IOCTL(9) | Kernel Developer's Manual | IOCTL(9)

# NAME

`ioctl` —
how to implement a new ioctl call to access device drivers

# SYNOPSIS

`#include <sys/ioctl.h>`
`#include <sys/ioccom.h>`

int
`ioctl`(int, unsigned long, ...);

# DESCRIPTION

`ioctl` are internally defined as

#define FOOIOCTL fun(t,n,pt)

where the different variables and functions are:

`FOOIOCTL`
  the name which will later be given in the ioctl(2) system call as second argument, e.g.,
  `ioctl(s, FOOIOCTL, ...).`
`fun`()
  a macro which can be one of
  _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.
  _IOR
    the call only reads parameters from the kernel and does not pass any to it
  _IOW
    the call only writes parameters to the kernel, but does not want anything back
  _IOWR
    the call writes data to the kernel and wants information back.
t
  This integer describes to which subsystem the ioctl applies. t can be one of
  '1'
    pulse-per-second interface
  'a'
    ISO networking
  'A'
    ac devices (hp300)
  'A'
    Advanced Power Management (hpcmips, i386, sparc), see apm(4)
  'A'
    ADB devices (mac68k, macppc)
  'A'
    audio(4)
  'b'
    tb(4)
  'b'
    Bluetooth HCI sockets, see bluetooth(4)
  'b'
    Bluetooth Hub Control, see bthub(4)
  'b'
    Bluetooth SCO audio driver, see btsco(4)
  'B'
    bell device (x68k)
  'B'
    bpf(4)
  'c'
    coda
  'c'
    cd(4)
  'c'
    ch(4)
  'C'
    clock devices (amiga, atari, hp300, x68k)
  'd'
    the disk subsystem
  'E'
    envsys(4)
  'f'
    files
  'F'
    Sun-compatible framebuffers
  'F'
    ccd(4) and vnd(4)
  'g'
    qdss framebuffers
  'G'
    grf devices (amiga, atari, hp300, mac68k, x68k)
  'h'
    HIL devices (hp300)
  'H'
    HIL devices (hp300)
  'H'
    HPc framebuffers
  'i'
    a (pseudo) interface
  'I'
    ite(4) (mac68k)
  'J'
    ISA joystick interface
  'k'
    Sun-compatible (and other) keyboards
  'l'
    leo devices (atari)
  'm'
    mtio(4)
  'M'
    mouse devices (atari)
  'M'
    mlx(4)
  'n'
    virtual console device (arm32)
  'n'
    SMB networking
  'O'
    OpenPROM and OpenFirmware
  'p'
    power control (x68k)
  'P'
    parallel port (amiga, x68k)
  'P'
    profiling (arm32)
  'P'
    printer/plotter interface (hp300)
  'P'
    pci(4)
  'P'
    compat/ossaudio and soundcard.h
  'P'
    magma(4) bpp (sparc)
  'q'
    altq(9)
  'q'
    pmax graphics devices
  'Q'
    altq(9)
  'Q'
    raw SCSI commands
  'r'
    the routing subsystem
  'r'
    md(4)
  'R'
    rnd(4)
  's'
    the socket layer
  'S'
    SCSI disks (arc, hp300, pmax)
  'S'
    watchdog devices (sh3)
  'S'
    ISA speaker devices
  'S'
    stic devices
  'S'
    scanners
  't'
    the tty layer
  'u'
    user defined ???
  'U'
    scsibus (see scsi(4))
  'v'
    Sun-compatible “firm events”
  'V'
    view device (amiga, atari)
  'V'
    sram device (x68k)
  'w'
    watchdog devices
  'W'
    wt devices
  'W'
    wscons devices
  'x'
    bt8xx devices
  'Z'
    ite devices (amiga, atari, x68k)
  'Z'
    passthrough ioctls
n
  This numbers the ioctl within the group. There may be only one n for a given t. This is an unsigned 8 bit number.
pt
  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 foo)

In order for the new ioctl to be known to the system it is installed in either ⟨sys/ioctl.h⟩ or one of the files that are reached from ⟨sys/ioctl.h⟩.

# RETURN VALUES

All `ioctl`() routines should return either 0 or a defined error code. The use of magic numbers such as -1, to indicate that a given ioctl code was not handled is strongly discouraged. The value -1 coincides with the historic value for `ERESTART` which was shown to produce user space code that never returned from a call to ioctl(2).

For ioctl codes that are not handled by a given routine, the pseudo error value `EPASSTHROUGH` is provided. `EPASSTHROUGH` indicates that no error occurred during processing (it did not fail), but neither was anything processed (it did not succeed). This supersedes the use of either `ENOTTY` (which is an explicit failure) or -1 (which has no contextual meaning) as a return value. `ENOTTY` will get passed directly back to user space and bypass any further processing by other ioctl layers. Only code that wishes to suppress possible further processing of an ioctl code (e.g., the tty line discipline code) should return `ENOTTY`. All other code should return `EPASSTHROUGH`, even if it knows that no other layers will be called upon.

If the value `EPASSTHROUGH` is returned to `sys_ioctl`(), then it will there be changed to `ENOTTY` to be returned to user space, thereby providing the proper error notification to the application.

# EXAMPLES

    #define	FOOIOCTL	_IOWR('i', 23, int)

    int a = 3;
    error = ioctl(s, FOOICTL, &a);

Within the `ioctl`()-routine of the driver, it can be then accessed like

    driver_ioctl(..., u_long cmd, void *data)
    {
    	...
    	switch (cmd) {

    	case FOOIOCTL:
    		int *a = (int *)data;
    		printf(" Value passed: %d\n", *a);
    		break;
    	}
    }

# NOTES

Note that if you for example try to read information from an ethernet driver where the name of the card is included in the third argument (e.g., ioctl(s, READFROMETH, struct ifreq *)), then you have to use the `_IOWR`() form not the `_IOR`(), as passing the name of the card to the kernel already consists of writing data.

# SEE ALSO

ioctl(2)

January 27, 2019 | NetBSD 9.4