a GET_FOO ioctl would be _IOR, although the kernel would actually write
data to user space.
-The first argument to _IO, _IOW, _IOR, or _IOWR is an identifying letter
-or number from the table below. Because of the large number of drivers,
-many drivers share a partial letter with other drivers.
+The first argument to the macros is an identifying letter or number from
+the table below. Because of the large number of drivers, many drivers
+share a partial letter with other drivers.
If you are writing a driver for a new device and need a letter, pick an
unused block with enough room for expansion: 32 to 256 ioctl commands
submitting the patch through :doc:`usual patch submission process
</process/submitting-patches>`.
-The second argument to _IO, _IOW, _IOR, or _IOWR is a sequence number
-to distinguish ioctls from each other. The third argument to _IOW,
-_IOR, or _IOWR is the type of the data going into the kernel or coming
-out of the kernel (e.g. 'int' or 'struct foo'). NOTE! Do NOT use
-sizeof(arg) as the third argument as this results in your ioctl thinking
-it passes an argument of type size_t.
+The second argument is a sequence number to distinguish ioctls from each
+other. The third argument (not applicable to _IO) is the type of the data
+going into the kernel or coming out of the kernel (e.g. 'int' or
+'struct foo').
+
+.. note::
+ Do NOT use sizeof(arg) as the third argument as this results in your
+ ioctl thinking it passes an argument of type size_t.
Some devices use their major number as the identifier; this is OK, as
long as it is unique. Some devices are irregular and don't follow any
error rather than some unexpected behaviour.
(2) The 'strace' build procedure automatically finds ioctl numbers
- defined with _IO, _IOW, _IOR, or _IOWR.
+ defined with the macros.
(3) 'strace' can decode numbers back into useful names when the
numbers are unique.