NAME

       ioctl — control device

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #include <sys/ioctl.h>

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

DESCRIPTION

       The  ioctl()  system  call 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 fd must be an open file descriptor.

       The third argument to ioctl() is traditionally named char *argp.  Most uses of ioctl(), however,  require
       the third argument to be a caddr_t or an int.

       An  ioctl() request has encoded in it whether the argument is an “in” argument or “out” argument, 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>.

GENERIC IOCTLS

       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 TCP sockets, these bytes
               have not yet been acknowledged by the other side of the connection.

       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.

RETURN VALUES

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

ERRORS

       The ioctl() system call will fail if:

       [EBADF]            The fd argument is not a valid descriptor.

       [ENOTTY]           The fd argument is not associated with a character special device.

       [ENOTTY]           The  specified  request  does  not  apply to the kind of object that the descriptor fd
                          references.

       [EINVAL]           The request or argp argument is not valid.

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

SEE ALSO

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

HISTORY

       The ioctl() function appeared in Version 7 AT&T UNIX.

Debian                                         September 11, 2013                                       IOCTL(2)