[jeffbee]

I don't think any part of your post is right. Aside from NFS, there should not be filesystems where this doesn't work. If there are, those are just bugs. The flags you mentioned are not required or relevant. Setting the fd offset to the end of the file atomically is the entire purpose of O_APPEND.

[ozgrakkurt]

It depends on what you mean by atomic. If it is only writing to page cache and you are writing a small amount then yes?

If there is a failure like a crash or power outage etc. then it doesn’t work like that.

You might as well be pushing into an in-memory data structure and writing to disk at program exit in terms of reliability

[jeffbee]

You are projecting imaginary features onto O_APPEND and then hypothesizing that your imaginary features might not work.

POSIX says that for a file opened with O_APPEND "the file offset shall be set to the end of the file prior to each write." That's it. That's all it does.