iounit – return size of atomic I/O unit for file descriptor
int iounit(int fd)
Reads and writes of files are transmitted using the 9P protocol (see
and in general, operations involving large amounts of data must be
broken into smaller pieces by the operating system.
The ‘I/O unit’ associated with each file descriptor records the maximum
size, in bytes, that may be read or written without breaking up the transfer.
routine uses the
interface to discover the I/O unit size for the file descriptor
and returns its value.
Certain file descriptors, particularly those associated with devices, may
have no specific I/O unit, in which case
Returns zero if any error occurs or if the I/O unit size of the fd is unspecified or unknown.