diff options
| -rw-r--r-- | Documentation/userspace-api/ioctl/ioctl-number.rst | 22 |
1 files changed, 12 insertions, 10 deletions
diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst index cbc65019879b..677456c31228 100644 --- a/Documentation/userspace-api/ioctl/ioctl-number.rst +++ b/Documentation/userspace-api/ioctl/ioctl-number.rst @@ -25,9 +25,9 @@ be _IOW, although the kernel would actually read data from user space; 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 @@ -35,12 +35,14 @@ should suffice. You can register the block by patching this file and 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 @@ -53,7 +55,7 @@ Following this convention is good because: 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. |