Only initialize the default context when it is requested

[platform/upstream/libusb.git] / libusb / core.c

/*
 * Core functions for libusb
 * Copyright (C) 2007-2008 Daniel Drake <dsd@gentoo.org>
 * Copyright (c) 2001 Johannes Erdfelt <johannes@erdfelt.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
 */

#include <config.h>

#include <errno.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

#include "os/poll_posix.h"

#include "libusbi.h"

#if defined(OS_LINUX)
const struct usbi_os_backend * const usbi_backend = &linux_usbfs_backend;
#elif defined(OS_DARWIN)
const struct usbi_os_backend * const usbi_backend = &darwin_backend;
#else
#error "Unsupported OS"
#endif

struct libusb_context *usbi_default_context = NULL;
static int default_context_refcnt = 0;
static usbi_mutex_static_t default_context_lock = USBI_MUTEX_INITIALIZER;

/**
 * \mainpage libusb-1.0 API Reference
 *
 * \section intro Introduction
 *
 * libusb is an open source library that allows you to communicate with USB
 * devices from userspace. For more info, see the
 * <a href="http://libusb.sourceforge.net">libusb homepage</a>.
 *
 * This documentation is aimed at application developers wishing to
 * communicate with USB peripherals from their own software. After reviewing
 * this documentation, feedback and questions can be sent to the
 * <a href="http://sourceforge.net/mail/?group_id=1674">libusb-devel mailing
 * list</a>.
 *
 * This documentation assumes knowledge of how to operate USB devices from
 * a software standpoint (descriptors, configurations, interfaces, endpoints,
 * control/bulk/interrupt/isochronous transfers, etc). Full information
 * can be found in the <a href="http://www.usb.org/developers/docs/">USB 2.0
 * Specification</a> which is available for free download. You can probably
 * find less verbose introductions by searching the web.
 *
 * \section features Library features
 *
 * - All transfer types supported (control/bulk/interrupt/isochronous)
 * - 2 transfer interfaces:
 *    -# Synchronous (simple)
 *    -# Asynchronous (more complicated, but more powerful)
 * - Thread safe (although the asynchronous interface means that you
 *   usually won't need to thread)
 * - Lightweight with lean API
 * - Compatible with libusb-0.1 through the libusb-compat-0.1 translation layer
 *
 * \section gettingstarted Getting Started
 *
 * To begin reading the API documentation, start with the Modules page which
 * links to the different categories of libusb's functionality.
 *
 * One decision you will have to make is whether to use the synchronous
 * or the asynchronous data transfer interface. The \ref io documentation
 * provides some insight into this topic.
 *
 * Some example programs can be found in the libusb source distribution under
 * the "examples" subdirectory. The libusb homepage includes a list of
 * real-life project examples which use libusb.
 *
 * \section errorhandling Error handling
 *
 * libusb functions typically return 0 on success or a negative error code
 * on failure. These negative error codes relate to LIBUSB_ERROR constants
 * which are listed on the \ref misc "miscellaneous" documentation page.
 *
 * \section msglog Debug message logging
 *
 * libusb does not log any messages by default. Your application is therefore
 * free to close stdout/stderr and those descriptors may be reused without
 * worry.
 *
 * The libusb_set_debug() function can be used to enable stdout/stderr logging
 * of certain messages. Under standard configuration, libusb doesn't really
 * log much at all, so you are advised to use this function to enable all
 * error/warning/informational messages. It will help you debug problems with
 * your software.
 *
 * The logged messages are unstructured. There is no one-to-one correspondence
 * between messages being logged and success or failure return codes from
 * libusb functions. There is no format to the messages, so you should not
 * try to capture or parse them. They are not and will not be localized.
 * These messages are not suitable for being passed to your application user;
 * instead, you should interpret the error codes returned from libusb functions
 * and provide appropriate notification to the user. The messages are simply
 * there to aid you as a programmer, and if you're confused because you're
 * getting a strange error code from a libusb function, enabling message
 * logging may give you a suitable explanation.
 *
 * The LIBUSB_DEBUG environment variable can be used to enable message logging
 * at run-time. This environment variable should be set to a number, which is
 * interpreted the same as the libusb_set_debug() parameter. When this
 * environment variable is set, the message logging verbosity level is fixed
 * and libusb_set_debug() effectively does nothing.
 *
 * libusb can be compiled without any logging functions, useful for embedded
 * systems. In this case, libusb_set_debug() and the LIBUSB_DEBUG environment
 * variable have no effects.
 *
 * libusb can also be compiled with verbose debugging messages. When the
 * library is compiled in this way, all messages of all verbosities are always
 * logged.  libusb_set_debug() and the LIBUSB_DEBUG environment variable have
 * no effects.
 *
 * \section remarks Other remarks
 *
 * libusb does have imperfections. The \ref caveats "caveats" page attempts
 * to document these.
 */

/**
 * \page caveats Caveats
 *
 * \section devresets Device resets
 *
 * The libusb_reset_device() function allows you to reset a device. If your
 * program has to call such a function, it should obviously be aware that
 * the reset will cause device state to change (e.g. register values may be
 * reset).
 *
 * The problem is that any other program could reset the device your program
 * is working with, at any time. libusb does not offer a mechanism to inform
 * you when this has happened, so if someone else resets your device it will
 * not be clear to your own program why the device state has changed.
 *
 * Ultimately, this is a limitation of writing drivers in userspace.
 * Separation from the USB stack in the underlying kernel makes it difficult
 * for the operating system to deliver such notifications to your program.
 * The Linux kernel USB stack allows such reset notifications to be delivered
 * to in-kernel USB drivers, but it is not clear how such notifications could
 * be delivered to second-class drivers that live in userspace.
 *
 * \section blockonly Blocking-only functionality
 *
 * The functionality listed below is only available through synchronous,
 * blocking functions. There are no asynchronous/non-blocking alternatives,
 * and no clear ways of implementing these.
 *
 * - Configuration activation (libusb_set_configuration())
 * - Interface/alternate setting activation (libusb_set_interface_alt_setting())
 * - Releasing of interfaces (libusb_release_interface())
 * - Clearing of halt/stall condition (libusb_clear_halt())
 * - Device resets (libusb_reset_device())
 *
 * \section nohotplug No hotplugging
 *
 * libusb-1.0 lacks functionality for providing notifications of when devices
 * are added or removed. This functionality is planned to be implemented
 * for libusb-1.1.
 *
 * That said, there is basic disconnection handling for open device handles:
 *  - If there are ongoing transfers, libusb's handle_events loop will detect
 *    disconnections and complete ongoing transfers with the
 *    LIBUSB_TRANSFER_NO_DEVICE status code.
 *  - Many functions such as libusb_set_configuration() return the special
 *    LIBUSB_ERROR_NO_DEVICE error code when the device has been disconnected.
 *
 * \section configsel Configuration selection and handling
 *
 * When libusb presents a device handle to an application, there is a chance
 * that the corresponding device may be in unconfigured state. For devices
 * with multiple configurations, there is also a chance that the configuration
 * currently selected is not the one that the application wants to use.
 *
 * The obvious solution is to add a call to libusb_set_configuration() early
 * on during your device initialization routines, but there are caveats to
 * be aware of:
 * -# If the device is already in the desired configuration, calling
 *    libusb_set_configuration() using the same configuration value will cause
 *    a lightweight device reset. This may not be desirable behaviour.
 * -# libusb will be unable to change configuration if the device is in
 *    another configuration and other programs or drivers have claimed
 *    interfaces under that configuration.
 * -# In the case where the desired configuration is already active, libusb
 *    may not even be able to perform a lightweight device reset. For example,
 *    take my USB keyboard with fingerprint reader: I'm interested in driving
 *    the fingerprint reader interface through libusb, but the kernel's
 *    USB-HID driver will almost always have claimed the keyboard interface.
 *    Because the kernel has claimed an interface, it is not even possible to
 *    perform the lightweight device reset, so libusb_set_configuration() will
 *    fail. (Luckily the device in question only has a single configuration.)
 *
 * One solution to some of the above problems is to consider the currently
 * active configuration. If the configuration we want is already active, then
 * we don't have to select any configuration:
\code
cfg = libusb_get_configuration(dev);
if (cfg != desired)
        libusb_set_configuration(dev, desired);
\endcode
 *
 * This is probably suitable for most scenarios, but is inherently racy:
 * another application or driver may change the selected configuration
 * <em>after</em> the libusb_get_configuration() call.
 *
 * Even in cases where libusb_set_configuration() succeeds, consider that other
 * applications or drivers may change configuration after your application
 * calls libusb_set_configuration().
 *
 * One possible way to lock your device into a specific configuration is as
 * follows:
 * -# Set the desired configuration (or use the logic above to realise that
 *    it is already in the desired configuration)
 * -# Claim the interface that you wish to use
 * -# Check that the currently active configuration is the one that you want
 *    to use.
 *
 * The above method works because once an interface is claimed, no application
 * or driver is able to select another configuration.
 *
 * \section earlycomp Early transfer completion
 *
 * NOTE: This section is currently Linux-centric. I am not sure if any of these
 * considerations apply to Darwin or other platforms.
 *
 * When a transfer completes early (i.e. when less data is received/sent in
 * any one packet than the transfer buffer allows for) then libusb is designed
 * to terminate the transfer immediately, not transferring or receiving any
 * more data unless other transfers have been queued by the user.
 *
 * On legacy platforms, libusb is unable to do this in all situations. After
 * the incomplete packet occurs, "surplus" data may be transferred. Prior to
 * libusb v1.0.2, this information was lost (and for device-to-host transfers,
 * the corresponding data was discarded). As of libusb v1.0.3, this information
 * is kept (the data length of the transfer is updated) and, for device-to-host
 * transfers, any surplus data was added to the buffer. Still, this is not
 * a nice solution because it loses the information about the end of the short
 * packet, and the user probably wanted that surplus data to arrive in the next
 * logical transfer.
 *
 * A previous workaround was to only ever submit transfers of size 16kb or
 * less.
 *
 * As of libusb v1.0.4 and Linux v2.6.32, this is fixed. A technical
 * explanation of this issue follows.
 *
 * When you ask libusb to submit a bulk transfer larger than 16kb in size,
 * libusb breaks it up into a number of smaller subtransfers. This is because
 * the usbfs kernel interface only accepts transfers of up to 16kb in size.
 * The subtransfers are submitted all at once so that the kernel can queue
 * them at the hardware level, therefore maximizing bus throughput.
 *
 * On legacy platforms, this caused problems when transfers completed early.
 * Upon this event, the kernel would terminate all further packets in that
 * subtransfer (but not any following ones). libusb would note this event and
 * immediately cancel any following subtransfers that had been queued,
 * but often libusb was not fast enough, and the following subtransfers had
 * started before libusb got around to cancelling them.
 *
 * Thanks to an API extension to usbfs, this is fixed with recent kernel and
 * libusb releases. The solution was to allow libusb to communicate to the
 * kernel where boundaries occur between logical libusb-level transfers. When
 * a short transfer (or other error) occurs, the kernel will cancel all the
 * subtransfers until the boundary without allowing those transfers to start.
 */

/**
 * \page contexts Contexts
 *
 * It is possible that libusb may be used simultaneously from two independent
 * libraries linked into the same executable. For example, if your application
 * has a plugin-like system which allows the user to dynamically load a range
 * of modules into your program, it is feasible that two independently
 * developed modules may both use libusb.
 *
 * libusb is written to allow for these multiple user scenarios. The two
 * "instances" of libusb will not interfere: libusb_set_debug() calls
 * from one user will not affect the same settings for other users, other
 * users can continue using libusb after one of them calls libusb_exit(), etc.
 *
 * This is made possible through libusb's <em>context</em> concept. When you
 * call libusb_init(), you are (optionally) given a context. You can then pass
 * this context pointer back into future libusb functions.
 *
 * In order to keep things simple for more simplistic applications, it is
 * legal to pass NULL to all functions requiring a context pointer (as long as
 * you're sure no other code will attempt to use libusb from the same process).
 * When you pass NULL, the default context will be used. The default context
 * is created the first time a process calls libusb_init() when no other
 * context is alive. Contexts are destroyed during libusb_exit().
 *
 * The default context is reference-counted and can be shared. That means that
 * if libusb_init(NULL) is called twice within the same process, the two
 * users end up sharing the same context. The deinitialization and freeing of
 * the default context will only happen when the last user calls libusb_exit().
 * In other words, the default context is created and initialized when its
 * reference count goes from 0 to 1, and is deinitialized and destroyed when
 * its reference count goes from 1 to 0.
 *
 * You may be wondering why only a subset of libusb functions require a
 * context pointer in their function definition. Internally, libusb stores
 * context pointers in other objects (e.g. libusb_device instances) and hence
 * can infer the context from those objects.
 */

/**
 * @defgroup lib Library initialization/deinitialization
 * This page details how to initialize and deinitialize libusb. Initialization
 * must be performed before using any libusb functionality, and similarly you
 * must not call any libusb functions after deinitialization.
 */

/**
 * @defgroup dev Device handling and enumeration
 * The functionality documented below is designed to help with the following
 * operations:
 * - Enumerating the USB devices currently attached to the system
 * - Choosing a device to operate from your software
 * - Opening and closing the chosen device
 *
 * \section nutshell In a nutshell...
 *
 * The description below really makes things sound more complicated than they
 * actually are. The following sequence of function calls will be suitable
 * for almost all scenarios and does not require you to have such a deep
 * understanding of the resource management issues:
 * \code
// discover devices
libusb_device **list;
libusb_device *found = NULL;
ssize_t cnt = libusb_get_device_list(NULL, &list);
ssize_t i = 0;
int err = 0;
if (cnt < 0)
        error();

for (i = 0; i < cnt; i++) {
        libusb_device *device = list[i];
        if (is_interesting(device)) {
                found = device;
                break;
        }
}

if (found) {
        libusb_device_handle *handle;

        err = libusb_open(found, &handle);
        if (err)
                error();
        // etc
}

libusb_free_device_list(list, 1);
\endcode
 *
 * The two important points:
 * - You asked libusb_free_device_list() to unreference the devices (2nd
 *   parameter)
 * - You opened the device before freeing the list and unreferencing the
 *   devices
 *
 * If you ended up with a handle, you can now proceed to perform I/O on the
 * device.
 *
 * \section devshandles Devices and device handles
 * libusb has a concept of a USB device, represented by the
 * \ref libusb_device opaque type. A device represents a USB device that
 * is currently or was previously connected to the system. Using a reference
 * to a device, you can determine certain information about the device (e.g.
 * you can read the descriptor data).
 *
 * The libusb_get_device_list() function can be used to obtain a list of
 * devices currently connected to the system. This is known as device
 * discovery.
 *
 * Just because you have a reference to a device does not mean it is
 * necessarily usable. The device may have been unplugged, you may not have
 * permission to operate such device, or another program or driver may be
 * using the device.
 *
 * When you've found a device that you'd like to operate, you must ask
 * libusb to open the device using the libusb_open() function. Assuming
 * success, libusb then returns you a <em>device handle</em>
 * (a \ref libusb_device_handle pointer). All "real" I/O operations then
 * operate on the handle rather than the original device pointer.
 *
 * \section devref Device discovery and reference counting
 *
 * Device discovery (i.e. calling libusb_get_device_list()) returns a
 * freshly-allocated list of devices. The list itself must be freed when
 * you are done with it. libusb also needs to know when it is OK to free
 * the contents of the list - the devices themselves.
 *
 * To handle these issues, libusb provides you with two separate items:
 * - A function to free the list itself
 * - A reference counting system for the devices inside
 *
 * New devices presented by the libusb_get_device_list() function all have a
 * reference count of 1. You can increase and decrease reference count using
 * libusb_ref_device() and libusb_unref_device(). A device is destroyed when
 * its reference count reaches 0.
 *
 * With the above information in mind, the process of opening a device can
 * be viewed as follows:
 * -# Discover devices using libusb_get_device_list().
 * -# Choose the device that you want to operate, and call libusb_open().
 * -# Unref all devices in the discovered device list.
 * -# Free the discovered device list.
 *
 * The order is important - you must not unreference the device before
 * attempting to open it, because unreferencing it may destroy the device.
 *
 * For convenience, the libusb_free_device_list() function includes a
 * parameter to optionally unreference all the devices in the list before
 * freeing the list itself. This combines steps 3 and 4 above.
 *
 * As an implementation detail, libusb_open() actually adds a reference to
 * the device in question. This is because the device remains available
 * through the handle via libusb_get_device(). The reference is deleted during
 * libusb_close().
 */

/** @defgroup misc Miscellaneous */

/* 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. */
#define DISCOVERED_DEVICES_SIZE_STEP 8

static struct discovered_devs *discovered_devs_alloc(void)
{
        struct discovered_devs *ret =
                malloc(sizeof(*ret) + (sizeof(void *) * DISCOVERED_DEVICES_SIZE_STEP));

        if (ret) {
                ret->len = 0;
                ret->capacity = DISCOVERED_DEVICES_SIZE_STEP;
        }
        return ret;
}

/* append a device to the discovered devices collection. may realloc itself,
 * returning new discdevs. returns NULL on realloc failure. */
struct discovered_devs *discovered_devs_append(
        struct discovered_devs *discdevs, struct libusb_device *dev)
{
        size_t len = discdevs->len;
        size_t capacity;

        /* if there is space, just append the device */
        if (len < discdevs->capacity) {
                discdevs->devices[len] = libusb_ref_device(dev);
                discdevs->len++;
                return discdevs;
        }

        /* exceeded capacity, need to grow */
        usbi_dbg("need to increase capacity");
        capacity = discdevs->capacity + DISCOVERED_DEVICES_SIZE_STEP;
        discdevs = realloc(discdevs,
                sizeof(*discdevs) + (sizeof(void *) * capacity));
        if (discdevs) {
                discdevs->capacity = capacity;
                discdevs->devices[len] = libusb_ref_device(dev);
                discdevs->len++;
        }

        return discdevs;
}

static void discovered_devs_free(struct discovered_devs *discdevs)
{
        size_t i;

        for (i = 0; i < discdevs->len; i++)
                libusb_unref_device(discdevs->devices[i]);

        free(discdevs);
}

/* Allocate a new device with a specific session ID. The returned device has
 * a reference count of 1. */
struct libusb_device *usbi_alloc_device(struct libusb_context *ctx,
        unsigned long session_id)
{
        size_t priv_size = usbi_backend->device_priv_size;
        struct libusb_device *dev = malloc(sizeof(*dev) + priv_size);
        int r;

        if (!dev)
                return NULL;

        r = usbi_mutex_init(&dev->lock, NULL);
        if (r) {
                free(dev);
                return NULL;
        }

        dev->ctx = ctx;
        dev->refcnt = 1;
        dev->session_data = session_id;
        memset(&dev->os_priv, 0, priv_size);

        usbi_mutex_lock(&ctx->usb_devs_lock);
        list_add(&dev->list, &ctx->usb_devs);
        usbi_mutex_unlock(&ctx->usb_devs_lock);
        return dev;
}

/* Perform some final sanity checks on a newly discovered device. If this
 * function fails (negative return code), the device should not be added
 * to the discovered device list. */
int usbi_sanitize_device(struct libusb_device *dev)
{
        int r;
        unsigned char raw_desc[DEVICE_DESC_LENGTH];
        uint8_t num_configurations;
        int host_endian;

        r = usbi_backend->get_device_descriptor(dev, raw_desc, &host_endian);
        if (r < 0)
                return r;

        num_configurations = raw_desc[DEVICE_DESC_LENGTH - 1];
        if (num_configurations > USB_MAXCONFIG) {
                usbi_err(DEVICE_CTX(dev), "too many configurations");
                return LIBUSB_ERROR_IO;
        } else if (num_configurations < 1) {
                usbi_dbg("no configurations?");
                return LIBUSB_ERROR_IO;
        }

        dev->num_configurations = num_configurations;
        return 0;
}

/* Examine libusb's internal list of known devices, looking for one with
 * a specific session ID. Returns the matching device if it was found, and
 * NULL otherwise. */
struct libusb_device *usbi_get_device_by_session_id(struct libusb_context *ctx,
        unsigned long session_id)
{
        struct libusb_device *dev;
        struct libusb_device *ret = NULL;

        usbi_mutex_lock(&ctx->usb_devs_lock);
        list_for_each_entry(dev, &ctx->usb_devs, list, struct libusb_device)
                if (dev->session_data == session_id) {
                        ret = dev;
                        break;
                }
        usbi_mutex_unlock(&ctx->usb_devs_lock);

        return ret;
}

/** @ingroup dev
 * Returns a list of USB devices currently attached to the system. This is
 * your entry point into finding a USB device to operate.
 *
 * You are expected to unreference all the devices when you are done with
 * them, and then free the list with libusb_free_device_list(). Note that
 * libusb_free_device_list() can unref all the devices for you. Be careful
 * not to unreference a device you are about to open until after you have
 * opened it.
 *
 * This return value of this function indicates the number of devices in
 * the resultant list. The list is actually one element larger, as it is
 * NULL-terminated.
 *
 * \param ctx the context to operate on, or NULL for the default context
 * \param list output location for a list of devices. Must be later freed with
 * libusb_free_device_list().
 * \returns the number of devices in the outputted list, or LIBUSB_ERROR_NO_MEM
 * on memory allocation failure.
 */
API_EXPORTED ssize_t libusb_get_device_list(libusb_context *ctx,
        libusb_device ***list)
{
        struct discovered_devs *discdevs = discovered_devs_alloc();
        struct libusb_device **ret;
        int r = 0;
        ssize_t i, len;
        USBI_GET_CONTEXT(ctx);
        usbi_dbg("");

        if (!discdevs)
                return LIBUSB_ERROR_NO_MEM;

        r = usbi_backend->get_device_list(ctx, &discdevs);
        if (r < 0) {
                len = r;
                goto out;
        }

        /* convert discovered_devs into a list */
        len = discdevs->len;
        ret = malloc(sizeof(void *) * (len + 1));
        if (!ret) {
                len = LIBUSB_ERROR_NO_MEM;
                goto out;
        }

        ret[len] = NULL;
        for (i = 0; i < len; i++) {
                struct libusb_device *dev = discdevs->devices[i];
                ret[i] = libusb_ref_device(dev);
        }
        *list = ret;

out:
        discovered_devs_free(discdevs);
        return len;
}

/** \ingroup dev
 * Frees a list of devices previously discovered using
 * libusb_get_device_list(). If the unref_devices parameter is set, the
 * reference count of each device in the list is decremented by 1.
 * \param list the list to free
 * \param unref_devices whether to unref the devices in the list
 */
API_EXPORTED void libusb_free_device_list(libusb_device **list,
        int unref_devices)
{
        if (!list)
                return;

        if (unref_devices) {
                int i = 0;
                struct libusb_device *dev;

                while ((dev = list[i++]) != NULL)
                        libusb_unref_device(dev);
        }
        free(list);
}

/** \ingroup dev
 * Get the number of the bus that a device is connected to.
 * \param dev a device
 * \returns the bus number
 */
API_EXPORTED uint8_t libusb_get_bus_number(libusb_device *dev)
{
        return dev->bus_number;
}

/** \ingroup dev
 * Get the address of the device on the bus it is connected to.
 * \param dev a device
 * \returns the device address
 */
API_EXPORTED uint8_t libusb_get_device_address(libusb_device *dev)
{
        return dev->device_address;
}

static const struct libusb_endpoint_descriptor *find_endpoint(
        struct libusb_config_descriptor *config, unsigned char endpoint)
{
        int iface_idx;
        for (iface_idx = 0; iface_idx < config->bNumInterfaces; iface_idx++) {
                const struct libusb_interface *iface = &config->interface[iface_idx];
                int altsetting_idx;

                for (altsetting_idx = 0; altsetting_idx < iface->num_altsetting;
                                altsetting_idx++) {
                        const struct libusb_interface_descriptor *altsetting
                                = &iface->altsetting[altsetting_idx];
                        int ep_idx;

                        for (ep_idx = 0; ep_idx < altsetting->bNumEndpoints; ep_idx++) {
                                const struct libusb_endpoint_descriptor *ep =
                                        &altsetting->endpoint[ep_idx];
                                if (ep->bEndpointAddress == endpoint)
                                        return ep;
                        }
                }
        }
        return NULL;
}

/** \ingroup dev
 * Convenience function to retrieve the wMaxPacketSize value for a particular
 * endpoint in the active device configuration.
 *
 * This function was originally intended to be of assistance when setting up
 * isochronous transfers, but a design mistake resulted in this function
 * instead. It simply returns the wMaxPacketSize value without considering
 * its contents. If you're dealing with isochronous transfers, you probably
 * want libusb_get_max_iso_packet_size() instead.
 *
 * \param dev a device
 * \param endpoint address of the endpoint in question
 * \returns the wMaxPacketSize value
 * \returns LIBUSB_ERROR_NOT_FOUND if the endpoint does not exist
 * \returns LIBUSB_ERROR_OTHER on other failure
 */
API_EXPORTED int libusb_get_max_packet_size(libusb_device *dev,
        unsigned char endpoint)
{
        struct libusb_config_descriptor *config;
        const struct libusb_endpoint_descriptor *ep;
        int r;

        r = libusb_get_active_config_descriptor(dev, &config);
        if (r < 0) {
                usbi_err(DEVICE_CTX(dev),
                        "could not retrieve active config descriptor");
                return LIBUSB_ERROR_OTHER;
        }

        ep = find_endpoint(config, endpoint);
        if (!ep)
                return LIBUSB_ERROR_NOT_FOUND;

        r = ep->wMaxPacketSize;
        libusb_free_config_descriptor(config);
        return r;
}

/** \ingroup dev
 * Calculate the maximum packet size which a specific endpoint is capable is
 * sending or receiving in the duration of 1 microframe
 *
 * Only the active configution is examined. The calculation is based on the
 * wMaxPacketSize field in the endpoint descriptor as described in section
 * 9.6.6 in the USB 2.0 specifications.
 *
 * If acting on an isochronous or interrupt endpoint, this function will
 * multiply the value found in bits 0:10 by the number of transactions per
 * microframe (determined by bits 11:12). Otherwise, this function just
 * returns the numeric value found in bits 0:10.
 *
 * This function is useful for setting up isochronous transfers, for example
 * you might pass the return value from this function to
 * libusb_set_iso_packet_lengths() in order to set the length field of every
 * isochronous packet in a transfer.
 *
 * Since v1.0.3.
 *
 * \param dev a device
 * \param endpoint address of the endpoint in question
 * \returns the maximum packet size which can be sent/received on this endpoint
 * \returns LIBUSB_ERROR_NOT_FOUND if the endpoint does not exist
 * \returns LIBUSB_ERROR_OTHER on other failure
 */
API_EXPORTED int libusb_get_max_iso_packet_size(libusb_device *dev,
        unsigned char endpoint)
{
        struct libusb_config_descriptor *config;
        const struct libusb_endpoint_descriptor *ep;
        enum libusb_transfer_type ep_type;
        uint16_t val;
        int r;

        r = libusb_get_active_config_descriptor(dev, &config);
        if (r < 0) {
                usbi_err(DEVICE_CTX(dev),
                        "could not retrieve active config descriptor");
                return LIBUSB_ERROR_OTHER;
        }

        ep = find_endpoint(config, endpoint);
        if (!ep)
                return LIBUSB_ERROR_NOT_FOUND;

        val = ep->wMaxPacketSize;
        ep_type = ep->bmAttributes & 0x3;
        libusb_free_config_descriptor(config);

        r = val & 0x07ff;
        if (ep_type == LIBUSB_TRANSFER_TYPE_ISOCHRONOUS
                        || ep_type == LIBUSB_TRANSFER_TYPE_INTERRUPT)
                r *= (1 + ((val >> 11) & 3));
        return r;
}

/** \ingroup dev
 * Increment the reference count of a device.
 * \param dev the device to reference
 * \returns the same device
 */
API_EXPORTED libusb_device *libusb_ref_device(libusb_device *dev)
{
        usbi_mutex_lock(&dev->lock);
        dev->refcnt++;
        usbi_mutex_unlock(&dev->lock);
        return dev;
}

/** \ingroup dev
 * Decrement the reference count of a device. If the decrement operation
 * causes the reference count to reach zero, the device shall be destroyed.
 * \param dev the device to unreference
 */
API_EXPORTED void libusb_unref_device(libusb_device *dev)
{
        int refcnt;

        if (!dev)
                return;

        usbi_mutex_lock(&dev->lock);
        refcnt = --dev->refcnt;
        usbi_mutex_unlock(&dev->lock);

        if (refcnt == 0) {
                usbi_dbg("destroy device %d.%d", dev->bus_number, dev->device_address);

                if (usbi_backend->destroy_device)
                        usbi_backend->destroy_device(dev);

                usbi_mutex_lock(&dev->ctx->usb_devs_lock);
                list_del(&dev->list);
                usbi_mutex_unlock(&dev->ctx->usb_devs_lock);

                usbi_mutex_destroy(&dev->lock);
                free(dev);
        }
}

/*
 * Interrupt the iteration of the event handling thread, so that it picks
 * up the new fd.
 */
void usbi_fd_notification(struct libusb_context *ctx)
{
        unsigned char dummy = 1;
        ssize_t r;

        if (ctx == NULL)
                return;

        /* record that we are messing with poll fds */
        usbi_mutex_lock(&ctx->pollfd_modify_lock);
        ctx->pollfd_modify++;
        usbi_mutex_unlock(&ctx->pollfd_modify_lock);

        /* write some data on control pipe to interrupt event handlers */
        r = usbi_write(ctx->ctrl_pipe[1], &dummy, sizeof(dummy));
        if (r <= 0) {
                usbi_warn(ctx, "internal signalling write failed");
                usbi_mutex_lock(&ctx->pollfd_modify_lock);
                ctx->pollfd_modify--;
                usbi_mutex_unlock(&ctx->pollfd_modify_lock);
                return;
        }

        /* take event handling lock */
        libusb_lock_events(ctx);

        /* read the dummy data */
        r = usbi_read(ctx->ctrl_pipe[0], &dummy, sizeof(dummy));
        if (r <= 0)
                usbi_warn(ctx, "internal signalling read failed");

        /* we're done with modifying poll fds */
        usbi_mutex_lock(&ctx->pollfd_modify_lock);
        ctx->pollfd_modify--;
        usbi_mutex_unlock(&ctx->pollfd_modify_lock);

        /* Release event handling lock and wake up event waiters */
        libusb_unlock_events(ctx);
}

/** \ingroup dev
 * Open a device and obtain a device handle. A handle allows you to perform
 * I/O on the device in question.
 *
 * Internally, this function adds a reference to the device and makes it
 * available to you through libusb_get_device(). This reference is removed
 * during libusb_close().
 *
 * This is a non-blocking function; no requests are sent over the bus.
 *
 * \param dev the device to open
 * \param handle output location for the returned device handle pointer. Only
 * populated when the return code is 0.
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NO_MEM on memory allocation failure
 * \returns LIBUSB_ERROR_ACCESS if the user has insufficient permissions
 * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
 * \returns another LIBUSB_ERROR code on other failure
 */
API_EXPORTED int libusb_open(libusb_device *dev, libusb_device_handle **handle)
{
        struct libusb_context *ctx = DEVICE_CTX(dev);
        struct libusb_device_handle *_handle;
        size_t priv_size = usbi_backend->device_handle_priv_size;
        int r;
        usbi_dbg("open %d.%d", dev->bus_number, dev->device_address);

        _handle = malloc(sizeof(*_handle) + priv_size);
        if (!_handle)
                return LIBUSB_ERROR_NO_MEM;

        r = usbi_mutex_init(&_handle->lock, NULL);
        if (r) {
                free(_handle);
                return LIBUSB_ERROR_OTHER;
        }

        _handle->dev = libusb_ref_device(dev);
        _handle->claimed_interfaces = 0;
        memset(&_handle->os_priv, 0, priv_size);

        r = usbi_backend->open(_handle);
        if (r < 0) {
                libusb_unref_device(dev);
                usbi_mutex_destroy(&_handle->lock);
                free(_handle);
                return r;
        }

        usbi_mutex_lock(&ctx->open_devs_lock);
        list_add(&_handle->list, &ctx->open_devs);
        usbi_mutex_unlock(&ctx->open_devs_lock);
        *handle = _handle;

        /* At this point, we want to interrupt any existing event handlers so
         * that they realise the addition of the new device's poll fd. One
         * example when this is desirable is if the user is running a separate
         * dedicated libusb events handling thread, which is running with a long
         * or infinite timeout. We want to interrupt that iteration of the loop,
         * so that it picks up the new fd, and then continues. */
        usbi_fd_notification(ctx);

        return 0;
}

/** \ingroup dev
 * Convenience function for finding a device with a particular
 * <tt>idVendor</tt>/<tt>idProduct</tt> combination. This function is intended
 * for those scenarios where you are using libusb to knock up a quick test
 * application - it allows you to avoid calling libusb_get_device_list() and
 * worrying about traversing/freeing the list.
 *
 * This function has limitations and is hence not intended for use in real
 * applications: if multiple devices have the same IDs it will only
 * give you the first one, etc.
 *
 * \param ctx the context to operate on, or NULL for the default context
 * \param vendor_id the idVendor value to search for
 * \param product_id the idProduct value to search for
 * \returns a handle for the first found device, or NULL on error or if the
 * device could not be found. */
API_EXPORTED libusb_device_handle *libusb_open_device_with_vid_pid(
        libusb_context *ctx, uint16_t vendor_id, uint16_t product_id)
{
        struct libusb_device **devs;
        struct libusb_device *found = NULL;
        struct libusb_device *dev;
        struct libusb_device_handle *handle = NULL;
        size_t i = 0;
        int r;

        if (libusb_get_device_list(ctx, &devs) < 0)
                return NULL;

        while ((dev = devs[i++]) != NULL) {
                struct libusb_device_descriptor desc;
                r = libusb_get_device_descriptor(dev, &desc);
                if (r < 0)
                        goto out;
                if (desc.idVendor == vendor_id && desc.idProduct == product_id) {
                        found = dev;
                        break;
                }
        }

        if (found) {
                r = libusb_open(found, &handle);
                if (r < 0)
                        handle = NULL;
        }

out:
        libusb_free_device_list(devs, 1);
        return handle;
}

static void do_close(struct libusb_context *ctx,
        struct libusb_device_handle *dev_handle)
{
        usbi_mutex_lock(&ctx->open_devs_lock);
        list_del(&dev_handle->list);
        usbi_mutex_unlock(&ctx->open_devs_lock);

        usbi_backend->close(dev_handle);
        libusb_unref_device(dev_handle->dev);
        usbi_mutex_destroy(&dev_handle->lock);
        free(dev_handle);
}

/** \ingroup dev
 * Close a device handle. Should be called on all open handles before your
 * application exits.
 *
 * Internally, this function destroys the reference that was added by
 * libusb_open() on the given device.
 *
 * This is a non-blocking function; no requests are sent over the bus.
 *
 * \param dev_handle the handle to close
 */
API_EXPORTED void libusb_close(libusb_device_handle *dev_handle)
{
        struct libusb_context *ctx;
        unsigned char dummy = 1;
        ssize_t r;

        if (!dev_handle)
                return;
        usbi_dbg("");

        ctx = HANDLE_CTX(dev_handle);

        /* Similarly to libusb_open(), we want to interrupt all event handlers
         * at this point. More importantly, we want to perform the actual close of
         * the device while holding the event handling lock (preventing any other
         * thread from doing event handling) because we will be removing a file
         * descriptor from the polling loop. */

        /* record that we are messing with poll fds */
        usbi_mutex_lock(&ctx->pollfd_modify_lock);
        ctx->pollfd_modify++;
        usbi_mutex_unlock(&ctx->pollfd_modify_lock);

        /* write some data on control pipe to interrupt event handlers */
        r = usbi_write(ctx->ctrl_pipe[1], &dummy, sizeof(dummy));
        if (r <= 0) {
                usbi_warn(ctx, "internal signalling write failed, closing anyway");
                do_close(ctx, dev_handle);
                usbi_mutex_lock(&ctx->pollfd_modify_lock);
                ctx->pollfd_modify--;
                usbi_mutex_unlock(&ctx->pollfd_modify_lock);
                return;
        }

        /* take event handling lock */
        libusb_lock_events(ctx);

        /* read the dummy data */
        r = usbi_read(ctx->ctrl_pipe[0], &dummy, sizeof(dummy));
        if (r <= 0)
                usbi_warn(ctx, "internal signalling read failed, closing anyway");

        /* Close the device */
        do_close(ctx, dev_handle);

        /* we're done with modifying poll fds */
        usbi_mutex_lock(&ctx->pollfd_modify_lock);
        ctx->pollfd_modify--;
        usbi_mutex_unlock(&ctx->pollfd_modify_lock);

        /* Release event handling lock and wake up event waiters */
        libusb_unlock_events(ctx);
}

/** \ingroup dev
 * Get the underlying device for a handle. This function does not modify
 * the reference count of the returned device, so do not feel compelled to
 * unreference it when you are done.
 * \param dev_handle a device handle
 * \returns the underlying device
 */
API_EXPORTED libusb_device *libusb_get_device(libusb_device_handle *dev_handle)
{
        return dev_handle->dev;
}

/** \ingroup dev
 * Determine the bConfigurationValue of the currently active configuration.
 *
 * You could formulate your own control request to obtain this information,
 * but this function has the advantage that it may be able to retrieve the
 * information from operating system caches (no I/O involved).
 *
 * If the OS does not cache this information, then this function will block
 * while a control transfer is submitted to retrieve the information.
 *
 * This function will return a value of 0 in the <tt>config</tt> output
 * parameter if the device is in unconfigured state.
 *
 * \param dev a device handle
 * \param config output location for the bConfigurationValue of the active
 * configuration (only valid for return code 0)
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
 * \returns another LIBUSB_ERROR code on other failure
 */
API_EXPORTED int libusb_get_configuration(libusb_device_handle *dev,
        int *config)
{
        int r = LIBUSB_ERROR_NOT_SUPPORTED;

        usbi_dbg("");
        if (usbi_backend->get_configuration)
                r = usbi_backend->get_configuration(dev, config);

        if (r == LIBUSB_ERROR_NOT_SUPPORTED) {
                uint8_t tmp = 0;
                usbi_dbg("falling back to control message");
                r = libusb_control_transfer(dev, LIBUSB_ENDPOINT_IN,
                        LIBUSB_REQUEST_GET_CONFIGURATION, 0, 0, &tmp, 1, 1000);
                if (r == 0) {
                        usbi_err(HANDLE_CTX(dev), "zero bytes returned in ctrl transfer?");
                        r = LIBUSB_ERROR_IO;
                } else if (r == 1) {
                        r = 0;
                        *config = tmp;
                } else {
                        usbi_dbg("control failed, error %d", r);
                }
        }

        if (r == 0)
                usbi_dbg("active config %d", *config);

        return r;
}

/** \ingroup dev
 * Set the active configuration for a device.
 *
 * The operating system may or may not have already set an active
 * configuration on the device. It is up to your application to ensure the
 * correct configuration is selected before you attempt to claim interfaces
 * and perform other operations.
 *
 * If you call this function on a device already configured with the selected
 * configuration, then this function will act as a lightweight device reset:
 * it will issue a SET_CONFIGURATION request using the current configuration,
 * causing most USB-related device state to be reset (altsetting reset to zero,
 * endpoint halts cleared, toggles reset).
 *
 * You cannot change/reset configuration if your application has claimed
 * interfaces - you should free them with libusb_release_interface() first.
 * You cannot change/reset configuration if other applications or drivers have
 * claimed interfaces.
 *
 * A configuration value of -1 will put the device in unconfigured state.
 * The USB specifications state that a configuration value of 0 does this,
 * however buggy devices exist which actually have a configuration 0.
 *
 * You should always use this function rather than formulating your own
 * SET_CONFIGURATION control request. This is because the underlying operating
 * system needs to know when such changes happen.
 *
 * This is a blocking function.
 *
 * \param dev a device handle
 * \param configuration the bConfigurationValue of the configuration you
 * wish to activate, or -1 if you wish to put the device in unconfigured state
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NOT_FOUND if the requested configuration does not exist
 * \returns LIBUSB_ERROR_BUSY if interfaces are currently claimed
 * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
 * \returns another LIBUSB_ERROR code on other failure
 */
API_EXPORTED int libusb_set_configuration(libusb_device_handle *dev,
        int configuration)
{
        usbi_dbg("configuration %d", configuration);
        return usbi_backend->set_configuration(dev, configuration);
}

/** \ingroup dev
 * Claim an interface on a given device handle. You must claim the interface
 * you wish to use before you can perform I/O on any of its endpoints.
 *
 * It is legal to attempt to claim an already-claimed interface, in which
 * case libusb just returns 0 without doing anything.
 *
 * Claiming of interfaces is a purely logical operation; it does not cause
 * any requests to be sent over the bus. Interface claiming is used to
 * instruct the underlying operating system that your application wishes
 * to take ownership of the interface.
 *
 * This is a non-blocking function.
 *
 * \param dev a device handle
 * \param interface_number the <tt>bInterfaceNumber</tt> of the interface you
 * wish to claim
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NOT_FOUND if the requested interface does not exist
 * \returns LIBUSB_ERROR_BUSY if another program or driver has claimed the
 * interface
 * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
 * \returns a LIBUSB_ERROR code on other failure
 */
API_EXPORTED int libusb_claim_interface(libusb_device_handle *dev,
        int interface_number)
{
        int r = 0;

        usbi_dbg("interface %d", interface_number);
        if (interface_number >= sizeof(dev->claimed_interfaces) * 8)
                return LIBUSB_ERROR_INVALID_PARAM;

        usbi_mutex_lock(&dev->lock);
        if (dev->claimed_interfaces & (1 << interface_number))
                goto out;

        r = usbi_backend->claim_interface(dev, interface_number);
        if (r == 0)
                dev->claimed_interfaces |= 1 << interface_number;

out:
        usbi_mutex_unlock(&dev->lock);
        return r;
}

/** \ingroup dev
 * Release an interface previously claimed with libusb_claim_interface(). You
 * should release all claimed interfaces before closing a device handle.
 *
 * This is a blocking function. A SET_INTERFACE control request will be sent
 * to the device, resetting interface state to the first alternate setting.
 *
 * \param dev a device handle
 * \param interface_number the <tt>bInterfaceNumber</tt> of the
 * previously-claimed interface
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NOT_FOUND if the interface was not claimed
 * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
 * \returns another LIBUSB_ERROR code on other failure
 */
API_EXPORTED int libusb_release_interface(libusb_device_handle *dev,
        int interface_number)
{
        int r;

        usbi_dbg("interface %d", interface_number);
        if (interface_number >= sizeof(dev->claimed_interfaces) * 8)
                return LIBUSB_ERROR_INVALID_PARAM;

        usbi_mutex_lock(&dev->lock);
        if (!(dev->claimed_interfaces & (1 << interface_number))) {
                r = LIBUSB_ERROR_NOT_FOUND;
                goto out;
        }

        r = usbi_backend->release_interface(dev, interface_number);
        if (r == 0)
                dev->claimed_interfaces &= ~(1 << interface_number);

out:
        usbi_mutex_unlock(&dev->lock);
        return r;
}

/** \ingroup dev
 * Activate an alternate setting for an interface. The interface must have
 * been previously claimed with libusb_claim_interface().
 *
 * You should always use this function rather than formulating your own
 * SET_INTERFACE control request. This is because the underlying operating
 * system needs to know when such changes happen.
 *
 * This is a blocking function.
 *
 * \param dev a device handle
 * \param interface_number the <tt>bInterfaceNumber</tt> of the
 * previously-claimed interface
 * \param alternate_setting the <tt>bAlternateSetting</tt> of the alternate
 * setting to activate
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NOT_FOUND if the interface was not claimed, or the
 * requested alternate setting does not exist
 * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
 * \returns another LIBUSB_ERROR code on other failure
 */
API_EXPORTED int libusb_set_interface_alt_setting(libusb_device_handle *dev,
        int interface_number, int alternate_setting)
{
        usbi_dbg("interface %d altsetting %d",
                interface_number, alternate_setting);
        if (interface_number >= sizeof(dev->claimed_interfaces) * 8)
                return LIBUSB_ERROR_INVALID_PARAM;

        usbi_mutex_lock(&dev->lock);
        if (!(dev->claimed_interfaces & (1 << interface_number))) {
                usbi_mutex_unlock(&dev->lock);
                return LIBUSB_ERROR_NOT_FOUND;
        }
        usbi_mutex_unlock(&dev->lock);

        return usbi_backend->set_interface_altsetting(dev, interface_number,
                alternate_setting);
}

/** \ingroup dev
 * Clear the halt/stall condition for an endpoint. Endpoints with halt status
 * are unable to receive or transmit data until the halt condition is stalled.
 *
 * You should cancel all pending transfers before attempting to clear the halt
 * condition.
 *
 * This is a blocking function.
 *
 * \param dev a device handle
 * \param endpoint the endpoint to clear halt status
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NOT_FOUND if the endpoint does not exist
 * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
 * \returns another LIBUSB_ERROR code on other failure
 */
API_EXPORTED int libusb_clear_halt(libusb_device_handle *dev,
        unsigned char endpoint)
{
        usbi_dbg("endpoint %x", endpoint);
        return usbi_backend->clear_halt(dev, endpoint);
}

/** \ingroup dev
 * Perform a USB port reset to reinitialize a device. The system will attempt
 * to restore the previous configuration and alternate settings after the
 * reset has completed.
 *
 * If the reset fails, the descriptors change, or the previous state cannot be
 * restored, the device will appear to be disconnected and reconnected. This
 * means that the device handle is no longer valid (you should close it) and
 * rediscover the device. A return code of LIBUSB_ERROR_NOT_FOUND indicates
 * when this is the case.
 *
 * This is a blocking function which usually incurs a noticeable delay.
 *
 * \param dev a handle of the device to reset
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NOT_FOUND if re-enumeration is required, or if the
 * device has been disconnected
 * \returns another LIBUSB_ERROR code on other failure
 */
API_EXPORTED int libusb_reset_device(libusb_device_handle *dev)
{
        usbi_dbg("");
        return usbi_backend->reset_device(dev);
}

/** \ingroup dev
 * Determine if a kernel driver is active on an interface. If a kernel driver
 * is active, you cannot claim the interface, and libusb will be unable to
 * perform I/O.
 *
 * \param dev a device handle
 * \param interface_number the interface to check
 * \returns 0 if no kernel driver is active
 * \returns 1 if a kernel driver is active
 * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
 * \returns another LIBUSB_ERROR code on other failure
 * \see libusb_detach_kernel_driver()
 */
API_EXPORTED int libusb_kernel_driver_active(libusb_device_handle *dev,
        int interface_number)
{
        usbi_dbg("interface %d", interface_number);
        if (usbi_backend->kernel_driver_active)
                return usbi_backend->kernel_driver_active(dev, interface_number);
        else
                return LIBUSB_ERROR_NOT_SUPPORTED;
}

/** \ingroup dev
 * Detach a kernel driver from an interface. If successful, you will then be
 * able to claim the interface and perform I/O.
 *
 * \param dev a device handle
 * \param interface_number the interface to detach the driver from
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NOT_FOUND if no kernel driver was active
 * \returns LIBUSB_ERROR_INVALID_PARAM if the interface does not exist
 * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
 * \returns another LIBUSB_ERROR code on other failure
 * \see libusb_kernel_driver_active()
 */
API_EXPORTED int libusb_detach_kernel_driver(libusb_device_handle *dev,
        int interface_number)
{
        usbi_dbg("interface %d", interface_number);
        if (usbi_backend->detach_kernel_driver)
                return usbi_backend->detach_kernel_driver(dev, interface_number);
        else
                return LIBUSB_ERROR_NOT_SUPPORTED;
}

/** \ingroup dev
 * Re-attach an interface's kernel driver, which was previously detached
 * using libusb_detach_kernel_driver().
 *
 * \param dev a device handle
 * \param interface_number the interface to attach the driver from
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NOT_FOUND if no kernel driver was active
 * \returns LIBUSB_ERROR_INVALID_PARAM if the interface does not exist
 * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
 * \returns LIBUSB_ERROR_BUSY if the driver cannot be attached because the
 * interface is claimed by a program or driver
 * \returns another LIBUSB_ERROR code on other failure
 * \see libusb_kernel_driver_active()
 */
API_EXPORTED int libusb_attach_kernel_driver(libusb_device_handle *dev,
        int interface_number)
{
        usbi_dbg("interface %d", interface_number);
        if (usbi_backend->attach_kernel_driver)
                return usbi_backend->attach_kernel_driver(dev, interface_number);
        else
                return LIBUSB_ERROR_NOT_SUPPORTED;
}

/** \ingroup lib
 * Set message verbosity.
 *  - Level 0: no messages ever printed by the library (default)
 *  - Level 1: error messages are printed to stderr
 *  - Level 2: warning and error messages are printed to stderr
 *  - Level 3: informational messages are printed to stdout, warning and error
 *    messages are printed to stderr
 *
 * The default level is 0, which means no messages are ever printed. If you
 * choose to increase the message verbosity level, ensure that your
 * application does not close the stdout/stderr file descriptors.
 *
 * You are advised to set level 3. libusb is conservative with its message
 * logging and most of the time, will only log messages that explain error
 * conditions and other oddities. This will help you debug your software.
 *
 * If the LIBUSB_DEBUG environment variable was set when libusb was
 * initialized, this function does nothing: the message verbosity is fixed
 * to the value in the environment variable.
 *
 * If libusb was compiled without any message logging, this function does
 * nothing: you'll never get any messages.
 *
 * If libusb was compiled with verbose debug message logging, this function
 * does nothing: you'll always get messages from all levels.
 *
 * \param ctx the context to operate on, or NULL for the default context
 * \param level debug level to set
 */
API_EXPORTED void libusb_set_debug(libusb_context *ctx, int level)
{
        USBI_GET_CONTEXT(ctx);
        if (!ctx->debug_fixed)
                ctx->debug = level;
}

/** \ingroup lib
 * Initialize libusb. This function must be called before calling any other
 * libusb function.
 *
 * If you do not provide an output location for a context pointer, a default
 * context will be created. If there was already a default context, it will
 * be reused (and nothing will be initialized/reinitialized).
 *
 * \param context Optional output location for context pointer.
 * Only valid on return code 0.
 * \returns 0 on success, or a LIBUSB_ERROR code on failure
 * \see contexts
 */
API_EXPORTED int libusb_init(libusb_context **context)
{
        char *dbg = getenv("LIBUSB_DEBUG");
        struct libusb_context *ctx;
        int r;

        usbi_mutex_static_lock(&default_context_lock);
        if (!context && usbi_default_context) {
                r = 0;
                usbi_dbg("reusing default context");
                default_context_refcnt++;
                usbi_mutex_static_unlock(&default_context_lock);
                return 0;
        }

        ctx = malloc(sizeof(*ctx));
        if (!ctx) {
                r = LIBUSB_ERROR_NO_MEM;
                goto err_unlock;
        }
        memset(ctx, 0, sizeof(*ctx));

        if (dbg) {
                ctx->debug = atoi(dbg);
                if (ctx->debug)
                        ctx->debug_fixed = 1;
        }

        usbi_dbg("");

        if (usbi_backend->init) {
                r = usbi_backend->init(ctx);
                if (r)
                        goto err_free_ctx;
        }

        usbi_mutex_init(&ctx->usb_devs_lock, NULL);
        usbi_mutex_init(&ctx->open_devs_lock, NULL);
        list_init(&ctx->usb_devs);
        list_init(&ctx->open_devs);

        r = usbi_io_init(ctx);
        if (r < 0) {
                if (usbi_backend->exit)
                        usbi_backend->exit();
                goto err_destroy_mutex;
        }

        if (context) {
                *context = ctx;
        } else if (!usbi_default_context) {
                usbi_dbg("created default context");
                usbi_default_context = ctx;
                default_context_refcnt++;
        }
        usbi_mutex_static_unlock(&default_context_lock);

        return 0;

err_destroy_mutex:
        usbi_mutex_destroy(&ctx->open_devs_lock);
        usbi_mutex_destroy(&ctx->usb_devs_lock);
err_free_ctx:
        free(ctx);
err_unlock:
        usbi_mutex_static_unlock(&default_context_lock);
        return r;
}

/** \ingroup lib
 * Deinitialize libusb. Should be called after closing all open devices and
 * before your application terminates.
 * \param ctx the context to deinitialize, or NULL for the default context
 */
API_EXPORTED void libusb_exit(struct libusb_context *ctx)
{
        usbi_dbg("");
        USBI_GET_CONTEXT(ctx);

        /* if working with default context, only actually do the deinitialization
         * if we're the last user */
        if (ctx == usbi_default_context) {
                usbi_mutex_static_lock(&default_context_lock);
                if (--default_context_refcnt > 0) {
                        usbi_dbg("not destroying default context");
                        usbi_mutex_static_unlock(&default_context_lock);
                        return;
                }
                usbi_dbg("destroying default context");
                usbi_default_context = NULL;
                usbi_mutex_static_unlock(&default_context_lock);
        }

        /* a little sanity check. doesn't bother with open_devs locking because
         * unless there is an application bug, nobody will be accessing this. */
        if (!list_empty(&ctx->open_devs))
                usbi_warn(ctx, "application left some devices open");

        usbi_io_exit(ctx);
        if (usbi_backend->exit)
                usbi_backend->exit();

        usbi_mutex_destroy(&ctx->open_devs_lock);
        usbi_mutex_destroy(&ctx->usb_devs_lock);
        free(ctx);
}

void usbi_log_v(struct libusb_context *ctx, enum usbi_log_level level,
        const char *function, const char *format, va_list args)
{
        FILE *stream = stdout;
        const char *prefix;

#ifndef ENABLE_DEBUG_LOGGING
        USBI_GET_CONTEXT(ctx);
        if (!ctx->debug)
                return;
        if (level == LOG_LEVEL_WARNING && ctx->debug < 2)
                return;
        if (level == LOG_LEVEL_INFO && ctx->debug < 3)
                return;
#endif

        switch (level) {
        case LOG_LEVEL_INFO:
                prefix = "info";
                break;
        case LOG_LEVEL_WARNING:
                stream = stderr;
                prefix = "warning";
                break;
        case LOG_LEVEL_ERROR:
                stream = stderr;
                prefix = "error";
                break;
        case LOG_LEVEL_DEBUG:
                stream = stderr;
                prefix = "debug";
                break;
        default:
                stream = stderr;
                prefix = "unknown";
                break;
        }

        fprintf(stream, "libusb:%s [%s] ", prefix, function);

        vfprintf(stream, format, args);

        fprintf(stream, "\n");
}

void usbi_log(struct libusb_context *ctx, enum usbi_log_level level,
        const char *function, const char *format, ...)
{
        va_list args;

        va_start (args, format);
        usbi_log_v(ctx, level, function, format, args);
        va_end (args);
}

/** \ingroup misc
 * Returns a constant NULL-terminated string with an English short description
 * of the given error code. The caller should never free() the returned pointer
 * since it points to a constant string.
 * The returned string is encoded in ASCII form and always starts with a
 * capital letter and ends without any punctuation.
 * Future versions of libusb may return NULL if the library is compiled without
 * these messages included (e.g. for embedded systems).
 * This function is intended to be used for debugging purposes only.
 *
 * \param errcode the error code whose description is desired
 * \returns a short description of the error code in English, or NULL if the
 * error descriptions are unavailable
 */
API_EXPORTED const char *libusb_strerror(enum libusb_error errcode)
{
        switch (errcode) {
        case LIBUSB_SUCCESS:
                return "Success";
        case LIBUSB_ERROR_IO:
                return "Input/output error";
        case LIBUSB_ERROR_INVALID_PARAM:
                return "Invalid parameter";
        case LIBUSB_ERROR_ACCESS:
                return "Access denied (insufficient permissions)";
        case LIBUSB_ERROR_NO_DEVICE:
                return "No such device (it may have been disconnected)";
        case LIBUSB_ERROR_NOT_FOUND:
                return "Entity not found";
        case LIBUSB_ERROR_BUSY:
                return "Resource busy";
        case LIBUSB_ERROR_TIMEOUT:
                return "Operation timed out";
        case LIBUSB_ERROR_OVERFLOW:
                return "Overflow";
        case LIBUSB_ERROR_PIPE:
                return "Pipe error";
        case LIBUSB_ERROR_INTERRUPTED:
                return "System call interrupted (perhaps due to signal)";
        case LIBUSB_ERROR_NO_MEM:
                return "Insufficient memory";
        case LIBUSB_ERROR_NOT_SUPPORTED:
                return "Operation not supported or unimplemented on this platform";
        case LIBUSB_ERROR_OTHER:
                return "Other error";
        }
        return "Unknown error";
}