Yes, you can. Then you don't write against the OS, but against the kernel. It sometimes works, because the kernel is a separate project, it sometimes doesn't, you gave an example yourself.

> In any case, file descriptors are definitely not a libc construct on Linux.

File descriptors come definitely from the kernel, but they do also exist as a concept in libc, and I was referring to them as such. I was saying that I depend on non-portable libc functions, even though I value portability, because the API is just so nice. I did not want to indicate, that I am doing syscalls directly.