November 6, 2004 IOCTL 2

### NAME

ioctl - control device

### LIBRARY

Standard C Library (libc, -lc)

### SYNOPSIS

```int` ioctl(```int d``, unsigned long request``, void *argp`)

### 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'' parameter or ``out'' parameter, and the size of the argument ```argp` in bytes. Macros and defines used in specifying an ioctl ```request` are located in the file ```sys/ioctl.h<.blm Pp`.`>`

### 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.

```FIONCLEX`
  Clear close-on-exec flag. The file will remain open across exec(3).

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 acknolwedged 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.

```FIOASYNC` `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.

```FIOSETOWN,` `FIOGETOWN` `int`
  Set/get the process or the process group (if negative) that should receive ```SIGIO` signals when data is available.

### 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.

[```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 ```argp` is not valid.

[```EFAULT`]
  ```argp` points outside the process's allocated address space.

### SEE ALSO

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

### HISTORY

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