A Stand-alone Library (abbreviated 'SAL') is a library that contains the
necessary code to
elaborate the Ada units that are included in the library. In contrast with
an ordinary library, which consists of all sources, objects and
files of the
library, a SAL may specify a restricted subset of compilation units
to serve as a library interface. In this case, the fully
self-sufficient set of files will normally consist of an objects
archive, the sources of interface units' specs, and the
files of interface units.
If an interface spec contains a generic unit or an inlined subprogram,
source must also be provided; if the units that must be provided in the source
form depend on other units, the source and
ALI files of those must
also be provided.
The main purpose of a SAL is to minimize the recompilation overhead of client applications when a new version of the library is installed. Specifically, if the interface sources have not changed, client applications do not need to be recompiled. If, furthermore, a SAL is provided in the shared form and its version, controlled by Library_Version attribute, is not changed, then the clients do not need to be relinked.
SALs also allow the library providers to minimize the amount of library source text exposed to the clients. Such 'information hiding' might be useful or necessary for various reasons.
Stand-alone libraries are also well suited to be used in an executable whose main routine is not written in Ada.