IOCTL(2) System Calls IOCTL(2)

ioctl - control device

#include <unistd.h>
#include <stropts.h>
int ioctl(int fildes, int request, /* arg */ ...);

The ioctl() function performs a variety of control functions on devices and streams. For non-streams files, the functions performed by this call are device-specific control functions. The request argument and an optional third argument with varying type are passed to the device file designated by fildes and are interpreted by the device driver.

For streams files, specific functions are performed by the ioctl() function as described in streamio(4I).

The fildes argument is an open file descriptor that refers to a device. The request argument selects the control function to be performed and depends on the device being addressed. The arg argument represents a third argument that has additional information that is needed by this specific device to perform the requested function. The data type of arg depends upon the particular control request, but it is either an int or a pointer to a device-specific data structure.

In addition to device-specific and streams functions, generic functions are provided by more than one device driver (for example, the general terminal interface or disk interfaces.) See termio(4I)), dkio(4I), etc..

Upon successful completion, the value returned depends upon the device control function, but must be a non-negative integer. Otherwise, −1 is returned and errno is set to indicate the error.

The ioctl() function will fail for any type of file if:


The fildes argument is not a valid open file descriptor.


A signal was caught during the execution of the ioctl() function.


The stream or multiplexer referenced by fildes is linked (directly or indirectly) downstream from a multiplexer.

The ioctl() function will also fail if the device driver detects an error. In this case, the error is passed through ioctl() without change to the caller. A particular driver might not have all of the following error cases. Under the following conditions, requests to device drivers may fail and set errno to indicate the error


The request argument requires a data transfer to or from a buffer pointed to by arg, but arg points to an illegal address.


The request or arg argument is not valid for this device. Many devices return ENOTTY for an unknown request.


Some physical I/O error has occurred.


The fildes argument is on a remote machine and the link to that machine is no longer active.


The fildes argument is not associated with a device that accepts control functions.

The device driver does not know the request command.


The request and arg arguments are valid for this device driver, but the service requested can not be performed on this particular subdevice.

The device driver does not support the ioctl() function.


The fildes argument refers to a valid streams device, but the corresponding device driver does not support the ioctl() function.

Streams errors are described in streamio(4I).

See attributes(7) for descriptions of the following attributes:

Interface Stability Standard

streamio(4I), termio(4I), attributes(7), standards(7)

June 18, 2020 OmniOS