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 fd 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 discussion.
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>.
Usually, on success zero is returned. A few ioctl() requests use the return value as an output parameter and return a nonnegative value on success. On error, -1 is returned, and errno is set appropriately.
No single standard. Arguments, returns, and semantics of ioctl() 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() system call appeared in Version 7 AT&T UNIX.