ioctl − control a character device


#include <sys/cred.h>
#include <sys/file.h>
#include <sys/types.h>
#include <sys/errno.h>
#include <sys/ddi.h>
#include <sys/sunddi.h>

int prefixioctl(dev_t dev, int cmd, intptr_t arg, int mode, cred_t *cred_p, int *rval_p);


Architecture independent level 1 (DDI/DKI). This entry point is optional.



Device number.


Command argument the driver ioctl() routine interprets as the operation to be performed.


Passes parameters between a user program and the driver. When used with terminals, the argument is the address of a user program structure containing driver or hardware settings. Alternatively, the argument may be a value that has meaning only to the driver. The interpretation of the argument is driver dependent and usually depends on the command type; the kernel does not interpret the argument.


A bit field that contains:

Information set when the device was opened. The driver may use it to determine if the device was opened for reading or writing. The driver can make this determination by checking the FREAD or FWRITE flags. See the flag argument description of the open() routine for further values.

Information on whether the caller is a 32-bit or 64-bit thread.

In some circumstances address space information about the arg argument. See below.


Pointer to the user credential structure.


Pointer to return value for calling process. The driver may elect to set the value which is valid only if the ioctl() succeeds.


ioctl() provides character-access drivers with an alternate entry point that can be used for almost any operation other than a simple transfer of characters in and out of buffers. Most often, ioctl() is used to control device hardware parameters and establish the protocol used by the driver in processing data.

The kernel determines that this is a character device, and looks up the entry point routines in cb_ops(9S). The kernel then packages the user request and arguments as integers and passes them to the driver’s ioctl() routine. The kernel itself does no processing of the passed command, so it is up to the user program and the driver to agree on what the arguments mean.

I/O control commands are used to implement the terminal settings passed from ttymon(1M) and stty(1), to format disk devices, to implement a trace driver for debugging, and to clean up character queues. Since the kernel does not interpret the command type that defines the operation, a driver is free to define its own commands.

Drivers that use an ioctl() routine typically have a command to ’’read’’ the current ioctl() settings, and at least one other that sets new settings. Drivers can use the mode argument to determine if the device unit was opened for reading or writing, if necessary, by checking the FREAD or FWRITE setting.

If the third argument, arg, is a pointer to a user buffer, the driver can call the copyin(9F) and copyout(9F) functions to transfer data between kernel and user space.

Other kernel subsystems may need to call into the drivers ioctl() routine. Drivers that intend to allow their ioctl() routine to be used in this way should publish the ddi-kernel-ioctl property on the associated devinfo node(s).

When the ddi-kernel-ioctl property is present, the mode argument is used to pass address space information about arg through to the driver. If the driver expects arg to contain a buffer address, and the FKIOCTL flag is set in mode, then the driver should assume that it is being handed a kernel buffer address. Otherwise, arg may be the address of a buffer from a user program. The driver can use ddi_copyin(9F) and ddi_copyout(9F) perform the correct type of copy operation for either kernel or user address spaces. See the example on ddi_copyout(9F).

Drivers have to interact with 32-bit and 64-bit applications. If a device driver shares data structures with the application (for example, through exported kernel memory) and the driver gets recompiled for a 64-bit kernel but the application remains 32-bit, binary layout of any data structures will be incompatible if they contain longs or pointers. The driver needs to know whether there is a model mismatch between the current thread and the kernel and take necessary action. The mode argument has additional bits set to determine the C Language Type Model which the current thread expects. mode has FILP32 set if the current thread expects 32-bit ( ILP32) semantics, or FLP64 if the current thread expects 64-bit ( LP64) semantics. mode is used in combination with ddi_model_convert_from(9F) and the FMODELS mask to determine whether there is a data model mismatch between the current thread and the device driver (see the example below). The device driver might have to adjust the shape of data structures before exporting them to a user thread which supports a different data model.

To implement I/O control commands for a driver the following two steps are required:


Define the I/O control command names and the associated value in the driver’s header and comment the commands.


Code the ioctl() routine in the driver that defines the functionality for each I/O control command name that is in the header.

The ioctl() routine is coded with instructions on the proper action to take for each command. It is commonly a switch statement, with each case definition corresponding to an ioctl() name to identify the action that should be taken. However, the command passed to the driver by the user process is an integer value associated with the command name in the header.


ioctl() should return 0 on success, or the appropriate error number. The driver may also set the value returned to the calling process through rval_p.


Example 1: ioctl() entry point

The following is an example of the ioctl() entry point and how to support 32-bit and 64-bit applications with the same device driver.

struct passargs32 {
        int len;
        caddr32_t addr;

struct passargs {
        int len;
        caddr_t addr;

xxioctl(dev_t dev, int cmd, intptr_t arg, int mode,
    cred_t *credp, int *rvalp) {
        struct passargs pa;

        switch (ddi_model_convert_from(mode & FMODELS)) {
            case DDI_MODEL_ILP32:
                struct passargs32 pa32;

                ddi_copyin(arg, &pa32, sizeof (struct passargs32), mode);
                pa.len = pa32.len;
                pa.address = pa32.address;
            case DDI_MODEL_NONE:
                ddi_copyin(arg, &pa, sizeof (struct passargs), mode);
#else /* _MULTI_DATAMODEL */
        ddi_copyin(arg, &pa, sizeof (struct passargs), mode);
#endif  /* _MULTI_DATAMODEL */



stty(1), ttymon(1M), dkio(7I), fbio(7I), termio(7I), open(9E), put(9E), srv(9E), copyin(9F), copyout(9F), ddi_copyin(9F), ddi_copyout(9F), ddi_model_convert_from(9F), cb_ops(9S)


Non-STREAMS driver ioctl() routines must make sure that user data is copied into or out of the kernel address space explicitly using copyin(9F), copyout(9F), ddi_copyin(9F), or ddi_copyout(9F), as appropriate.

It is a severe error to simply dereference pointers to the user address space, even when in user context.

Failure to use the appropriate copying routines can result in panics under load on some platforms, and reproducible panics on others.


STREAMS drivers do not have ioctl() routines. The stream head converts I/O control commands to M_IOCTL messages, which are handled by the driver’s put(9E) or srv(9E) routine.