The system directory is used to store system dependent files. Its
main purpose is to allow different platforms sharing the same file
system to share system independent files, such as .pl and .qof
files, but still be able to locate the necessary system dependent
files.
Generally, several system directories are defined by
file_search_path/2 facts to include different
host and operating system combinations. The reason for this is that
certain system files are only machine dependent and can be shared by
applications running on the same type of machine, but some system
files are operating system dependent also.
The most commonly used system files are object files that are loaded
by load_foreign_files/2 and load_foreign_executable/1 predicates.
Quintus Prolog libraries, for example, make frequent
use of system dependent foreign code, but also contain system independent files
that are shared by all platforms.
Therefore, the system dependent object files are stored in
a system directory for each platform. The system directories are located
under the library directory, and the library files use calls like these:
:- load_foreign_files([system('file')], []).
:- load_foreign_executable(system('file')).
to specify files located in system directories. The library object files are compiled for each platform at installation time, and placed in the system directory specific to the platform.
The file_search_path/2 list in ref-fdi-fsp-pre, shows the different combinations of system
file_search_path facts for a Sun 4 running SunOS 5.x. The order
of search starts at the most specific system definition, which includes
the host and full operating system, and proceeds to less specific
forms of the system definitions. Finally, if none of the system
directories exist, or none contain the specified file, the current
working directory is tried. This way, if Quintus Prolog is to run on
a single platform, the system dependent files may reside in the same
directory as the system independent files.