io: Track device in usbi_transfer

[platform/upstream/libusb.git] / libusb / libusbi.h

/*
 * Internal header for libusb
 * Copyright © 2007-2009 Daniel Drake <dsd@gentoo.org>
 * Copyright © 2001 Johannes Erdfelt <johannes@erdfelt.com>
 * Copyright © 2019 Nathan Hjelm <hjelmn@cs.umm.edu>
 * Copyright © 2019-2020 Google LLC. All rights reserved.
 * Copyright © 2020 Chris Dickens <christopher.a.dickens@gmail.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#ifndef LIBUSBI_H
#define LIBUSBI_H

#include <config.h>

#include <assert.h>
#include <inttypes.h>
#include <stdarg.h>
#include <stddef.h>
#include <stdlib.h>
#ifdef HAVE_SYS_TIME_H
#include <sys/time.h>
#endif

#include "libusb.h"

/* Not all C standard library headers define static_assert in assert.h
 * Additionally, Visual Studio treats static_assert as a keyword.
 */
#if !defined(__cplusplus) && !defined(static_assert) && !defined(_MSC_VER)
#define static_assert(cond, msg) _Static_assert(cond, msg)
#endif

#ifdef NDEBUG
#define ASSERT_EQ(expression, value)    (void)expression
#define ASSERT_NE(expression, value)    (void)expression
#else
#define ASSERT_EQ(expression, value)    assert(expression == value)
#define ASSERT_NE(expression, value)    assert(expression != value)
#endif

#define container_of(ptr, type, member) \
        ((type *)((uintptr_t)(ptr) - (uintptr_t)offsetof(type, member)))

#ifndef ARRAYSIZE
#define ARRAYSIZE(array) (sizeof(array) / sizeof(array[0]))
#endif

#ifndef CLAMP
#define CLAMP(val, min, max) \
        ((val) < (min) ? (min) : ((val) > (max) ? (max) : (val)))
#endif

#ifndef MIN
#define MIN(a, b)       ((a) < (b) ? (a) : (b))
#endif

#ifndef MAX
#define MAX(a, b)       ((a) > (b) ? (a) : (b))
#endif

/* The following is used to silence warnings for unused variables */
#if defined(UNREFERENCED_PARAMETER)
#define UNUSED(var)     UNREFERENCED_PARAMETER(var)
#else
#define UNUSED(var)     do { (void)(var); } while(0)
#endif

/* Macro to align a value up to the next multiple of the size of a pointer */
#define PTR_ALIGN(v) \
        (((v) + (sizeof(void *) - 1)) & ~(sizeof(void *) - 1))

/* Atomic operations
 *
 * Useful for reference counting or when accessing a value without a lock
 *
 * The following atomic operations are defined:
 *   usbi_atomic_load() - Atomically read a variable's value
 *   usbi_atomic_store() - Atomically write a new value value to a variable
 *   usbi_atomic_inc() - Atomically increment a variable's value and return the new value
 *   usbi_atomic_dec() - Atomically decrement a variable's value and return the new value
 *
 * All of these operations are ordered with each other, thus the effects of
 * any one operation is guaranteed to be seen by any other operation.
 */
#ifdef _MSC_VER
typedef volatile LONG usbi_atomic_t;
#define usbi_atomic_load(a)     (*(a))
#define usbi_atomic_store(a, v) (*(a)) = (v)
#define usbi_atomic_inc(a)      InterlockedIncrement((a))
#define usbi_atomic_dec(a)      InterlockedDecrement((a))
#else
#include <stdatomic.h>
typedef atomic_long usbi_atomic_t;
#define usbi_atomic_load(a)     atomic_load((a))
#define usbi_atomic_store(a, v) atomic_store((a), (v))
#define usbi_atomic_inc(a)      (atomic_fetch_add((a), 1) + 1)
#define usbi_atomic_dec(a)      (atomic_fetch_add((a), -1) - 1)
#endif

/* Internal abstractions for event handling and thread synchronization */
#if defined(PLATFORM_POSIX)
#include "os/events_posix.h"
#include "os/threads_posix.h"
#elif defined(PLATFORM_WINDOWS)
#include "os/events_windows.h"
#include "os/threads_windows.h"
#endif

/* Inside the libusb code, mark all public functions as follows:
 *   return_type API_EXPORTED function_name(params) { ... }
 * But if the function returns a pointer, mark it as follows:
 *   DEFAULT_VISIBILITY return_type * LIBUSB_CALL function_name(params) { ... }
 * In the libusb public header, mark all declarations as:
 *   return_type LIBUSB_CALL function_name(params);
 */
#define API_EXPORTED LIBUSB_CALL DEFAULT_VISIBILITY

#ifdef __cplusplus
extern "C" {
#endif

#define USB_MAXENDPOINTS        32
#define USB_MAXINTERFACES       32
#define USB_MAXCONFIG           8

/* Backend specific capabilities */
#define USBI_CAP_HAS_HID_ACCESS                 0x00010000
#define USBI_CAP_SUPPORTS_DETACH_KERNEL_DRIVER  0x00020000

/* Maximum number of bytes in a log line */
#define USBI_MAX_LOG_LEN        1024
/* Terminator for log lines */
#define USBI_LOG_LINE_END       "\n"

struct list_head {
        struct list_head *prev, *next;
};

/* Get an entry from the list
 *  ptr - the address of this list_head element in "type"
 *  type - the data type that contains "member"
 *  member - the list_head element in "type"
 */
#define list_entry(ptr, type, member) \
        container_of(ptr, type, member)

#define list_first_entry(ptr, type, member) \
        list_entry((ptr)->next, type, member)

#define list_next_entry(ptr, type, member) \
        list_entry((ptr)->member.next, type, member)

/* Get each entry from a list
 *  pos - A structure pointer has a "member" element
 *  head - list head
 *  member - the list_head element in "pos"
 *  type - the type of the first parameter
 */
#define list_for_each_entry(pos, head, member, type)                    \
        for (pos = list_first_entry(head, type, member);                \
                 &pos->member != (head);                                \
                 pos = list_next_entry(pos, type, member))

#define list_for_each_entry_safe(pos, n, head, member, type)            \
        for (pos = list_first_entry(head, type, member),                \
                 n = list_next_entry(pos, type, member);                \
                 &pos->member != (head);                                \
                 pos = n, n = list_next_entry(n, type, member))

/* Helper macros to iterate over a list. The structure pointed
 * to by "pos" must have a list_head member named "list".
 */
#define for_each_helper(pos, head, type) \
        list_for_each_entry(pos, head, list, type)

#define for_each_safe_helper(pos, n, head, type) \
        list_for_each_entry_safe(pos, n, head, list, type)

#define list_empty(entry) ((entry)->next == (entry))

static inline void list_init(struct list_head *entry)
{
        entry->prev = entry->next = entry;
}

static inline void list_add(struct list_head *entry, struct list_head *head)
{
        entry->next = head->next;
        entry->prev = head;

        head->next->prev = entry;
        head->next = entry;
}

static inline void list_add_tail(struct list_head *entry,
        struct list_head *head)
{
        entry->next = head;
        entry->prev = head->prev;

        head->prev->next = entry;
        head->prev = entry;
}

static inline void list_del(struct list_head *entry)
{
        entry->next->prev = entry->prev;
        entry->prev->next = entry->next;
        entry->next = entry->prev = NULL;
}

static inline void list_cut(struct list_head *list, struct list_head *head)
{
        if (list_empty(head)) {
                list_init(list);
                return;
        }

        list->next = head->next;
        list->next->prev = list;
        list->prev = head->prev;
        list->prev->next = list;

        list_init(head);
}

static inline void list_splice_front(struct list_head *list, struct list_head *head)
{
        list->next->prev = head;
        list->prev->next = head->next;
        head->next->prev = list->prev;
        head->next = list->next;
}

static inline void *usbi_reallocf(void *ptr, size_t size)
{
        void *ret = realloc(ptr, size);

        if (!ret)
                free(ptr);
        return ret;
}

#if !defined(USEC_PER_SEC)
#define USEC_PER_SEC    1000000L
#endif

#if !defined(NSEC_PER_SEC)
#define NSEC_PER_SEC    1000000000L
#endif

#define TIMEVAL_IS_VALID(tv)                                            \
        ((tv)->tv_sec >= 0 &&                                           \
         (tv)->tv_usec >= 0 && (tv)->tv_usec < USEC_PER_SEC)

#define TIMESPEC_IS_SET(ts)     ((ts)->tv_sec || (ts)->tv_nsec)
#define TIMESPEC_CLEAR(ts)      (ts)->tv_sec = (ts)->tv_nsec = 0
#define TIMESPEC_CMP(a, b, CMP)                                         \
        (((a)->tv_sec == (b)->tv_sec)                                   \
         ? ((a)->tv_nsec CMP (b)->tv_nsec)                              \
         : ((a)->tv_sec CMP (b)->tv_sec))
#define TIMESPEC_SUB(a, b, result)                                      \
        do {                                                            \
                (result)->tv_sec = (a)->tv_sec - (b)->tv_sec;           \
                (result)->tv_nsec = (a)->tv_nsec - (b)->tv_nsec;        \
                if ((result)->tv_nsec < 0L) {                           \
                        --(result)->tv_sec;                             \
                        (result)->tv_nsec += NSEC_PER_SEC;              \
                }                                                       \
        } while (0)

#if defined(PLATFORM_WINDOWS)
#define TIMEVAL_TV_SEC_TYPE     long
#else
#define TIMEVAL_TV_SEC_TYPE     time_t
#endif

/* Some platforms don't have this define */
#ifndef TIMESPEC_TO_TIMEVAL
#define TIMESPEC_TO_TIMEVAL(tv, ts)                                     \
        do {                                                            \
                (tv)->tv_sec = (TIMEVAL_TV_SEC_TYPE) (ts)->tv_sec;      \
                (tv)->tv_usec = (ts)->tv_nsec / 1000L;                  \
        } while (0)
#endif

#ifdef ENABLE_LOGGING

#if defined(_MSC_VER) && (_MSC_VER < 1900)
#include <stdio.h>
#define snprintf usbi_snprintf
#define vsnprintf usbi_vsnprintf
int usbi_snprintf(char *dst, size_t size, const char *format, ...);
int usbi_vsnprintf(char *dst, size_t size, const char *format, va_list args);
#define LIBUSB_PRINTF_WIN32
#endif /* defined(_MSC_VER) && (_MSC_VER < 1900) */

void usbi_log(struct libusb_context *ctx, enum libusb_log_level level,
        const char *function, const char *format, ...) PRINTF_FORMAT(4, 5);

#define _usbi_log(ctx, level, ...) usbi_log(ctx, level, __func__, __VA_ARGS__)

#define usbi_err(ctx, ...)      _usbi_log(ctx, LIBUSB_LOG_LEVEL_ERROR, __VA_ARGS__)
#define usbi_warn(ctx, ...)     _usbi_log(ctx, LIBUSB_LOG_LEVEL_WARNING, __VA_ARGS__)
#define usbi_info(ctx, ...)     _usbi_log(ctx, LIBUSB_LOG_LEVEL_INFO, __VA_ARGS__)
#define usbi_dbg(ctx ,...)              _usbi_log(ctx, LIBUSB_LOG_LEVEL_DEBUG, __VA_ARGS__)

#else /* ENABLE_LOGGING */

#define usbi_err(ctx, ...)      UNUSED(ctx)
#define usbi_warn(ctx, ...)     UNUSED(ctx)
#define usbi_info(ctx, ...)     UNUSED(ctx)
#define usbi_dbg(ctx, ...)      do {} while (0)

#endif /* ENABLE_LOGGING */

#define DEVICE_CTX(dev)         ((dev)->ctx)
#define HANDLE_CTX(handle)      ((handle) ? DEVICE_CTX((handle)->dev) : NULL)
#define ITRANSFER_CTX(itransfer) \
        ((itransfer)->dev ? DEVICE_CTX((itransfer)->dev) : NULL)
#define TRANSFER_CTX(transfer) \
        (ITRANSFER_CTX(LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer)))

#define IS_EPIN(ep)             (0 != ((ep) & LIBUSB_ENDPOINT_IN))
#define IS_EPOUT(ep)            (!IS_EPIN(ep))
#define IS_XFERIN(xfer)         (0 != ((xfer)->endpoint & LIBUSB_ENDPOINT_IN))
#define IS_XFEROUT(xfer)        (!IS_XFERIN(xfer))

struct libusb_context {
#if defined(ENABLE_LOGGING) && !defined(ENABLE_DEBUG_LOGGING)
        enum libusb_log_level debug;
        int debug_fixed;
        libusb_log_cb log_handler;
#endif

        /* used for signalling occurrence of an internal event. */
        usbi_event_t event;

#ifdef HAVE_OS_TIMER
        /* used for timeout handling, if supported by OS.
         * this timer is maintained to trigger on the next pending timeout */
        usbi_timer_t timer;
#endif

        struct list_head usb_devs;
        usbi_mutex_t usb_devs_lock;

        /* A list of open handles. Backends are free to traverse this if required.
         */
        struct list_head open_devs;
        usbi_mutex_t open_devs_lock;

        /* A list of registered hotplug callbacks */
        struct list_head hotplug_cbs;
        libusb_hotplug_callback_handle next_hotplug_cb_handle;
        usbi_mutex_t hotplug_cbs_lock;

        /* A flag to indicate that the context is ready for hotplug notifications */
        usbi_atomic_t hotplug_ready;

        /* this is a list of in-flight transfer handles, sorted by timeout
         * expiration. URBs to timeout the soonest are placed at the beginning of
         * the list, URBs that will time out later are placed after, and urbs with
         * infinite timeout are always placed at the very end. */
        struct list_head flying_transfers;
        /* Note paths taking both this and usbi_transfer->lock must always
         * take this lock first */
        usbi_mutex_t flying_transfers_lock;

#if !defined(PLATFORM_WINDOWS)
        /* user callbacks for pollfd changes */
        libusb_pollfd_added_cb fd_added_cb;
        libusb_pollfd_removed_cb fd_removed_cb;
        void *fd_cb_user_data;
#endif

        /* ensures that only one thread is handling events at any one time */
        usbi_mutex_t events_lock;

        /* used to see if there is an active thread doing event handling */
        int event_handler_active;

        /* A thread-local storage key to track which thread is performing event
         * handling */
        usbi_tls_key_t event_handling_key;

        /* used to wait for event completion in threads other than the one that is
         * event handling */
        usbi_mutex_t event_waiters_lock;
        usbi_cond_t event_waiters_cond;

        /* A lock to protect internal context event data. */
        usbi_mutex_t event_data_lock;

        /* A bitmask of flags that are set to indicate specific events that need to
         * be handled. Protected by event_data_lock. */
        unsigned int event_flags;

        /* A counter that is set when we want to interrupt and prevent event handling,
         * in order to safely close a device. Protected by event_data_lock. */
        unsigned int device_close;

        /* A list of currently active event sources. Protected by event_data_lock. */
        struct list_head event_sources;

        /* A list of event sources that have been removed since the last time
         * event sources were waited on. Protected by event_data_lock. */
        struct list_head removed_event_sources;

        /* A pointer and count to platform-specific data used for monitoring event
         * sources. Only accessed during event handling. */
        void *event_data;
        unsigned int event_data_cnt;

        /* A list of pending hotplug messages. Protected by event_data_lock. */
        struct list_head hotplug_msgs;

        /* A list of pending completed transfers. Protected by event_data_lock. */
        struct list_head completed_transfers;

        struct list_head list;
};

extern struct libusb_context *usbi_default_context;

extern struct list_head active_contexts_list;
extern usbi_mutex_static_t active_contexts_lock;

static inline struct libusb_context *usbi_get_context(struct libusb_context *ctx)
{
        return ctx ? ctx : usbi_default_context;
}

enum usbi_event_flags {
        /* The list of event sources has been modified */
        USBI_EVENT_EVENT_SOURCES_MODIFIED = 1U << 0,

        /* The user has interrupted the event handler */
        USBI_EVENT_USER_INTERRUPT = 1U << 1,

        /* A hotplug callback deregistration is pending */
        USBI_EVENT_HOTPLUG_CB_DEREGISTERED = 1U << 2,

        /* One or more hotplug messages are pending */
        USBI_EVENT_HOTPLUG_MSG_PENDING = 1U << 3,

        /* One or more completed transfers are pending */
        USBI_EVENT_TRANSFER_COMPLETED = 1U << 4,

        /* A device is in the process of being closed */
        USBI_EVENT_DEVICE_CLOSE = 1U << 5,
};

/* Macros for managing event handling state */
static inline int usbi_handling_events(struct libusb_context *ctx)
{
        return usbi_tls_key_get(ctx->event_handling_key) != NULL;
}

static inline void usbi_start_event_handling(struct libusb_context *ctx)
{
        usbi_tls_key_set(ctx->event_handling_key, ctx);
}

static inline void usbi_end_event_handling(struct libusb_context *ctx)
{
        usbi_tls_key_set(ctx->event_handling_key, NULL);
}

struct libusb_device {
        usbi_atomic_t refcnt;

        struct libusb_context *ctx;
        struct libusb_device *parent_dev;

        uint8_t bus_number;
        uint8_t port_number;
        uint8_t device_address;
        enum libusb_speed speed;

        struct list_head list;
        unsigned long session_data;

        struct libusb_device_descriptor device_descriptor;
        usbi_atomic_t attached;
};

struct libusb_device_handle {
        /* lock protects claimed_interfaces */
        usbi_mutex_t lock;
        unsigned long claimed_interfaces;

        struct list_head list;
        struct libusb_device *dev;
        int auto_detach_kernel_driver;
};

/* Function called by backend during device initialization to convert
 * multi-byte fields in the device descriptor to host-endian format.
 */
static inline void usbi_localize_device_descriptor(struct libusb_device_descriptor *desc)
{
        desc->bcdUSB = libusb_le16_to_cpu(desc->bcdUSB);
        desc->idVendor = libusb_le16_to_cpu(desc->idVendor);
        desc->idProduct = libusb_le16_to_cpu(desc->idProduct);
        desc->bcdDevice = libusb_le16_to_cpu(desc->bcdDevice);
}

#ifdef HAVE_CLOCK_GETTIME
static inline void usbi_get_monotonic_time(struct timespec *tp)
{
        ASSERT_EQ(clock_gettime(CLOCK_MONOTONIC, tp), 0);
}
static inline void usbi_get_real_time(struct timespec *tp)
{
        ASSERT_EQ(clock_gettime(CLOCK_REALTIME, tp), 0);
}
#else
/* If the platform doesn't provide the clock_gettime() function, the backend
 * must provide its own clock implementations.  Two clock functions are
 * required:
 *
 *   usbi_get_monotonic_time(): returns the time since an unspecified starting
 *                              point (usually boot) that is monotonically
 *                              increasing.
 *   usbi_get_real_time(): returns the time since system epoch.
 */
void usbi_get_monotonic_time(struct timespec *tp);
void usbi_get_real_time(struct timespec *tp);
#endif

/* in-memory transfer layout:
 *
 * 1. os private data
 * 2. struct usbi_transfer
 * 3. struct libusb_transfer (which includes iso packets) [variable size]
 *
 * from a libusb_transfer, you can get the usbi_transfer by rewinding the
 * appropriate number of bytes.
 */

struct usbi_transfer {
        int num_iso_packets;
        struct list_head list;
        struct list_head completed_list;
        struct timespec timeout;
        int transferred;
        uint32_t stream_id;
        uint32_t state_flags;   /* Protected by usbi_transfer->lock */
        uint32_t timeout_flags; /* Protected by the flying_stransfers_lock */

        /* The device reference is held until destruction for logging
         * even after dev_handle is set to NULL.  */
        struct libusb_device *dev;

        /* this lock is held during libusb_submit_transfer() and
         * libusb_cancel_transfer() (allowing the OS backend to prevent duplicate
         * cancellation, submission-during-cancellation, etc). the OS backend
         * should also take this lock in the handle_events path, to prevent the user
         * cancelling the transfer from another thread while you are processing
         * its completion (presumably there would be races within your OS backend
         * if this were possible).
         * Note paths taking both this and the flying_transfers_lock must
         * always take the flying_transfers_lock first */
        usbi_mutex_t lock;

        void *priv;
};

enum usbi_transfer_state_flags {
        /* Transfer successfully submitted by backend */
        USBI_TRANSFER_IN_FLIGHT = 1U << 0,

        /* Cancellation was requested via libusb_cancel_transfer() */
        USBI_TRANSFER_CANCELLING = 1U << 1,

        /* Operation on the transfer failed because the device disappeared */
        USBI_TRANSFER_DEVICE_DISAPPEARED = 1U << 2,
};

enum usbi_transfer_timeout_flags {
        /* Set by backend submit_transfer() if the OS handles timeout */
        USBI_TRANSFER_OS_HANDLES_TIMEOUT = 1U << 0,

        /* The transfer timeout has been handled */
        USBI_TRANSFER_TIMEOUT_HANDLED = 1U << 1,

        /* The transfer timeout was successfully processed */
        USBI_TRANSFER_TIMED_OUT = 1U << 2,
};

#define USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer)     \
        ((struct libusb_transfer *)                     \
         ((unsigned char *)(itransfer)                  \
          + PTR_ALIGN(sizeof(struct usbi_transfer))))
#define LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer)      \
        ((struct usbi_transfer *)                       \
         ((unsigned char *)(transfer)                   \
          - PTR_ALIGN(sizeof(struct usbi_transfer))))

#ifdef _MSC_VER
#pragma pack(push, 1)
#endif

/* All standard descriptors have these 2 fields in common */
struct usbi_descriptor_header {
        uint8_t  bLength;
        uint8_t  bDescriptorType;
} LIBUSB_PACKED;

struct usbi_device_descriptor {
        uint8_t  bLength;
        uint8_t  bDescriptorType;
        uint16_t bcdUSB;
        uint8_t  bDeviceClass;
        uint8_t  bDeviceSubClass;
        uint8_t  bDeviceProtocol;
        uint8_t  bMaxPacketSize0;
        uint16_t idVendor;
        uint16_t idProduct;
        uint16_t bcdDevice;
        uint8_t  iManufacturer;
        uint8_t  iProduct;
        uint8_t  iSerialNumber;
        uint8_t  bNumConfigurations;
} LIBUSB_PACKED;

struct usbi_configuration_descriptor {
        uint8_t  bLength;
        uint8_t  bDescriptorType;
        uint16_t wTotalLength;
        uint8_t  bNumInterfaces;
        uint8_t  bConfigurationValue;
        uint8_t  iConfiguration;
        uint8_t  bmAttributes;
        uint8_t  bMaxPower;
} LIBUSB_PACKED;

struct usbi_interface_descriptor {
        uint8_t  bLength;
        uint8_t  bDescriptorType;
        uint8_t  bInterfaceNumber;
        uint8_t  bAlternateSetting;
        uint8_t  bNumEndpoints;
        uint8_t  bInterfaceClass;
        uint8_t  bInterfaceSubClass;
        uint8_t  bInterfaceProtocol;
        uint8_t  iInterface;
} LIBUSB_PACKED;

struct usbi_string_descriptor {
        uint8_t  bLength;
        uint8_t  bDescriptorType;
        uint16_t wData[ZERO_SIZED_ARRAY];
} LIBUSB_PACKED;

struct usbi_bos_descriptor {
        uint8_t  bLength;
        uint8_t  bDescriptorType;
        uint16_t wTotalLength;
        uint8_t  bNumDeviceCaps;
} LIBUSB_PACKED;

#ifdef _MSC_VER
#pragma pack(pop)
#endif

union usbi_config_desc_buf {
        struct usbi_configuration_descriptor desc;
        uint8_t buf[LIBUSB_DT_CONFIG_SIZE];
        uint16_t align;         /* Force 2-byte alignment */
};

union usbi_string_desc_buf {
        struct usbi_string_descriptor desc;
        uint8_t buf[255];       /* Some devices choke on size > 255 */
        uint16_t align;         /* Force 2-byte alignment */
};

union usbi_bos_desc_buf {
        struct usbi_bos_descriptor desc;
        uint8_t buf[LIBUSB_DT_BOS_SIZE];
        uint16_t align;         /* Force 2-byte alignment */
};

enum usbi_hotplug_flags {
        /* This callback is interested in device arrivals */
        USBI_HOTPLUG_DEVICE_ARRIVED = LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED,

        /* This callback is interested in device removals */
        USBI_HOTPLUG_DEVICE_LEFT = LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT,

        /* IMPORTANT: The values for the below entries must start *after*
         * the highest value of the above entries!!!
         */

        /* The vendor_id field is valid for matching */
        USBI_HOTPLUG_VENDOR_ID_VALID = (1U << 3),

        /* The product_id field is valid for matching */
        USBI_HOTPLUG_PRODUCT_ID_VALID = (1U << 4),

        /* The dev_class field is valid for matching */
        USBI_HOTPLUG_DEV_CLASS_VALID = (1U << 5),

        /* This callback has been unregistered and needs to be freed */
        USBI_HOTPLUG_NEEDS_FREE = (1U << 6),
};

struct usbi_hotplug_callback {
        /* Flags that control how this callback behaves */
        uint8_t flags;

        /* Vendor ID to match (if flags says this is valid) */
        uint16_t vendor_id;

        /* Product ID to match (if flags says this is valid) */
        uint16_t product_id;

        /* Device class to match (if flags says this is valid) */
        uint8_t dev_class;

        /* Callback function to invoke for matching event/device */
        libusb_hotplug_callback_fn cb;

        /* Handle for this callback (used to match on deregister) */
        libusb_hotplug_callback_handle handle;

        /* User data that will be passed to the callback function */
        void *user_data;

        /* List this callback is registered in (ctx->hotplug_cbs) */
        struct list_head list;
};

struct usbi_hotplug_message {
        /* The hotplug event that occurred */
        libusb_hotplug_event event;

        /* The device for which this hotplug event occurred */
        struct libusb_device *device;

        /* List this message is contained in (ctx->hotplug_msgs) */
        struct list_head list;
};

/* shared data and functions */

void usbi_hotplug_init(struct libusb_context *ctx);
void usbi_hotplug_exit(struct libusb_context *ctx);
void usbi_hotplug_notification(struct libusb_context *ctx, struct libusb_device *dev,
        libusb_hotplug_event event);
void usbi_hotplug_process(struct libusb_context *ctx, struct list_head *hotplug_msgs);

int usbi_io_init(struct libusb_context *ctx);
void usbi_io_exit(struct libusb_context *ctx);

struct libusb_device *usbi_alloc_device(struct libusb_context *ctx,
        unsigned long session_id);
struct libusb_device *usbi_get_device_by_session_id(struct libusb_context *ctx,
        unsigned long session_id);
int usbi_sanitize_device(struct libusb_device *dev);
void usbi_handle_disconnect(struct libusb_device_handle *dev_handle);

int usbi_handle_transfer_completion(struct usbi_transfer *itransfer,
        enum libusb_transfer_status status);
int usbi_handle_transfer_cancellation(struct usbi_transfer *itransfer);
void usbi_signal_transfer_completion(struct usbi_transfer *itransfer);

void usbi_connect_device(struct libusb_device *dev);
void usbi_disconnect_device(struct libusb_device *dev);

struct usbi_event_source {
        struct usbi_event_source_data {
                usbi_os_handle_t os_handle;
                short poll_events;
        } data;
        struct list_head list;
};

int usbi_add_event_source(struct libusb_context *ctx, usbi_os_handle_t os_handle,
        short poll_events);
void usbi_remove_event_source(struct libusb_context *ctx, usbi_os_handle_t os_handle);

struct usbi_option {
  int is_set;
  union {
    int ival;
  } arg;
};

/* OS event abstraction */

int usbi_create_event(usbi_event_t *event);
void usbi_destroy_event(usbi_event_t *event);
void usbi_signal_event(usbi_event_t *event);
void usbi_clear_event(usbi_event_t *event);

#ifdef HAVE_OS_TIMER
int usbi_create_timer(usbi_timer_t *timer);
void usbi_destroy_timer(usbi_timer_t *timer);
int usbi_arm_timer(usbi_timer_t *timer, const struct timespec *timeout);
int usbi_disarm_timer(usbi_timer_t *timer);
#endif

static inline int usbi_using_timer(struct libusb_context *ctx)
{
#ifdef HAVE_OS_TIMER
        return usbi_timer_valid(&ctx->timer);
#else
        UNUSED(ctx);
        return 0;
#endif
}

struct usbi_reported_events {
        union {
                struct {
                        unsigned int event_triggered:1;
#ifdef HAVE_OS_TIMER
                        unsigned int timer_triggered:1;
#endif
                };
                unsigned int event_bits;
        };
        void *event_data;
        unsigned int event_data_count;
        unsigned int num_ready;
};

int usbi_alloc_event_data(struct libusb_context *ctx);
int usbi_wait_for_events(struct libusb_context *ctx,
        struct usbi_reported_events *reported_events, int timeout_ms);

/* accessor functions for structure private data */

static inline void *usbi_get_context_priv(struct libusb_context *ctx)
{
        return (unsigned char *)ctx + PTR_ALIGN(sizeof(*ctx));
}

static inline void *usbi_get_device_priv(struct libusb_device *dev)
{
        return (unsigned char *)dev + PTR_ALIGN(sizeof(*dev));
}

static inline void *usbi_get_device_handle_priv(struct libusb_device_handle *dev_handle)
{
        return (unsigned char *)dev_handle + PTR_ALIGN(sizeof(*dev_handle));
}

static inline void *usbi_get_transfer_priv(struct usbi_transfer *itransfer)
{
        return itransfer->priv;
}

/* device discovery */

/* we traverse usbfs without knowing how many devices we are going to find.
 * so we create this discovered_devs model which is similar to a linked-list
 * which grows when required. it can be freed once discovery has completed,
 * eliminating the need for a list node in the libusb_device structure
 * itself. */
struct discovered_devs {
        size_t len;
        size_t capacity;
        struct libusb_device *devices[ZERO_SIZED_ARRAY];
};

struct discovered_devs *discovered_devs_append(
        struct discovered_devs *discdevs, struct libusb_device *dev);

/* OS abstraction */

/* This is the interface that OS backends need to implement.
 * All fields are mandatory, except ones explicitly noted as optional. */
struct usbi_os_backend {
        /* A human-readable name for your backend, e.g. "Linux usbfs" */
        const char *name;

        /* Binary mask for backend specific capabilities */
        uint32_t caps;

        /* Perform initialization of your backend. You might use this function
         * to determine specific capabilities of the system, allocate required
         * data structures for later, etc.
         *
         * This function is called when a libusb user initializes the library
         * prior to use. Mutual exclusion with other init and exit calls is
         * guaranteed when this function is called.
         *
         * Return 0 on success, or a LIBUSB_ERROR code on failure.
         */
        int (*init)(struct libusb_context *ctx);

        /* Deinitialization. Optional. This function should destroy anything
         * that was set up by init.
         *
         * This function is called when the user deinitializes the library.
         * Mutual exclusion with other init and exit calls is guaranteed when
         * this function is called.
         */
        void (*exit)(struct libusb_context *ctx);

        /* Set a backend-specific option. Optional.
         *
         * This function is called when the user calls libusb_set_option() and
         * the option is not handled by the core library.
         *
         * Return 0 on success, or a LIBUSB_ERROR code on failure.
         */
        int (*set_option)(struct libusb_context *ctx, enum libusb_option option,
                va_list args);

        /* Enumerate all the USB devices on the system, returning them in a list
         * of discovered devices.
         *
         * Your implementation should enumerate all devices on the system,
         * regardless of whether they have been seen before or not.
         *
         * When you have found a device, compute a session ID for it. The session
         * ID should uniquely represent that particular device for that particular
         * connection session since boot (i.e. if you disconnect and reconnect a
         * device immediately after, it should be assigned a different session ID).
         * If your OS cannot provide a unique session ID as described above,
         * presenting a session ID of (bus_number << 8 | device_address) should
         * be sufficient. Bus numbers and device addresses wrap and get reused,
         * but that is an unlikely case.
         *
         * After computing a session ID for a device, call
         * usbi_get_device_by_session_id(). This function checks if libusb already
         * knows about the device, and if so, it provides you with a reference
         * to a libusb_device structure for it.
         *
         * If usbi_get_device_by_session_id() returns NULL, it is time to allocate
         * a new device structure for the device. Call usbi_alloc_device() to
         * obtain a new libusb_device structure with reference count 1. Populate
         * the bus_number and device_address attributes of the new device, and
         * perform any other internal backend initialization you need to do. At
         * this point, you should be ready to provide device descriptors and so
         * on through the get_*_descriptor functions. Finally, call
         * usbi_sanitize_device() to perform some final sanity checks on the
         * device. Assuming all of the above succeeded, we can now continue.
         * If any of the above failed, remember to unreference the device that
         * was returned by usbi_alloc_device().
         *
         * At this stage we have a populated libusb_device structure (either one
         * that was found earlier, or one that we have just allocated and
         * populated). This can now be added to the discovered devices list
         * using discovered_devs_append(). Note that discovered_devs_append()
         * may reallocate the list, returning a new location for it, and also
         * note that reallocation can fail. Your backend should handle these
         * error conditions appropriately.
         *
         * This function should not generate any bus I/O and should not block.
         * If I/O is required (e.g. reading the active configuration value), it is
         * OK to ignore these suggestions :)
         *
         * This function is executed when the user wishes to retrieve a list
         * of USB devices connected to the system.
         *
         * If the backend has hotplug support, this function is not used!
         *
         * Return 0 on success, or a LIBUSB_ERROR code on failure.
         */
        int (*get_device_list)(struct libusb_context *ctx,
                struct discovered_devs **discdevs);

        /* Apps which were written before hotplug support, may listen for
         * hotplug events on their own and call libusb_get_device_list on
         * device addition. In this case libusb_get_device_list will likely
         * return a list without the new device in there, as the hotplug
         * event thread will still be busy enumerating the device, which may
         * take a while, or may not even have seen the event yet.
         *
         * To avoid this libusb_get_device_list will call this optional
         * function for backends with hotplug support before copying
         * ctx->usb_devs to the user. In this function the backend should
         * ensure any pending hotplug events are fully processed before
         * returning.
         *
         * Optional, should be implemented by backends with hotplug support.
         */
        void (*hotplug_poll)(void);

        /* Wrap a platform-specific device handle for I/O and other USB
         * operations. The device handle is preallocated for you.
         *
         * Your backend should allocate any internal resources required for I/O
         * and other operations so that those operations can happen (hopefully)
         * without hiccup. This is also a good place to inform libusb that it
         * should monitor certain file descriptors related to this device -
         * see the usbi_add_event_source() function.
         *
         * Your backend should also initialize the device structure
         * (dev_handle->dev), which is NULL at the beginning of the call.
         *
         * This function should not generate any bus I/O and should not block.
         *
         * This function is called when the user attempts to wrap an existing
         * platform-specific device handle for a device.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_ACCESS if the user has insufficient permissions
         * - another LIBUSB_ERROR code on other failure
         *
         * Do not worry about freeing the handle on failed open, the upper layers
         * do this for you.
         */
        int (*wrap_sys_device)(struct libusb_context *ctx,
                struct libusb_device_handle *dev_handle, intptr_t sys_dev);

        /* Open a device for I/O and other USB operations. The device handle
         * is preallocated for you, you can retrieve the device in question
         * through handle->dev.
         *
         * Your backend should allocate any internal resources required for I/O
         * and other operations so that those operations can happen (hopefully)
         * without hiccup. This is also a good place to inform libusb that it
         * should monitor certain file descriptors related to this device -
         * see the usbi_add_event_source() function.
         *
         * This function should not generate any bus I/O and should not block.
         *
         * This function is called when the user attempts to obtain a device
         * handle for a device.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_ACCESS if the user has insufficient permissions
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since
         *   discovery
         * - another LIBUSB_ERROR code on other failure
         *
         * Do not worry about freeing the handle on failed open, the upper layers
         * do this for you.
         */
        int (*open)(struct libusb_device_handle *dev_handle);

        /* Close a device such that the handle cannot be used again. Your backend
         * should destroy any resources that were allocated in the open path.
         * This may also be a good place to call usbi_remove_event_source() to
         * inform libusb of any event sources associated with this device that
         * should no longer be monitored.
         *
         * This function is called when the user closes a device handle.
         */
        void (*close)(struct libusb_device_handle *dev_handle);

        /* Get the ACTIVE configuration descriptor for a device.
         *
         * The descriptor should be retrieved from memory, NOT via bus I/O to the
         * device. This means that you may have to cache it in a private structure
         * during get_device_list enumeration. You may also have to keep track
         * of which configuration is active when the user changes it.
         *
         * This function is expected to write len bytes of data into buffer, which
         * is guaranteed to be big enough. If you can only do a partial write,
         * return an error code.
         *
         * This function is expected to return the descriptor in bus-endian format
         * (LE).
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if the device is in unconfigured state
         * - another LIBUSB_ERROR code on other failure
         */
        int (*get_active_config_descriptor)(struct libusb_device *device,
                void *buffer, size_t len);

        /* Get a specific configuration descriptor for a device.
         *
         * The descriptor should be retrieved from memory, NOT via bus I/O to the
         * device. This means that you may have to cache it in a private structure
         * during get_device_list enumeration.
         *
         * The requested descriptor is expressed as a zero-based index (i.e. 0
         * indicates that we are requesting the first descriptor). The index does
         * not (necessarily) equal the bConfigurationValue of the configuration
         * being requested.
         *
         * This function is expected to write len bytes of data into buffer, which
         * is guaranteed to be big enough. If you can only do a partial write,
         * return an error code.
         *
         * This function is expected to return the descriptor in bus-endian format
         * (LE).
         *
         * Return the length read on success or a LIBUSB_ERROR code on failure.
         */
        int (*get_config_descriptor)(struct libusb_device *device,
                uint8_t config_index, void *buffer, size_t len);

        /* Like get_config_descriptor but then by bConfigurationValue instead
         * of by index.
         *
         * Optional, if not present the core will call get_config_descriptor
         * for all configs until it finds the desired bConfigurationValue.
         *
         * Returns a pointer to the raw-descriptor in *buffer, this memory
         * is valid as long as device is valid.
         *
         * Returns the length of the returned raw-descriptor on success,
         * or a LIBUSB_ERROR code on failure.
         */
        int (*get_config_descriptor_by_value)(struct libusb_device *device,
                uint8_t bConfigurationValue, void **buffer);

        /* Get the bConfigurationValue for the active configuration for a device.
         * Optional. This should only be implemented if you can retrieve it from
         * cache (don't generate I/O).
         *
         * If you cannot retrieve this from cache, either do not implement this
         * function, or return LIBUSB_ERROR_NOT_SUPPORTED. This will cause
         * libusb to retrieve the information through a standard control transfer.
         *
         * This function must be non-blocking.
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - LIBUSB_ERROR_NOT_SUPPORTED if the value cannot be retrieved without
         *   blocking
         * - another LIBUSB_ERROR code on other failure.
         */
        int (*get_configuration)(struct libusb_device_handle *dev_handle, uint8_t *config);

        /* Set the active configuration for a device.
         *
         * A configuration value of -1 should put the device in unconfigured state.
         *
         * This function can block.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if the configuration does not exist
         * - LIBUSB_ERROR_BUSY if interfaces are currently claimed (and hence
         *   configuration cannot be changed)
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure.
         */
        int (*set_configuration)(struct libusb_device_handle *dev_handle, int config);

        /* Claim an interface. When claimed, the application can then perform
         * I/O to an interface's endpoints.
         *
         * This function should not generate any bus I/O and should not block.
         * Interface claiming is a logical operation that simply ensures that
         * no other drivers/applications are using the interface, and after
         * claiming, no other drivers/applications can use the interface because
         * we now "own" it.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if the interface does not exist
         * - LIBUSB_ERROR_BUSY if the interface is in use by another driver/app
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*claim_interface)(struct libusb_device_handle *dev_handle, uint8_t interface_number);

        /* Release a previously claimed interface.
         *
         * This function should also generate a SET_INTERFACE control request,
         * resetting the alternate setting of that interface to 0. It's OK for
         * this function to block as a result.
         *
         * You will only ever be asked to release an interface which was
         * successfully claimed earlier.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*release_interface)(struct libusb_device_handle *dev_handle, uint8_t interface_number);

        /* Set the alternate setting for an interface.
         *
         * You will only ever be asked to set the alternate setting for an
         * interface which was successfully claimed earlier.
         *
         * It's OK for this function to block.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if the alternate setting does not exist
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*set_interface_altsetting)(struct libusb_device_handle *dev_handle,
                uint8_t interface_number, uint8_t altsetting);

        /* Clear a halt/stall condition on an endpoint.
         *
         * It's OK for this function to block.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if the endpoint does not exist
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*clear_halt)(struct libusb_device_handle *dev_handle,
                unsigned char endpoint);

        /* Perform a USB port reset to reinitialize a device. Optional.
         *
         * If possible, the device handle should still be usable after the reset
         * completes, assuming that the device descriptors did not change during
         * reset and all previous interface state can be restored.
         *
         * If something changes, or you cannot easily locate/verify the reset
         * device, return LIBUSB_ERROR_NOT_FOUND. This prompts the application
         * to close the old handle and re-enumerate the device.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if re-enumeration is required, or if the device
         *   has been disconnected since it was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*reset_device)(struct libusb_device_handle *dev_handle);

        /* Alloc num_streams usb3 bulk streams on the passed in endpoints */
        int (*alloc_streams)(struct libusb_device_handle *dev_handle,
                uint32_t num_streams, unsigned char *endpoints, int num_endpoints);

        /* Free usb3 bulk streams allocated with alloc_streams */
        int (*free_streams)(struct libusb_device_handle *dev_handle,
                unsigned char *endpoints, int num_endpoints);

        /* Allocate persistent DMA memory for the given device, suitable for
         * zerocopy. May return NULL on failure. Optional to implement.
         */
        void *(*dev_mem_alloc)(struct libusb_device_handle *handle, size_t len);

        /* Free memory allocated by dev_mem_alloc. */
        int (*dev_mem_free)(struct libusb_device_handle *handle, void *buffer,
                size_t len);

        /* Determine if a kernel driver is active on an interface. Optional.
         *
         * The presence of a kernel driver on an interface indicates that any
         * calls to claim_interface would fail with the LIBUSB_ERROR_BUSY code.
         *
         * Return:
         * - 0 if no driver is active
         * - 1 if a driver is active
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*kernel_driver_active)(struct libusb_device_handle *dev_handle,
                uint8_t interface_number);

        /* Detach a kernel driver from an interface. Optional.
         *
         * After detaching a kernel driver, the interface should be available
         * for claim.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if no kernel driver was active
         * - LIBUSB_ERROR_INVALID_PARAM if the interface does not exist
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - another LIBUSB_ERROR code on other failure
         */
        int (*detach_kernel_driver)(struct libusb_device_handle *dev_handle,
                uint8_t interface_number);

        /* Attach a kernel driver to an interface. Optional.
         *
         * Reattach a kernel driver to the device.
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NOT_FOUND if no kernel driver was active
         * - LIBUSB_ERROR_INVALID_PARAM if the interface does not exist
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
         *   was opened
         * - LIBUSB_ERROR_BUSY if a program or driver has claimed the interface,
         *   preventing reattachment
         * - another LIBUSB_ERROR code on other failure
         */
        int (*attach_kernel_driver)(struct libusb_device_handle *dev_handle,
                uint8_t interface_number);

        /* Destroy a device. Optional.
         *
         * This function is called when the last reference to a device is
         * destroyed. It should free any resources allocated in the get_device_list
         * path.
         */
        void (*destroy_device)(struct libusb_device *dev);

        /* Submit a transfer. Your implementation should take the transfer,
         * morph it into whatever form your platform requires, and submit it
         * asynchronously.
         *
         * This function must not block.
         *
         * This function gets called with the flying_transfers_lock locked!
         *
         * Return:
         * - 0 on success
         * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
         * - another LIBUSB_ERROR code on other failure
         */
        int (*submit_transfer)(struct usbi_transfer *itransfer);

        /* Cancel a previously submitted transfer.
         *
         * This function must not block. The transfer cancellation must complete
         * later, resulting in a call to usbi_handle_transfer_cancellation()
         * from the context of handle_events.
         */
        int (*cancel_transfer)(struct usbi_transfer *itransfer);

        /* Clear a transfer as if it has completed or cancelled, but do not
         * report any completion/cancellation to the library. You should free
         * all private data from the transfer as if you were just about to report
         * completion or cancellation.
         *
         * This function might seem a bit out of place. It is used when libusb
         * detects a disconnected device - it calls this function for all pending
         * transfers before reporting completion (with the disconnect code) to
         * the user. Maybe we can improve upon this internal interface in future.
         */
        void (*clear_transfer_priv)(struct usbi_transfer *itransfer);

        /* Handle any pending events on event sources. Optional.
         *
         * Provide this function when event sources directly indicate device
         * or transfer activity. If your backend does not have such event sources,
         * implement the handle_transfer_completion function below.
         *
         * This involves monitoring any active transfers and processing their
         * completion or cancellation.
         *
         * The function is passed a pointer that represents platform-specific
         * data for monitoring event sources (size count). This data is to be
         * (re)allocated as necessary when event sources are modified.
         * The num_ready parameter indicates the number of event sources that
         * have reported events. This should be enough information for you to
         * determine which actions need to be taken on the currently active
         * transfers.
         *
         * For any cancelled transfers, call usbi_handle_transfer_cancellation().
         * For completed transfers, call usbi_handle_transfer_completion().
         * For control/bulk/interrupt transfers, populate the "transferred"
         * element of the appropriate usbi_transfer structure before calling the
         * above functions. For isochronous transfers, populate the status and
         * transferred fields of the iso packet descriptors of the transfer.
         *
         * This function should also be able to detect disconnection of the
         * device, reporting that situation with usbi_handle_disconnect().
         *
         * When processing an event related to a transfer, you probably want to
         * take usbi_transfer.lock to prevent races. See the documentation for
         * the usbi_transfer structure.
         *
         * Return 0 on success, or a LIBUSB_ERROR code on failure.
         */
        int (*handle_events)(struct libusb_context *ctx,
                void *event_data, unsigned int count, unsigned int num_ready);

        /* Handle transfer completion. Optional.
         *
         * Provide this function when there are no event sources available that
         * directly indicate device or transfer activity. If your backend does
         * have such event sources, implement the handle_events function above.
         *
         * Your backend must tell the library when a transfer has completed by
         * calling usbi_signal_transfer_completion(). You should store any private
         * information about the transfer and its completion status in the transfer's
         * private backend data.
         *
         * During event handling, this function will be called on each transfer for
         * which usbi_signal_transfer_completion() was called.
         *
         * For any cancelled transfers, call usbi_handle_transfer_cancellation().
         * For completed transfers, call usbi_handle_transfer_completion().
         * For control/bulk/interrupt transfers, populate the "transferred"
         * element of the appropriate usbi_transfer structure before calling the
         * above functions. For isochronous transfers, populate the status and
         * transferred fields of the iso packet descriptors of the transfer.
         *
         * Return 0 on success, or a LIBUSB_ERROR code on failure.
         */
        int (*handle_transfer_completion)(struct usbi_transfer *itransfer);

        /* Number of bytes to reserve for per-context private backend data.
         * This private data area is accessible by calling
         * usbi_get_context_priv() on the libusb_context instance.
         */
        size_t context_priv_size;

        /* Number of bytes to reserve for per-device private backend data.
         * This private data area is accessible by calling
         * usbi_get_device_priv() on the libusb_device instance.
         */
        size_t device_priv_size;

        /* Number of bytes to reserve for per-handle private backend data.
         * This private data area is accessible by calling
         * usbi_get_device_handle_priv() on the libusb_device_handle instance.
         */
        size_t device_handle_priv_size;

        /* Number of bytes to reserve for per-transfer private backend data.
         * This private data area is accessible by calling
         * usbi_get_transfer_priv() on the usbi_transfer instance.
         */
        size_t transfer_priv_size;
};

extern const struct usbi_os_backend usbi_backend;

#define for_each_context(c) \
        for_each_helper(c, &active_contexts_list, struct libusb_context)

#define for_each_device(ctx, d) \
        for_each_helper(d, &(ctx)->usb_devs, struct libusb_device)

#define for_each_device_safe(ctx, d, n) \
        for_each_safe_helper(d, n, &(ctx)->usb_devs, struct libusb_device)

#define for_each_open_device(ctx, h) \
        for_each_helper(h, &(ctx)->open_devs, struct libusb_device_handle)

#define __for_each_transfer(list, t) \
        for_each_helper(t, (list), struct usbi_transfer)

#define for_each_transfer(ctx, t) \
        __for_each_transfer(&(ctx)->flying_transfers, t)

#define __for_each_transfer_safe(list, t, n) \
        for_each_safe_helper(t, n, (list), struct usbi_transfer)

#define for_each_transfer_safe(ctx, t, n) \
        __for_each_transfer_safe(&(ctx)->flying_transfers, t, n)

#define __for_each_completed_transfer_safe(list, t, n) \
        list_for_each_entry_safe(t, n, (list), completed_list, struct usbi_transfer)

#define for_each_event_source(ctx, e) \
        for_each_helper(e, &(ctx)->event_sources, struct usbi_event_source)

#define for_each_removed_event_source(ctx, e) \
        for_each_helper(e, &(ctx)->removed_event_sources, struct usbi_event_source)

#define for_each_removed_event_source_safe(ctx, e, n) \
        for_each_safe_helper(e, n, &(ctx)->removed_event_sources, struct usbi_event_source)

#define for_each_hotplug_cb(ctx, c) \
        for_each_helper(c, &(ctx)->hotplug_cbs, struct usbi_hotplug_callback)

#define for_each_hotplug_cb_safe(ctx, c, n) \
        for_each_safe_helper(c, n, &(ctx)->hotplug_cbs, struct usbi_hotplug_callback)

#ifdef __cplusplus
}
#endif

#endif