>But I also don't think the auther meant Things you can do in Linux but not Unix
I wasn't claiming that. I just thought the ggp had a useful comment about renameat2() which led to gp's "correction" which wasn't 100% accurate.
IBM z/OS UNIX also has renameat2(). It doesn't have the Linux specific flag RENAME_EXCHANGE.
https://www.ibm.com/docs/en/zos/3.1.0?topic=functions-rename...
pedantic but z/OS isn't a unix, it can just pretend to be one enough for the open group to call it one. IBM has a unix still, AIX.
In recent versions, z/OS has been copying lots of Linux-specific APIs (e.g. unshare [0]) in order to support the z/OS port of Kubernetes.
If Kubernetes starts using renameat2(RENAME_EXCHANGE), they could very plausibly add it.
[0] https://www.ibm.com/docs/en/zos/3.2.0?topic=csd-unshare-bpx1...