pragma Extend_System ([Name =>] IDENTIFIER);
This pragma is used to provide backwards compatibility with other implementations that extend the facilities of package System. In GNAT, System contains only the definitions that are present in the Ada RM. However, other implementations, notably the DEC Ada 83 implementation, provide many extensions to package System.
For each such implementation accommodated by this pragma, GNAT provides a package Aux_`xxx`, e.g., Aux_DEC for the DEC Ada 83 implementation, which provides the required additional definitions. You can use this package in two ways. You can with it in the normal way and access entities either by selection or using a use clause. In this case no special processing is required.
However, if existing code contains references such as System.`xxx` where xxx is an entity in the extended definitions provided in package System, you may use this pragma to extend visibility in System in a non-standard way that provides greater compatibility with the existing code. Pragma Extend_System is a configuration pragma whose single argument is the name of the package containing the extended definition (e.g., Aux_DEC for the DEC Ada case). A unit compiled under control of this pragma will be processed using special visibility processing that looks in package System.Aux_`xxx` where Aux_`xxx` is the pragma argument for any entity referenced in package System, but not found in package System.
You can use this pragma either to access a predefined System extension supplied with the compiler, for example Aux_DEC or you can construct your own extension unit following the above definition. Note that such a package is a child of System and thus is considered part of the implementation. To compile it you will have to use the `-gnatg' switch for compiling System units, as explained in the GNAT User's Guide.