Filtering keycodes with EVIOCSMASK

Question

I would like to receive only EV_KEY events with codes 224 and 225 when reading from a /dev/input/event* file. How do I do that?

---

As far as I can tell, I need to run ioctl(fd, EVIOCSMASK, &mask), but I don't understand how to set up a struct input_mask with my specifications.

I tried setting type to EV_KEY, but that seems to filter out EV_KEY, and if I set anything in codes_* (such as an array of codes and a pointer to it), then ioctl returns -1.

Answer 1

Look at include/linux/input.h:

The event mask is a per-client mask that specifies which events are forwarded to the client. Each event code is represented by a single bit in the event mask. If the bit is set, the event is passed to the client normally.

In other words:

#include <stdlib.h>
#include <limits.h>
#include <sys/ioctl.h>
#include <linux/input.h>
#include <errno.h>

#define  ULONG_BITS  (CHAR_BIT * sizeof (unsigned long))

static void set_bit(unsigned long bits[], size_t bit)
{
    bits[bit / ULONG_BITS] |= 1UL << (bit % ULONG_BITS);
}

static void set_keymask(int fd)
{
    unsigned long      bits[(KEY_CNT + ULONG_BITS - 1) / ULONG_BITS] = { 0 };
    struct input_mask  mask;

    mask.type = EV_KEY;
    mask.codes_size = KEY_CNT;
    mask.codes_ptr = &bits;

    /* Only report KEY_BRIGHTNESSDOWN and KEY_BRIGHTNESSUP */
    set_bit(bits, KEY_BRIGHTNESSDOWN);
    set_bit(bits, KEY_BRIGHTNESSUP);

    if (ioctl(fd, EVIOSMASK, &mask))
        return errno;

    return 0;
}