Device hotplug event notification

Introduction

Version 1.0.16, LIBUSBX_API_VERSION >= 0x01000102, has added support for hotplug events on some platforms (you should test if your platform supports hotplug notification by calling libusb_has_capability() with parameter LIBUSB_CAP_HAS_HOTPLUG).

This interface allows you to request notification for the arrival and departure of matching USB devices.

To receive hotplug notification you register a callback by calling libusb_hotplug_register_callback(). This function will optionally return a callback handle that can be passed to libusb_hotplug_deregister_callback().

A callback function must return an int (0 or 1) indicating whether the callback is expecting additional events. Returning 0 will rearm the callback and 1 will cause the callback to be deregistered. Note that when callbacks are called from libusb_hotplug_register_callback() because of the LIBUSB_HOTPLUG_ENUMERATE flag, the callback return value is ignored. In other words, you cannot cause a callback to be deregistered by returning 1 when it is called from libusb_hotplug_register_callback().

Callbacks for a particular context are automatically deregistered by libusb_exit().

As of 1.0.16 there are two supported hotplug events:

• LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED: A device has arrived and is ready to use
• LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT: A device has left and is no longer available

A hotplug event can listen for either or both of these events.

Note: If you receive notification that a device has left and you have any libusb_device_handles for the device it is up to you to call libusb_close() on each device handle to free up any remaining resources associated with the device. Once a device has left any libusb_device_handle associated with the device are invalid and will remain so even if the device comes back.

When handling a LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED event it is considered safe to call any libusb function that takes a libusb_device. It also safe to open a device and submit asynchronous transfers. However, most other functions that take a libusb_device_handle are not safe to call. Examples of such functions are any of the synchronous API functions or the blocking functions that retrieve various USB descriptors. These functions must be used outside of the context of the hotplug callback.

When handling a LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT event the only safe function is libusb_get_device_descriptor().

The following code provides an example of the usage of the hotplug interface:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <libusb.h>
 
static int count = 0;
 
int hotplug_callback(struct libusb_context *ctx, struct libusb_device *dev,
                     libusb_hotplug_event event, void *user_data) {
  static libusb_device_handle *dev_handle = NULL;
  struct libusb_device_descriptor desc;
  int rc;
 
  (void)libusb_get_device_descriptor(dev, &desc);
 
  if (LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED == event) {
    rc = libusb_open(dev, &dev_handle);
    if (LIBUSB_SUCCESS != rc) {
      printf("Could not open USB device\n");
    }
  } else if (LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT == event) {
    if (dev_handle) {
      libusb_close(dev_handle);
      dev_handle = NULL;
    }
  } else {
    printf("Unhandled event %d\n", event);
  }
  count++;
 
  return 0;
}
 
int main (void) {
  libusb_hotplug_callback_handle callback_handle;
  int rc;
 
  libusb_init_context(NULL, NULL, 0);
 
  rc = libusb_hotplug_register_callback(NULL, LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED |
                                        LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT, 0, 0x045a, 0x5005,
                                        LIBUSB_HOTPLUG_MATCH_ANY, hotplug_callback, NULL,
                                        &callback_handle);
  if (LIBUSB_SUCCESS != rc) {
    printf("Error creating a hotplug callback\n");
    libusb_exit(NULL);
    return EXIT_FAILURE;
  }
 
  while (count < 2) {
    libusb_handle_events_completed(NULL, NULL);
    nanosleep(&(struct timespec){0, 10000000UL}, NULL);
  }
 
  libusb_hotplug_deregister_callback(NULL, callback_handle);
  libusb_exit(NULL);
 
  return 0;
}