> Any modern OS you'll find will buffer write calls in one way or the other.
This is simply not true as a general rule. It depends on the nature of the file descriptor. Yes, if the file descriptor refers to the file system, it will in all likelihood be buffered by the OS (not with O_DIRECT, however). But on "any modern OS", file descriptors can refer to things that are not files, and the buffering situation there will vary from case to case.
This is simply not true as a general rule. It depends on the nature of the file descriptor. Yes, if the file descriptor refers to the file system, it will in all likelihood be buffered by the OS (not with O_DIRECT, however). But on "any modern OS", file descriptors can refer to things that are not files, and the buffering situation there will vary from case to case.