3.2.10 Compiling Ada 83 Programs

-gnat83

Although GNAT is primarily an Ada 95 compiler, it accepts this switch to specify that an Ada 83 program is to be compiled in Ada 83 mode. If you specify this switch, GNAT rejects most Ada 95 extensions and applies Ada 83 semantics where this can be done easily. It is not possible to guarantee this switch does a perfect job; for example, some subtle tests, such as are found in earlier ACVC tests (and that have been removed from the ACATS suite for Ada 95), might not compile correctly. Nevertheless, this switch may be useful in some circumstances, for example where, due to contractual reasons, legacy code needs to be maintained using only Ada 83 features.

With few exceptions (most notably the need to use <> on unconstrained generic formal parameters, the use of the new Ada 95 reserved words, and the use of packages with optional bodies), it is not necessary to use the -gnat83 switch when compiling Ada 83 programs, because, with rare exceptions, Ada 95 is upwardly compatible with Ada 83. This means that a correct Ada 83 program is usually also a correct Ada 95 program. For further information, please refer to Compatibility and Porting Guide.