14.2.2 Limitations of qsave_program

There are three areas that require special attention when using qsave_program/[1,2].

• If the program is an embedded Prolog application or uses the foreign language interface, care has to be taken to restore the appropriate foreign context. See section 14.2.3 for details.

• If the program uses directives (:- goal. lines) that perform other actions than setting predicate attributes (dynamic/1, volatile/1, etc.) or loading files (use_module/1, etc.). Goals that need to be executed when the state is started must use initialization/1 (ISO standard) or initialization/2 (SWI extension that provides more control over when the goal is executed). For example, initialization/2 can be used to start the application:

  :- initialization(go, main).

• Blobs used as references to the database (see clause/3, recorded/3), streams, threads, etc. can not be saved. This implies that (dynamic) clauses may not contain such references at the moment the qsave_program/2 is called. Note that the required foreign context (stream, etc.) cannot be present in the state anyway, making it pointless to save such references. An attempt to save such objects results in a warning.

  The volatile/1 directive may be used to prevent saving the clauses of predicates that hold such references. The saved program must reinitialise such references using the normal program initialization techniques: use initialization/1,2 directives, explicitly create them by the entry point or make the various components recreate the context lazily when required.

• Blobs that properly implement the save() and load() callbacks can be saved and restored. By default a blob is saved as an array of bytes, of the internal form of the blob. This means that any saved program using such a blob is probably not portable to a different architecture.