4.4.2 Saved States

saved states are just a special case of PO files. The save_program/[1,2] predicate will save the execution state in a file. 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 those marked as volatile in ref-lps-flg, any blackboard data (see ref-mdb-bbd), database data (see ref-mdb-bas), and as of release 4.2, information for source-linked debugging. See mpg-ref-save_program.

A saved state, can be restored using the restore/1 predicate from within Prolog:

| ?- restore(File).

which will replace the current program state by the one in File. See mpg-ref-restore.

A saved state can also be given as an option to the sicstus command:

% sicstus -r File

which will start execution by restoring File.

The location (i.e. directory) of the saved state, when it is created with save_program/[1,2] and loaded with restore/1, is treated in a special way in order to make it possible locate files when the saved state has been moved. See Saving for more information.

The save_program/2 predicate can be used to specify an initial goal that will be run when the saved state is restored. For example:

| ?- save_program(saved_state,initial_goal([a,b,c])).

When saved_state is loaded initial_goal/1 will be called. This allows saved states to be generated that will immediately start running the user’s program when they are restored. In addition to this save_program/2 facility, see also the initialization/1 facility to declare goal to be executed upon loading (see Initializations).