2.19 System limits

2.19.1 Limits on memory areas

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.

2.19.1.1 The heap

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.

2.19.2 Other Limits

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.

2.19.3 Reserved Names

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.