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IOCTL(2) NetBSD System Calls Manual IOCTL(2)
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ioctl -- control device
Standard C Library (libc, -lc)
ioctl(int d, unsigned long request, void *argp);
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.
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>.
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:
Get the number of bytes that are immediately available for read-
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
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.
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.
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 argp is not valid.
[EFAULT] argp points outside the process's allocated address
mt(1), execve(2), fcntl(2), intro(4), tty(4)
An ioctl() function call appeared in Version 7 AT&T UNIX.
NetBSD 5.0 November 6, 2004 NetBSD 5.0