iounit - return size of atomic I/O unit for file descriptor #include <u.h>

#include <libc.h>

int iounit(int fd)

Reads and writes of files are transmitted using the 9P protocol (see intro (5)) 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.

The iounit routine uses the dup (3) interface to discover the I/O unit size for the file descriptor fd and returns its value. Certain file descriptors, particularly those associated with devices, may have no specific I/O unit, in which case iounit will return 0 .

/sys/src/libc/9sys dup (3), read (5) Returns zero if any error occurs or if the I/O unit size of the fd is unspecified or unknown.