IOCTL(2) BSD System Calls Manual IOCTL(2)
ioctl -- control device
ioctl(int d, unsigned long request, ...);
The ioctl() function manipulates the underlying device parameters of spe-
cial files. In particular, many operating characteristics of character
special files (e.g., terminals) may be controlled with ioctl() requests.
The argument d must be an open file descriptor. The third argument is
called arg and contains additional information needed by this device to
perform the requested function. arg is either an int or a pointer to a
device-specific data structure, depending upon the given request.
An ioctl request has encoded in it whether the argument is an ``in''
parameter or ``out'' parameter, and the size of the third argument (arg)
in bytes. Macros and defines used in specifying an ioctl request are
located in the file <sys/ioctl.h>.
Some ioctls are applicable to any file descriptor. These include:
Set close-on-exec flag. The file will be closed when exec(3) is
Clear close-on-exec flag. The file will remain open across
Some generic ioctls are not implemented for all types of file descrip-
tors. These include:
FIONREAD int *
Get the number of bytes that are immediately available for read-
FIONBIO int *
Set non-blocking I/O mode if the argument is non-zero. In non-
blocking mode, read(2) or write(2) calls return -1 and set errno
to EAGAIN immediately when no data is available.
FIOASYNC int *
Set asynchronous I/O mode if the argument is non-zero. In asyn-
chronous mode, the process or process group specified by
FIOSETOWN will start receiving SIGIO signals when data is avail-
able. The SIGIO signal will be delivered when data is available
on the file descriptor.
FIOSETOWN, FIOGETOWN int *
Set/get the process or the process group (if negative) that
should receive SIGIO signals when data is available.
If an error has occurred, a value of -1 is returned and errno is set to
indicate the error.
ioctl() will fail if:
[EBADF] d is not a valid descriptor.
[ENOTTY] d is not associated with a character special device.
[ENOTTY] The specified request does not apply to the kind of
object that the descriptor d references.
[EINVAL] request or arg is not valid.
[EFAULT] arg points outside the process's allocated address
cdio(1), chio(1), mt(1), execve(2), fcntl(2), intro(4), tty(4)
An ioctl() function call appeared in Version 7 AT&T UNIX.
BSD March 27, 2017 BSD