[dmoy]

Here is my list of numbers: 1,Here is my list of letters: a,b,2,3,d

[stevenhuang]

Although not a POSIX requirement, in practice for unix-like systems, file writes are atomic across concurrent writers.

You maybe thinking of stdio buffering, where calls to printf etc get split into multiple write calls. Then in those cases, it's possible to get errant interleaved writes.