Surprisingly, OpenBSD does not have a library (neither a public API nor even just routines which are copied project-to-project as is common with OpenBSD utiliti

Surprisingly, OpenBSD does not have a library (neither a public API nor even just routines which are copied project-to-project as is common with OpenBSD utilities and daemons) to handle arithmetic overflow. The closest might be malloc/realloc extensions, like reallocarray, that handle common scenarios where arithmetic overflow is seen.