int ioctl(int d, unsigned long request, ...);
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 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.
d is not a valid descriptor.
argp references an inaccessible memory area.
request or argp is not valid.
d is not associated with a character special device.
The specified request does not apply to the kind of object that the descriptor d references.
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() function call appeared in Version 7 AT&T UNIX.
In order to use this call, one needs an open file descriptor. Often the open(2) call has unwanted side effects, that can be avoided under Linux by giving it the O_NONBLOCK flag.
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