IOCTL(2)

IOCTL(2) | System Calls Manual | IOCTL(2)

# NAME

`ioctl` —
control device

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <sys/ioctl.h>`

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

# DESCRIPTION

The `ioctl`() 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.

An `ioctl`() request has encoded in it whether the argument is an “in”, “out”, or “inout” parameter, and the size of the first variadic argument in bytes. Note that there can be only one variadic argument but cannot be represented as a void * argument in the prototype because this would require a cast to pass integral types without warnings. Macros and defines used in specifying an `ioctl`() request are located in the header `<sys/ioctl.h>`.

# GENERIC IOCTLS

Some ioctls are applicable to any file descriptor. These include:

`FIOCLEX`
  Set close-on-exec flag. The file will be closed when exec(3) is invoked (This is equivalent to `fcntl`() `F_SETFD` `FD_CLOEXEC` and the `fcntl`() form should be preferred).
`FIONCLEX`
  Clear close-on-exec flag. The file will remain open across exec(3) (This is equivalent to `fcntl`() `F_SETFD` `0` and the `fcntl`() form should be preferred).

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

`FIONREAD int`
  Get the number of bytes that are immediately available for reading.
`FIONWRITE int`
  Get the number of bytes in the descriptor's send queue. These bytes are data which has been written to the descriptor but which are being held by the kernel for further processing. The nature of the required processing depends on the underlying device. For tty devices, these bytes are typically queued for delivery to the tty hardware. For TCP sockets, these bytes have not yet been acknowledged by the other side of the connection. For files, this operation always returns zero as files do not have send queues.
`FIONSPACE int`
  Get the free space in the descriptor's send queue. This value is the size of the send queue minus the number of bytes being held in the queue. Note: while this value represents the number of bytes that may be added to the queue, other resource limitations may cause a write not larger than the send queue's space to be blocked. One such limitation would be a lack of network buffers for a write to a network connection.
`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 (This is equivalent to `fcntl`() `F_SETFL` `O_NONBLOCK` and the `fcntl`() form should be preferred).
`FIOASYNC int`
  Set asynchronous I/O mode if the argument is non-zero (This is equivalent to `fcntl`() `F_SETFL` `O_ASYNC` and the `fcntl`() form should be preferred). 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.
`FIOSETOWN, FIOGETOWN int`
  Set/get the process or the process group (if negative) that should receive `SIGIO` signals when data is available (This is equivalent to `fcntl`() `F_SETOWN` pid_t and the fcntl form should be preferred).

# RETURN VALUES

If an error has occurred, a value of -1 is returned and errno is set to indicate the error.

# ERRORS

`ioctl`() will fail if:

[`EBADF`]
  d is not a valid descriptor.
[`EFAULT`]
  argp points outside the process's allocated address space.
[`EINVAL`]
  request or argp is not valid.
[`ENOTTY`]
  d is not associated with a character special device; or the specified request does not apply to the kind of object that the descriptor d references.

# SEE ALSO

mt(1), execve(2), fcntl(2), intro(4), tty(4)

# HISTORY

An `ioctl`() function call appeared in Version 7 AT&T UNIX.

December 19, 2010 | NetBSD 9.5