I'm on a laptop with 2 connected keyboards (built-in and USB). I'm obtaining these connected keyboards with libudev and using epoll to poll them for input via the evdev interface:
// Compile with $(gcc udev.c -ludev) #include <stdbool.h> #include <stdio.h> #include <string.h> #include <sys/epoll.h> #include <sys/poll.h> #include <sys/types.h> #include <fcntl.h> #include <unistd.h> #include <linux/input.h> #include <time.h> #include <libudev.h> #define BILLION 1000000000L long timespec_diff(struct timespec *start, struct timespec *end) { return (BILLION * (end->tv_sec - start->tv_sec)) + (end->tv_nsec - start->tv_nsec); } bool want_to_run = true; int main(int argc, char *argv[]) { int epoll_fd = epoll_create1(0); struct udev *udev_obj = udev_new(); struct udev_enumerate *udev_enum = udev_enumerate_new(udev_obj); udev_enumerate_add_match_subsystem(udev_enum, "input"); udev_enumerate_scan_devices(udev_enum); struct udev_list_entry *udev_entries = udev_enumerate_get_list_entry(udev_enum); struct udev_list_entry *udev_entry = NULL; udev_list_entry_foreach(udev_entry, udev_entries) { char const *udev_entry_syspath = udev_list_entry_get_name(udev_entry); struct udev_device *device = udev_device_new_from_syspath(udev_obj, udev_entry_syspath); char const *dev_prop = \ udev_device_get_property_value(device, "ID_INPUT_KEYBOARD"); if (dev_prop != NULL && strcmp(dev_prop, "1") == 0) { const char *dev_path = udev_device_get_devnode(device); if (dev_path != NULL) { int dev_fd = open(dev_path, O_RDWR | O_NONBLOCK); struct epoll_event event = {}; event.events = EPOLLIN; event.data.fd = dev_fd; epoll_ctl(epoll_fd, EPOLL_CTL_ADD, dev_fd, &event); } } udev_device_unref(device); } udev_enumerate_unref(udev_enum); struct timespec prev_timespec = {}; clock_gettime(CLOCK_MONOTONIC_RAW, &prev_timespec); while (want_to_run) { struct epoll_event epoll_events[5] = {0}; int num_epoll_events = epoll_wait(epoll_fd, epoll_events, 5, 0); for (int epoll_event_i = 0; epoll_event_i < num_epoll_events; ++epoll_event_i) { int dev_fd = epoll_events[epoll_event_i].data.fd; struct input_event dev_events[4] = {0}; int dev_event_bytes_read = read(dev_fd, dev_events, sizeof(dev_events)); int num_dev_events = dev_event_bytes_read / sizeof(dev_events[0]); for (int dev_event_i = 0; dev_event_i < num_dev_events; ++dev_event_i) { int dev_event_type = dev_events[dev_event_i].type; int dev_event_code = dev_events[dev_event_i].code; int dev_event_value = dev_events[dev_event_i].value; bool is_released = (dev_event_type == EV_KEY ? dev_event_value == 0 : false); bool is_down = (dev_event_type == EV_KEY ? dev_event_value == 1 : false); bool was_down = (dev_event_type == EV_KEY ? dev_event_value == 2 : false); bool w = (dev_event_code == KEY_W); bool a = (dev_event_code == KEY_A); bool s = (dev_event_code == KEY_S); bool d = (dev_event_code == KEY_D); bool q = (dev_event_code == KEY_Q); bool e = (dev_event_code == KEY_E); bool up = (dev_event_code == KEY_UP); bool down = (dev_event_code == KEY_DOWN); bool left = (dev_event_code == KEY_LEFT); bool right = (dev_event_code == KEY_RIGHT); bool escape = (dev_event_code == KEY_ESC); bool space = (dev_event_code == KEY_SPACE); bool enter = (dev_event_code == KEY_ENTER); bool ctrl = (dev_event_code == KEY_LEFTCTRL); if (q) want_to_run = false; } } struct timespec end_timespec = {}; clock_gettime(CLOCK_MONOTONIC_RAW, &end_timespec); printf("ns per frame: %lu\n", timespec_diff(&prev_timespec, &end_timespec)); prev_timespec = end_timespec; } return 0; }
Experimenting by entering keys on each keyboard I experience some serious lag/stall in the following circumstances (I encourage you to compile and try yourself):
- If I start entering keys on one keyboard and then switch to the other, the program stalls briefly.
- If I simultaneously enter keys on each keyboard the program stalls indefinitely until I stop entering keys.
What is going on here?