What does the suffix _r mean?

Question

I know that _r suffix is ​​used to indicate something thread safe. For example strtok and strtok_r or libmysql.so and libmysql_r.so . But I can’t find what it really means? For example, _t means "type" and what does the suffix _r mean?

Answer 1

See chapter A.4.16 Thread-Safety at http://pubs.opengroup.org/onlinepubs/000095399/xrat/xbd_chap04.html :

The suffix "_r" is historical, where "r" means "repeated."

Answer 2

The suffix _r means re-entry.

Answer 3

_r means reentrant, although these functions may not be completely reentrant. These functions simply guarantee that no static storage is used.