read (midde) mouse button state every 5ms

mouse.c

#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <linux/input.h>
#include <stdint.h>

#define test_bit(p, n) !!(p[n>>3] & (1u << (n&7)))

int isButtonPressed(int fd, unsigned short code) {
	uint8_t keys[(KEY_MAX<<3)+1];
	memset(keys, 0, sizeof(keys));
	ioctl(fd, EVIOCGKEY(sizeof(keys)), keys);
	return test_bit(keys, code);
}

int main(int argc, char* argv[])
{
	char * dev;
	int fd;
	if (argc < 2) {
		fprintf(stderr, "Usage: %s /dev/input/eventX\n", argv[0]);
		return 1;
	}
	dev = argv[1];
	printf("using %s\n", dev);
	if ((fd = open(dev, O_RDWR)) < 0) {
		perror("open event device");
		return 1;
	}

	while (1) {
		write(1, isButtonPressed(fd, BTN_MIDDLE) ? "X" : ".", 1);
		usleep(5000);
	}
}