particular an effort has been made to be compatible with existing Ada 83 usage to the greatest extent possible.

A few cases exist in which Ada 83 compiler behavior is incompatible with the requirements in Ada 95 (and thus also Ada 2005). These are instances of intentional or accidental dependence on specific implementation dependent characteristics of these Ada 83 compilers. The following is a list of the cases most likely to arise in existing Ada 83 code.

  • Implicit Packing

    Some Ada 83 compilers allowed a Size specification to cause implicit packing of an array or record. This could cause expensive implicit conversions for change of representation in the presence of derived types, and the Ada design intends to avoid this possibility. Subsequent AI’s were issued to make it clear that such implicit change of representation in response to a Size clause is inadvisable, and this recommendation is represented explicitly in the Ada 95 (and Ada 2005) Reference Manuals as implementation advice that is followed by GNAT. The problem will show up as an error message rejecting the size clause. The fix is simply to provide the explicit pragma Pack, or for more fine tuned control, provide a Component_Size clause.

  • Meaning of Size Attribute

    The Size attribute in Ada 95 (and Ada 2005) for discrete types is defined as the minimal number of bits required to hold values of the type. For example, on a 32-bit machine, the size of Natural will typically be 31 and not 32 (since no sign bit is required). Some Ada 83 compilers gave 31, and some 32 in this situation. This problem will usually show up as a compile time error, but not always. It is a good idea to check all uses of the ‘Size attribute when porting Ada 83 code. The GNAT specific attribute Object_Size can provide a useful way of duplicating the behavior of some Ada 83 compiler systems.

  • Size of Access Types

    A common assumption in Ada 83 code is that an access type is in fact a pointer, and that therefore it will be the same size as a System.Address value. This assumption is true for GNAT in most cases with one exception. For the case of a pointer to an unconstrained array type (where the bounds may vary from one value of the access type to another), the default is to use a ‘fat pointer’, which is represented as two separate pointers, one to the bounds, and one to the array. This representation has a number of advantages, including improved efficiency. However, it may cause some difficulties in porting existing Ada 83 code which makes the assumption that, for example, pointers fit in 32 bits on a machine with 32-bit addressing.

    To get around this problem, GNAT also permits the use of ‘thin pointers’ for access types in this case (where the designated type is an unconstrained array type). These thin pointers are indeed the same size as a System.Address value. To specify a thin pointer, use a size clause for the type, for example:

    type X is access all String;
    for X'Size use Standard'Address_Size;
    

    which will cause the type X to be represented using a single pointer. When using this representation, the bounds are right behind the array. This representation is slightly less efficient, and does not allow quite such flexibility in the use of foreign pointers or in using the Unrestricted_Access attribute to create pointers to non-aliased objects. But for any standard portable use of the access type it will work in a functionally correct manner and allow porting of existing code. Note that another way of forcing a thin pointer representation is to use a component size clause for the element size in an array, or a record representation clause for an access field in a record.

    See the documentation of Unrestricted_Access in the GNAT RM for a full discussion of possible problems using this attribute in conjunction with thin pointers.

18.7. Compatibility with HP Ada 83

All the HP Ada 83 pragmas and attributes are recognized, although only a subset of them can sensibly be implemented. The description of pragmas in Implementation Defined Pragmas indicates whether or not they are applicable to GNAT.

  • Default floating-point representation

    In GNAT, the default floating-point format is IEEE, whereas in HP Ada 83, it is VMS format.

  • System

    the package System in GNAT exactly corresponds to the definition in the Ada 95 reference manual, which means that it excludes many of the HP Ada 83 extensions. However, a separate package Aux_DEC is provided that contains the additional definitions, and a special pragma, Extend_System allows this package to be treated transparently as an extension of package System.