fflush and std::endl/std::flush are not synonymous with the fsync syscall. None of the standard libraries I tested above call fsync at all when using those func

fflush and std::endl/std::flush are not synonymous with the fsync syscall. None of the standard libraries I tested above call fsync at all when using those functions. They flush the accumulated memory "to the file" in the sense of making a write syscall (or equivalent, e.g., writev), and they do not attempt to enforce a flush to disk, that's just left to the OS. The "line buffering" in question is the C/C++ library accumulated memory, and the relevant kind of "unbuffering/flushing" is just write-like syscalls. I meant to reply much sooner, with more info from empirical tests. But now I'm happy to leave it at just the write stuff and the note about the meaning of stdio/iostream "flushing". I think you have an incorrect or esoteric understanding of what the "buffering" in question is, but it doesn't matter to me. My argument is about what syscalls happen, and I don't care if you disagree with the (I think, standard) descriptor/terminology I'm using.