устройство управления (control device)
Имя (Name)
ioctl - control device
Синопсис (Synopsis)
#include <sys/ioctl.h>
int ioctl(int fd, unsigned long request, ...);
Описание (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.
Возвращаемое значение (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 to
indicate the error.
Ошибки (Error)
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.
Стандарты (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.
Примечание (Note)
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.
Смотри также (See also)
execve(2), fcntl(2), ioctl_console(2), ioctl_fat(2),
ioctl_ficlonerange(2), ioctl_fideduperange(2), ioctl_fslabel(2),
ioctl_getfsmap(2), ioctl_iflags(2), ioctl_ns(2), ioctl_tty(2),
ioctl_userfaultfd(2), open(2), sd(4), tty(4)
