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ioctl() - Unix, Linux System Call
ioctl - control device
int ioctl(int d, int request, ...);
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. Its traditionally
char *argp (from the days before
void * was valid C), and will be so named for this discussion.
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
Usually, on success zero is returned.
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.
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
In order to use this call, one needs an open file descriptor.
call has unwanted side effects, that can be avoided under Linux
by giving it the O_NONBLOCK flag.
No single standard.
Arguments, returns, and semantics of
vary according to the device driver in question (the call is used as a
catch-all for operations that dont cleanly fit the Unix stream I/O
for a list of many of the known
ioctl() function call appeared in Version 7 AT&T Unix.