git: purgatorio

ref: 87d72e7e8614d96b4f61adae5fb05b0534231c4c
dir: /man/2/sys-iounit/

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.TH SYS-IOUNIT 2
.SH NAME
iounit \- return size of atomic I/O unit for file descriptor
.SH SYNOPSIS
.EX
include "sys.m";
sys := load Sys Sys->PATH;

iounit:	fn(fd: ref FD): int;
.EE
.SH DESCRIPTION
Reads and writes of files are transmitted using the 9P protocol (see
.IR 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.
.PP
The
.B iounit
system call returns the I/O unit size for the open file
.IR fd .
Certain file descriptors, particularly those associated with devices, may
have no specific I/O unit, in which case
.B iounit
will return zero.
.SH SOURCE
.B /emu/port/inferno.c
.br
.B /os/port/inferno.c
.SH SEE ALSO
.IR sys-dup (2),
.IR read (5)
.SH DIAGNOSTICS
Returns zero if any error occurs or if the I/O unit size of the fd is unspecified or unknown.