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IOCTL(2) System Calls Manual IOCTL(2)

ioctlcontrol device

#include <sys/ioctl.h>

ioctl(int d, unsigned long request, ...);

The () function manipulates the underlying device parameters of special 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 execve(2) is invoked.
Clear close-on-exec flag. The file will remain open across execve(2).

Some generic ioctls are not implemented for all types of file descriptors. These include:

int *
Get the number of bytes that are immediately available for reading.
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.
int *
Set asynchronous I/O mode if the argument is non-zero. In asynchronous mode, the process or process group specified by FIOSETOWN will start receiving SIGIO signals when data is available. The SIGIO signal will be delivered when data is available on the file descriptor.
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:

d is not a valid descriptor.
d is not associated with a character special device.
The specified request does not apply to the kind of object that the descriptor d references.
request or arg is not valid.
arg points outside the process's allocated address space.

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.

September 11, 2022 OpenBSD-7.5