Man pages matching name pthread_once in section 3:

pthread_once.3.en	debian 10 / glibc-doc
pthread_once.3.ja	debian 10 / manpages-ja-dev
pthread_once.3.en	debian 11 / glibc-doc
pthread_once.3.fr	debian 11 / manpages-fr-dev
pthread_once.3.ja	debian 11 / manpages-ja-dev
pthread_once.3.en	debian 12 / glibc-doc
pthread_once.3.de	debian 12 / manpages-de-dev
pthread_once.3.fr	debian 12 / manpages-fr-dev
pthread_once.3.ja	debian 12 / manpages-ja-dev