get_input_event_key_state.c

#include <stdio.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/input.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>

#define EVDEV "/dev/input/event0"

#define BUTTON_CODE 42
#define PRESSED 0
#define RELEASED 1

int main(int argc, char **argv) {
    unsigned char key_states[KEY_MAX/8 + 1];
    struct input_event evt;
    int fd;

    memset(key_states, 0, sizeof(key_states));
    fd = open(EVDEV, O_RDONLY);
    int err = errno;

    if (fd < 0) {
        printf("failed open " EVDEV ": %s\n", strerror(err));
        return -1;
    }

    int rv = ioctl(fd, EVIOCGKEY(sizeof(key_states)), key_states);
    err = errno;
    close(fd);
    if (rv < 0) {
        printf("Failed ioctl: %s\n", strerror(err));
        return -1;
    }

    int byte = BUTTON_CODE / 8;
    int bit = BUTTON_CODE % 8;
    int state = !!(key_states[byte] & (1 << bit));
    if (state == PRESSED) {
        printf("key is pressed\n");
    } else {
        printf("key is released\n");
    }
}