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