4.10.7.3 Permanent Atoms

Atom garbage collection scans all Prolog’s dynamic data areas when looking for atoms that are in use. Scanning finds atoms in the Prolog stacks and in all compiled and interpreted code that has been dynamically loaded into Prolog via consult/1, use_module/1, assert/2, etc. However, there are certain potential sources of atoms in the Prolog image from which atoms cannot be reclaimed. Atoms for Prolog code that has been statically linked with either the Prolog Development Environment or the Runtime Environment have been placed in the text space, making them (and the code that contains them) effectively permanent. Although such code can be abolished, its space can never be reclaimed.

These atoms are internally flagged as permanent by the system and are always retained by atom garbage collection. An atom that has become permanent cannot be made non-permanent, so can never be reclaimed.