IOCTL

Section: Linux Programmer's Manual (2)
Updated: 2020-04-11
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NAME

ioctl - control device

SYNOPSIS

#include <sys/ioctl.h>

int ioctl(int fd, unsigned long request, ...);

DESCRIPTION

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>. See NOTES.

RETURN VALUE

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.

ERRORS

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.

CONFORMING TO

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.

NOTES

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.

ioctl structure

Ioctl command values are 32-bit constants. In principle these constants are completely arbitrary, but people have tried to build some structure into them.

The old Linux situation was that of mostly 16-bit constants, where the last byte is a serial number, and the preceding byte(s) give a type indicating the driver. Sometimes the major number was used: 0x03 for the HDIO_* ioctls, 0x06 for the LP* ioctls. And sometimes one or more ASCII letters were used. For example, TCGETS has value 0x00005401, with 0x54 = 'T' indicating the terminal driver, and CYGETTIMEOUT has value 0x00435906, with 0x43 0x59 = 'C' 'Y' indicating the cyclades driver.

Later (0.98p5) some more information was built into the number. One has 2 direction bits (00: none, 01: write, 10: read, 11: read/write) followed by 14 size bits (giving the size of the argument), followed by an 8-bit type (collecting the ioctls in groups for a common purpose or a common driver), and an 8-bit serial number.

The macros describing this structure live in <asm/ioctl.h> and are _IO(type,nr) and {_IOR,_IOW,_IOWR}(type,nr,size). They use sizeof(size) so that size is a misnomer here: this third argument is a data type.

Note that the size bits are very unreliable: in lots of cases they are wrong, either because of buggy macros using sizeof(sizeof(struct)), or because of legacy values.

Thus, it seems that the new structure only gave disadvantages: it does not help in checking, but it causes varying values for the various architectures.

COLOPHON

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