Once a program has been read, the system will have available all the information necessary for its execution. This information is called a program state.
The state of a program may be saved on disk for future execution. The
state consists of all predicates and modules
except built-in predicates and clauses of volatile predicates,
the current operator declarations, the current character-conversion mapping,
the values of all writable Prolog
flags except debugging, source_info, and the user_*
stream aliases (see State Info), any blackboard data
(see Blackboard Primitives), internal database data
(see Database), and profiling data (see Profiling), but no
information for source-linked debugging.
To save a program into a file File, type the following query. On UNIX platforms, the file becomes executable:
| ?- save_program(File).
You can also specify a goal to be run when a saved program is restored. This is done by:
| ?- save_program(File, start).
where start/0 is the predicate to be called.
Once a program has been saved into a file File, the following query will restore the system to the saved state:
| ?- restore(File).
If a saved state has been moved or copied to another machine, the path names of foreign resources and other files needed upon restore are typically different at restore time from their save time values. To solve this problem, certain atoms will be relocated during restore as follows:
$SP_PATH/library (the name of the directory
containing the Prolog Library) as prefix at save time will have that
prefix replaced by the corresponding restore time value.
The purpose of this procedure is to be able to build and deploy an application consisting of a saved state and other files as a directory tree with the saved state at the root: as long as the other files maintain their relative position in the deployed copy, they can still be found upon restore.
NOTE: Foreign resources, see Calling C, are unloaded by
save_program/[1,2]. The names and paths of the
resources, typically $SP_PATH/library relative,
are however included in the saved state. After the save, and
after restoring a saved state, this information is used to reload
the foreign resources again. The state of the foreign resource in terms
of global C variables and allocated memory is thus not
preserved. Foreign resources may define init and deinit
functions to take special action upon loading and unloading; see Init and Deinit Functions.
As of SICStus Prolog 3.8, partial saved states corresponding to a set of
source files, modules, and predicates can be created by the built-in
predicates save_files/2, save_modules/2, and
save_predicates/2 respectively. These predicates create files in
a binary format, by default with the prefix .po (for Prolog
object file), which can be loaded by load_files/[1,2].
For example, to compile a program split into several source files into a
single object file, type:
| ?- compile(Files), save_files(Files, Object).
For each filename given, the first goal will try to locate a source file
with the default suffix .pl and compile it into memory. The
second goal will save the program just compiled into an object file
whose default suffix is .po. Thus the object file will contain a
partial memory image.
NOTE: Prolog object files can be created with any suffix, but cannot be
loaded unless the suffix is .po!