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ioctl - control device


#include <sys/ioctl.h>
int ioctl(int fd, unsigned long request, ...);


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.


EBADF fd is not a valid file descriptor.
EFAULT argp references an inaccessible memory area.
EINVAL request or argp is not valid.
ENOTTY fd is not associated with a character special device.
ENOTTY The specified request does not apply to the kind of object that the file descriptor fd 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). The ioctl() system 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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