ioctl -- I/O control command


cc ... -lc

#include <unistd.h>

int ioctl (fildes, request, arg) int fildes, request;


The ioctl system call 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 arguments request and arg are passed to the file designated by fildes and are interpreted by the device driver. Basic input/output functions in STREAMS are performed with the read(S) and write(S) system calls.

fildes is an open file descriptor that refers to a device. request selects the control function to be performed and depends on the device being addressed. arg represents 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 integer 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 (see termio(M)).


The ioctl system call fails for any type of file if one or more of the following is true:

Future error.

fildes is not a valid open file descriptor.

A signal was caught during the ioctl system call.

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

The ioctl system call also fails 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. Other requests to device drivers fails if one or more of the following is true:

request requires a data transfer to or from a buffer pointed to by arg, but some part of the buffer is outside the process's allocated space.

request or arg is not valid for this device.

Some physical I/O error has occurred.

fildes is on a remote machine and the link to that machine is no longer active.

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

STREAMS errors are described in streamio(M).

Return value

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

See also


Standards conformance

ioctl is conformant with:

Intel386 Binary Compatibility Specification, Edition 2 (iBCSe2) .

© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003