The SWI-Prolog engine uses three stacks the local stack
(also called environment stack) stores the environment frames
used to call predicates as well as choice points. The
global stack (also called heap) contains terms,
floats, strings and large integers. Finally, the trail stack
records variable bindings and assignments to support
backtracking. Except for available memory, there is no
hard limit for the sizes of the stacks.42As
of version 9.3.6. Older versions have a hard limit on 32-bit hardware of
128Mb for each stack.
The combined stack size (per thread) has a soft limit
implemented by the writeable flag stack_limit
or the command line option --stack-limit. Currently the
default limit is 1Gb. Considering portability, applications that need to
modify the default limits are advised to do so using the Prolog flag stack_limit.
| Area name | Description |
| local stack | The local stack is used
to store the execution environments of procedure invocations. The space
for an environment is reclaimed when it fails, exits without leaving
choice points, the alternatives are cut off with the !/0 predicate or no
choice points have been created since the invocation and the last
subclause is started (last call optimisation). |
| global stack | The global stack is used to store
terms, strings, big integers, rational numbers and floating numbers
created during Prolog's execution. Data on this stack is reclaimed by
backtracking to a point before the data was created or by garbage
collection (provided the data is no longer referenced). |
| trail stack | The trail stack is used to store
assignments during execution. Entries on this stack remain alive until
backtracking before the point of creation or the garbage collector
determines they are no longer needed.
As the trail and global stacks are garbage collected together, a
small trail can cause an excessive amount of garbage collections. To
avoid this, the trail is automatically resized to be at least 1/6th of
the size of the global stack. |
Table 2 : Memory areas
With
the heap, we refer to the memory area used by malloc() and
friends. SWI-Prolog uses the area to store atoms, functors, predicates
and their clauses, records and other dynamic data. No limits are imposed
on the addresses returned by malloc() and friends.
- Clauses
- The only limit on clauses is their arity (the number of arguments to the
head), which is limited to 1024. Raising this limit is easy and
relatively cheap; removing it is harder.
- Atoms and Strings
- SWI-Prolog has no limits on the length of atoms or strings. The number
of atoms is unlimited. Atoms are subject to garbage collection. See
section 12.4.2.1.
Both atoms and strings can represent all Unicode code points, including
0 (
\u0000). Currently, SWI-Prolog uses a separate
representation for ISO Latin 1 text (code points
0 ... 255) and text that includes higher code points. The
latter is represented using the C wchar_t type. On most
systems this implies UCS-4, i.e., 32-bit unsigned integers. On Windows
wchar_t uses UTF-16, which implies that it cannot represent
the code points reserved for surrogate pairs as single code
points. Future versions may switch to using UTF-8 throughout.
- Nesting of terms
- Most built-in predicates that process Prolog terms create an explicitly
managed stack and perform optimization for processing the last argument
of a term. This implies they can process deeply nested terms at constant
and low usage of the C stack, and the system raises a resource error if
no more stack can be allocated. Currently only read/1
and write/1
(and all variations thereof) still use the C stack and may cause the
system to crash in an uncontrolled way (i.e., not mapped to a Prolog
exception that can be caught).
- Integers
- SWI-Prolog has two integer representations. Tagged integers are
currently limited to 57 bits.43Before
version 9.3.6, tagged integers on 32-bit systems had 25 bits and
there was a third representation for 64 bit integers.
Unbounded integers are by default provided by the GNU GMP library.
Alternatively, they may be provided by the bundled LibBf library. The
system can be built without support for unbounded integers.
- Floating point numbers
- Floating point numbers are represented as C-native double precision
floats, 64-bit IEEE on most machines.
The boot compiler (see -b option) does not support
the module system. As large parts of the system are written in Prolog
itself we need some way to avoid name clashes with the user's
predicates, database keys, etc. Like Edinburgh C-Prolog Pereira,
1986 all predicates, database keys, etc., that should be
hidden from the user start with a dollar ($) sign.
