2.20.1 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. The internal data representation limits these stacks to 128 MB (each) on 32-bit processors. More generally to 2 ** bits-per-pointer - 5 bytes, which implies they are virtually unlimited on 64-bit machines.

As of version 7.7.14, the stacks are restricted by the writeable flag stack_limit or the command line option --stack-limit. This flag limits the combined size of the three stacks per thread. The default limit is currently 512 Mbytes on 32-bit machines, which imposes no additional limit considering the 128 Mbytes hard limit on 32-bit and 1 Gbytes on 64-bit machines.

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 created during Prolog's execution. Terms on this stack will be reclaimed by backtracking to a point before the term was created or by garbage collection (provided the term 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.

2.20.1.1 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.