SWI-Prolog 5.11.18 Reference Manual: Section 8.6

## 8.6 Unbounded thread creation

(SWI-)Prolog threads are rather heavyweight objects, notably on 32-bit systems, because every thread uses a considerable amount of virtual address space. SWI-Prolog threads claim the stack limit in virtual address space for each of the runtime stacks, while on 32-bit systems this resource is generally limited somewhere between 1GB and 3.5 GB, depending on the operating system and operating configuration.

If SWI-Prolog starts a thread it copies the initial goal and starts a POSIX thread which allocates a new Prolog engine that starts proving the given goal. If allocation of the engine fails, typically due to lack of virtual memory space, the thread is still created with minimal (8 Kbyte) stacks and immediately calls its exit handlers. See the option `at_exit(Goal)`. Although this mechanism allows for handling this type of error gracefully it is not safe to rely on it. Allocating an engine that nearly exhausts virtual address space may cause failures in normal memory allocation that can appear anywhere in Prolog or the foreign libraries used by it. Such errors typically kill the process with a fatal error.

Especially on 32-bit hardware, the design of the application must consider this issue and avoid ungraceful termination being conservative with the dynamic creation of new threads.