> Unix does offer an API for writing C-standard in-memory text strings

Why on bloody Earth should a presumably generic-purpose OS provide a special API for dealing with internal representation of some data structure in a (particular) implementation of a (particular) programming language?

Besides, it doesn't offer such an API anyhow; you need to take care to manually pass the result of a strlen() call instead of sizeof()'s as the value for the len parameter of a write() call, otherwise a NUL-terminator will get written into the file as well.

And C says nothing about what constitutes a line break, by the way. Nor does it have any concept of a "line", or any utilities for working with lines specifically, it only knows of strings, and that's all. The concept of "text line" is POSIX.