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ioctl — control device
Standard C Library (libc, -lc)
ioctl(int d, unsigned long request, ...);
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 d 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 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
Some generic ioctls are not implemented for all types of file
descriptors. These include:
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 connection.
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.
If an error has occurred, a value of -1 is returned and errno is set to
indicate the error.
The ioctl() system call will fail if:
[EBADF] The d argument is not a valid descriptor.
[ENOTTY] The d argument is not associated with a character
[ENOTTY] The specified request does not apply to the kind of
object that the descriptor d references.
[EINVAL] The request or argp argument is not valid.
[EFAULT] The argp argument points outside the process's
allocated address space.
execve(2), fcntl(2), intro(4), tty(4)
The ioctl() function appeared in Version 7 AT&T UNIX.