What does _r suffix mean?

I know _r suffix is used to indicate something thread-safe. For example strtok and strtok_r or libmysql.so and libmysql_r.so. But I cannot find anywhere what this actually means ? For example, _t means 'type' and what does _r suffix mean?

Solution

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

The suffix "_r" is historical, where the 'r' stood for "reentrant".

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