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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, ...);
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'',
``out'', or ``inout'' parameter, and the size of the first variadic argu-
ment 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
Some ioctls are applicable to any file descriptor. These include:
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).
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 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
acknowledged 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 (This is equiva-
lent to fcntl() F_SETFL O_NONBLOCK and the fcntl() form should be
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
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.
[EFAULT] argp points outside the process's allocated address
[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.
mt(1), execve(2), fcntl(2), intro(4), tty(4)
An ioctl() function call appeared in Version 7 AT&T UNIX.
NetBSD 9.3 December 19, 2010 NetBSD 9.3