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.\" $NetBSD: ioctl.9,v 1.26 2008/11/12 12:35:54 ad Exp $
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.\" by Heiko W.Rupp <hwr@pilhuhn.de>
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.Dd October 4, 2018
.Dt IOCTL 9
.Os
.Sh NAME
.Nm ioctl ,
.Nm _IO ,
.Nm _IOR ,
.Nm _IOW ,
.Nm _IOWR
.Nd "how to implement a new ioctl call to access device drivers"
.Sh SYNOPSIS
.In sys/ioccom.h
.Fn _IO "g" "t"
.Fn _IOR "g" "n" "t"
.Fn _IOW "g" "n" "t"
.Fn _IOWR "g" "n" "t"
.Sh DESCRIPTION
Whenever an
.Xr ioctl 2
call is made, the kernel dispatches it to the device driver
which can then interpret the request number and data in a specialized
manner.
Ioctls are defined as:
.Bd -literal
#define MYDEVIOCTL   fun(g, n, t)
.Ed
.Pp
where the different symbols correspond to:
.Bl -tag -width ".Dv MYDEVIOCTL"
.It Dv MYDEVIOCTL
The name which will later be given in the
.Xr ioctl 2
system call as second argument, e.g.,
.Bd -literal
ioctl(fd, MYDEVIOCTL, ...)
.Ed
.It Fn fun
A macro which can be one of:
.Bl -tag -width ".Fn _IOWR"
.It Fn _IO
The call is a simple message to the kernel by itself.
It does not copy anything into the kernel, nor does it want anything back.
.It Fn _IOR
The call only reads parameters from the kernel and does not
pass any to it.
.It Fn _IOW
The call only writes parameters to the kernel, but does not want anything
back.
.It Fn _IOWR
The call writes data to the kernel and wants information back.
.El
.Pp
We always consider reading or writing to the kernel, from the user perspective.
.It Fa g
This integer describes to which subsystem the ioctl applies.
Here are some examples:
.Pp
.Bl -tag -width xxxxx -compact
.It '8'
.Xr aac 4
.It 'a'
.Xr nata 4
.It 'B'
.Xr bpf 4
.It 'C'
.Xr cam 4
.It 'C'
.Xr ciss 4
.It 'd'
.Xr disklabel 5
.It 'd'
diskslice
.It 'd'
.Xr drm 4
.It 'f'
generic file-descriptor
.It 'F'
frame buffer
.It 'h'
.Xr HAMMER 5
.It 'i'
.Xr iic 4
.It 'i'
.Xr carp 4
.It 'i'
.Xr gre 4
.It 'k'
.Xr keyboard 4
and
.Xr syscons 4
.It 'm'
.Xr mem 4
.It 'm'
.Pa /dev/midi
.It 'm'
.Xr mtio 4
.It 'M'
.Xr sound 4
mixer
.It 'n'
.Xr smb 4
.It 'p'
.Pa /dev/dsp
and
.Pa /dev/audio
.It 'p'
.Xr pci 4
.It 'p'
.Xr ppbus 4
.It 'p'
.Xr procfs 5
.It 'q'
.Pa /dev/sequencer
.It 'r'
random number generator
.It 't'
.Xr tty 4
.It 't'
.Xr tap 4
.It 't'
.Xr tun 4
.It 't'
SLIP ttys
.It 'T'
.Xr snp 4
.El
.It Fa n
This number uniquely identifies the ioctl within the group.
That said, two subsystems may share the same
.Fa g ,
but there may be only one
.Fa n
for a given
.Fa g .
This is an unsigned 8 bit number.
.It Fa t
This specifies the type of the passed parameter.
This one gets internally transformed to the size of the parameter, so
for example, if you want to pass a structure, then you have to specify that
structure and not a pointer to it or sizeof(struct MYDEV).
.El
.Pp
In order for the new ioctl to be visible to the system, it is installed
in either
.In sys/ioctl.h or one of the files that are reached from
.In sys/ioctl.h .
.Sh RETURN VALUES
A distinction must be made at this point.
All
.Fn *_ioctl
routines from
.Em within kernel
should return either 0 for success
or a defined error code, as described in
.In sys/errno.h .
At the libc level though a conversion takes place, so that eventually
.Xr ioctl 2
returns either 0 for success or -1 for failure, in which case the
.Va errno
variable is set accordingly.
.Pp
The use of magic numbers such as -1, to indicate that a given ioctl
code was not handled, is strongly discouraged.
The value -1 is bound to the
.Er ERESTART
pseudo-error, which is returned inside kernel to modify return to process.
.Sh EXAMPLES
Let's suppose that we want to pass an integer value to the kernel.
From the user point of view, this is like writing to the kernel.
So we define the ioctl as:
.Bd -literal -offset indent
#define	MYDEVIOCTL	_IOW('i', 25, int)
.Ed
.Pp
Within the
.Fn *_ioctl
routine of the driver, it can be then accessed like:
.Bd -literal -offset indent
int
mydev_ioctl(struct dev_ioctl_args *ap)
{
	int error;
	int *a;

	switch (ap->a_cmd) {
	case MYDEVIOCTL:
		a = (int *)ap->data;
		kprintf("Value passed from userspace: %d\\n", *a);
		return (0);    /* Success */
		break;

	/* Handle other ioctls here */

        default:
                /* Inappropriate ioctl for device */
                error = ENOTTY;
		break;
	}

	return (error);
}
.Ed
.Pp
In userspace:
.Bd -literal -offset indent
int a = 101;
if (ioctl(fd, MYDEVIOCTL, \*[Am]a) == -1) {
	/* Handle failure */
}
.Ed
.Sh SEE ALSO
.Xr ioctl 2