The ioctl function 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 second argument is a device-dependent request code. The third argument is an
untyped pointer to memory. It's traditionally char *argp (from
the days before void * was valid C), and will be so named for this
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>.
No single standard. Arguments, returns, and semantics of ioctl(2) vary
according to the device driver in question (the call is used as a catch-all
for operations that don't cleanly fit the Unix stream I/O model). See
ioctl_list(2) for a list of many of the known ioctl calls. The
ioctl function call appeared in Version 7 AT&T Unix.