IOCTL Linux device driver
The ioctl
function is useful for implementing a device driver to set the configuration on the device. e.g. a printer that has configuration options to check and set the font family, font size etc. ioctl
could be used to get the current font as well as set the font to a new one. A user application uses ioctl
to send a code to a printer telling it to return the current font or to set the font to a new one.
int ioctl(int fd, int request, ...)
fd
is file descriptor, the one returned byopen
;request
is request code. e.gGETFONT
will get the current font from the printer,SETFONT
will set the font on the printer;- the third argument is
void *
. Depending on the second argument, the third may or may not be present, e.g. if the second argument isSETFONT
, the third argument can be the font name such as"Arial"
;
int request
is not just a macro. A user application is required to generate a request code and the device driver module to determine which configuration on device must be played with. The application sends the request code using ioctl
and then uses the request code in the device driver module to determine which action to perform.
A request code has 4 main parts
1. A Magic number - 8 bits
2. A sequence number - 8 bits
3. Argument type (typically 14 bits), if any.
4. Direction of data transfer (2 bits).
If the request code is SETFONT
to set font on a printer, the direction for data transfer will be from user application to device driver module (The user application sends the font name "Arial"
to the printer).
If the request code is GETFONT
, direction is from printer to the user application.
In order to generate a request code, Linux provides some predefined function-like macros.
1._IO(MAGIC, SEQ_NO)
both are 8 bits, 0 to 255, e.g. let us say we want to pause printer.
This does not require a data transfer. So we would generate the request code as below
#define PRIN_MAGIC 'P'
#define NUM 0
#define PAUSE_PRIN __IO(PRIN_MAGIC, NUM)
and now use ioctl
as
ret_val = ioctl(fd, PAUSE_PRIN);
The corresponding system call in the driver module will receive the code and pause the printer.
__IOW(MAGIC, SEQ_NO, TYPE)
MAGIC
andSEQ_NO
are the same as above, andTYPE
gives the type of the next argument, recall the third argument ofioctl
isvoid *
. W in__IOW
indicates that the data flow is from user application to driver module. As an example, suppose we want to set the printer font to"Arial"
.
#define PRIN_MAGIC 'S'
#define SEQ_NO 1
#define SETFONT __IOW(PRIN_MAGIC, SEQ_NO, unsigned long)
further,
char *font = "Arial";
ret_val = ioctl(fd, SETFONT, font);
Now font
is a pointer, which means it is an address best represented as unsigned long
, hence the third part of _IOW
mentions type as such. Also, this address of font is passed to corresponding system call implemented in device driver module as unsigned long
and we need to cast it to proper type before using it. Kernel space can access user space and hence this works. other two function-like macros are __IOR(MAGIC, SEQ_NO, TYPE)
and __IORW(MAGIC, SEQ_NO, TYPE)
where the data flow will be from kernel space to user space and both ways respectively.
Please let me know if this helps!
An ioctl
, which means "input-output control" is a kind of device-specific system call. There are only a few system calls in Linux (300-400), which are not enough to express all the unique functions devices may have. So a driver can define an ioctl which allows a userspace application to send it orders. However, ioctls are not very flexible and tend to get a bit cluttered (dozens of "magic numbers" which just work... or not), and can also be insecure, as you pass a buffer into the kernel - bad handling can break things easily.
An alternative is the sysfs
interface, where you set up a file under /sys/
and read/write that to get information from and to the driver. An example of how to set this up:
static ssize_t mydrvr_version_show(struct device *dev,
struct device_attribute *attr, char *buf)
{
return sprintf(buf, "%s\n", DRIVER_RELEASE);
}
static DEVICE_ATTR(version, S_IRUGO, mydrvr_version_show, NULL);
And during driver setup:
device_create_file(dev, &dev_attr_version);
You would then have a file for your device in /sys/
, for example, /sys/block/myblk/version
for a block driver.
Another method for heavier use is netlink, which is an IPC (inter-process communication) method to talk to your driver over a BSD socket interface. This is used, for example, by the WiFi drivers. You then communicate with it from userspace using the libnl
or libnl3
libraries.