Section: Linux Programmer's Manual (2)
ioctl - control device
int ioctl(int fd, unsigned long request, ...);
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
must be an open file descriptor.
The second argument is a device-dependent request code.
The third argument is an untyped pointer to memory.
(from the days before
was valid C), and will be so named for this discussion.
has encoded in it whether the argument is an
parameter, and the size of the argument
Macros and defines used in specifying an
are located in the file
Usually, on success zero is returned.
requests use the return value as an output parameter
and return a nonnegative value on success.
On error, -1 is returned, and
is set appropriately.
is not a valid file descriptor.
references an inaccessible memory area.
is not valid.
is not associated with a character special device.
The specified request does not apply to the kind of object that the
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 don't cleanly fit the UNIX stream I/O
system call appeared in Version 7 AT&T UNIX.
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
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
ioctls, 0x06 for the
one or more ASCII letters were used.
0x00005401, with 0x54 = 'T' indicating the terminal driver, and
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
The macros describing this structure live in
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
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.
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