A SWI-Prolog saved state is a resource archive that contains the compiled program in a machine-independent format,237Although the compiled code is independent from the CPU and operating system, 32-bit compiled code does not run on the 64-bit emulator, nor the other way around. Conditionally compiled code (see if/1) may also reduce platform independence. startup options, optionally shared objects/DLLs and optionally additional resource files. As of version 7.7.13, the resource archive format is ZIP. A resource file is normally created using the commandline option -c:
swipl -o mystate option ... -c file.pl ...
The above causes SWI-Prolog to load the given Prolog files and call qsave_program/2 using options created from the option ... in the command above.
A saved state may be executed in several ways. The basic mechanism is to use the -x:
swipl -x mystate app-arg ...
Saved states may have an arbitrary payload at the start.
This allows combining a (shell) script or the emulator with the state to
turn the state into a single file executable. By default a state starts
with a shell script (Unix) or the emulator (Windows).238As
the default emulator is a short program while the true emulator is in a
DLL this keeps the state short. The options emulator(File)
and
stand_alone(Bool) control what is added at the start of the
state. Finally, C/C++ programs that embed Prolog may use a static C
string that embeds the state into the executable. See
PL_set_resource_db_mem().
The predicates in this section support creating a saved state. Note that states are commonly created from the commandline using the -c, for example:
swipl -o mystate --foreign=save -c load.pl
Long (--) options are translated into options for
qsave_program/2.
This transformation uses the same conventions as used by argv_options/3,
except that the transformation is guided by the option type. This
implies that integer and callable options need to have valid syntax and
boolean options may be abbreviated to simply
--autoload or --no-autoload as shorthands for
--autoload=true and --autoload=false.
stand_alone option, the resource is headed by the emulator,
a Unix shell script or nothing. Options is a list of
additional options:
prolog starts the Prolog
toplevel and default runs halt/0
if there are initialization goals and the prolog/0
toplevel otherwise.
main, the default toplevel is to enter the Prolog
interactive shell unless a goal has been specified using
goal(Callable).
runtime (default), read resources from the state and
disconnect the code loaded into the state from the original source. If
development, save the predicates in their current state and
keep reading resources from their source (if present). See also
open_resource/3.
true (default), run autoload/0
first. If the class is
runtime and autoload is true, the
state is supposed to be self contained and autoloading is disabled in
the restored state.
save (default) to save the current operator table or standard
to use the initial table of the emulator.
true, the emulator is the first part of the state. If
the emulator is started it tests whether a saved state is attached to
itself and load this state. Provided the application has all libraries
loaded, the resulting executable is completely independent from the
runtime environment or location where it was built. See also
section 2.11.2.4.
/usr/lib/swipl/lib/x86_64-linux/swipl. To
create a saved state based on the public executable such that it can run
on multiple archirectures one can use e.g.
$ swipl -o myexe --emulator=$(which swipl) -c myload.pl
save, include shared objects (DLLs) for the current
architecture into the saved state. See current_foreign_library/2,
and current_prolog_flag(arch, Arch). If the program strip is
available, this is first used to reduce the size of the shared object.
If a state is started, use_foreign_library/1
first tries to locate the foreign resource in the resource database.
When found it copies the content of the resource to a temporary file and
loads it. If possible (Unix), the temporary object is deleted
immediately after opening.239This
option is experimental and currently disabled by default. It will become
the default if it proves robust.240Creating
a temporary file is the most portable way to load a shared object from a
zip file but requires write access to the file system. Future versions
may provide shortcuts for specific platforms that bypass the file
system.
If Action is of the form arch(ListOfArches)
then the shared objects for the specified architectures are stored in
the saved state. On the command line, the list of architectures can be
passed as --foreign=<CommaSepArchesList>.
In order to obtain the shared object file for the specified
architectures, qsave_program/2
calls a user defined hook:
qsave:arch_shlib(+Arch, +FileSpec, -SoPath). This hook
needs to unify SoPath with the absolute path to the shared
object for the specified architecture. FileSpec is of the
form foreign(Name).
At runtime, SWI-Prolog will try to load the shared library which is
compatible with the current architecture, obtained by calling
current_prolog_flag(arch, Arch). An architecture is
compatible if one of the two following conditions is true (tried in
order):
qsave:compat_arch(Arch1, Arch2) hook
succeeds.
This last one is useful when one wants to produce one shared object file that works for multiple architectures, usually compiling for the lowest common denominator of a certain CPU type. For example, it is common to compile for armv7 if even if the code will be running on newer arm CPUs. It is also useful to provide highly-optimized shared objects for particular architectures.
ignore (default) or error.
In the latter case creating the state is aborted with a message that
indicates the undefines predicates and from where they are called.
true (default false), replace predicate
names with generated symbols to make the code harder to assess for
reverse engineering. See section
14.6.1.
true (default false), report progress and
status, notably regarding auto loading.
qsave_program(File, []).This predicate is used by qsave_program/[1,2] to ensure the saved state does not depend on availability of the libraries. The predicate autoload_all/0 examines all clauses of the loaded program (obtained with clause/2) and analyzes the body for referenced goals. Such an analysis cannot be complete in Prolog, which allows for the creation of arbitrary terms at runtime and the use of them as a goal. The current analysis is limited to the following:
The analysis of meta-predicate arguments is limited to cases where the argument appears literally in the clause or is assigned using =/2 before the meta-call. That is, the following fragment is processed correctly:
...,
Goal = prove(Theory),
forall(current_theory(Theory),
Goal)),
But, the calls to prove_simple/1 and prove_complex/1 in the example below are not discovered by the analysis and therefore the modules that define these predicates must be loaded explicitly using use_module/[1,2].
...,
member(Goal, [ prove_simple(Theory),
prove_complex(Theory)
]),
forall(current_theory(Theory),
Goal)),
It is good practice to use gxref/0 to make sure that the program has sufficient declarations such that the analysis tools can verify that all required predicates can be resolved and that all code is called. See meta_predicate/1, dynamic/1, public/1 and prolog:called_by/2.
There are three areas that require special attention when using qsave_program/[1,2].
:- 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).
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.
Many applications use packages that include foreign language
components compiled to shared objects or DLLs. This code is normally
loaded using
use_foreign_library/1
and the foreign file search path. Below is an example from
the socket library.
:- use_foreign_library(foreign(socket)).
There are two options to handle shared objects in runtime
applications. The first is to use the foreign(save) option
of qsave_program/2
or the --foreign=save commandline option. This causes
the dependent shared objects to be included into the resource archive.
The use_foreign_library/1
directive first attempts to find the foreign file in the resource
archive. Alternatively, the shared objects may be placed in a directory
that is distributed with the application. In this cases the file search
path foreign must be setup to point at this directory. For
example, we can place the shared objects in the same directory as the
executable using the definition below. This may be refined further by
adding subdirectories depending on the architecture as available from
the Prolog flag arch.
:- multifile user:file_search_path/2.
user:file_search_path(foreign, Dir) :-
current_prolog_flag(executable, Exe),
file_directory_name(Exe, Dir).