— control device
Standard C Library (libc, -lc)
, unsigned long
() 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
() requests. The argument d
be an open file descriptor.
The third argument to ioctl
() is traditionally named
. Most uses of ioctl
however, require the third argument to be a caddr_t
has encoded in it
whether the argument is an “in” argument or “out”
argument, and the size of the argument argp
Macros and defines used in specifying an ioctl request
are located in the file
Some generic ioctls are not implemented for all types of file descriptors. These
- Get the number of bytes that are immediately available for
- 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
- 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
If an error has occurred, a value of -1 is returned and
is set to indicate the error.
() system call will fail if:
- The d argument is not a valid
- The d argument is not associated with
a character special device.
- The specified request does not apply to the kind of object
that the descriptor d references.
- The request or
argp argument is not valid.
- The argp argument points outside the
process's allocated address space.
() function appeared in
Version 7 AT&T UNIX