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+/*
+ * I/O functions for libusb
+ * Copyright (C) 2007-2009 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 <signal.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+
+#ifdef HAVE_SYS_TIME_H
+#include <sys/time.h>
+#endif
+
+#ifdef USBI_TIMERFD_AVAILABLE
+#include <sys/timerfd.h>
+#endif
+
+#include "libusbi.h"
+
+/**
+ * \page io Synchronous and asynchronous device I/O
+ *
+ * \section intro Introduction
+ *
+ * If you're using libusb in your application, you're probably wanting to
+ * perform I/O with devices - you want to perform USB data transfers.
+ *
+ * libusb offers two separate interfaces for device I/O. This page aims to
+ * introduce the two in order to help you decide which one is more suitable
+ * for your application. You can also choose to use both interfaces in your
+ * application by considering each transfer on a case-by-case basis.
+ *
+ * Once you have read through the following discussion, you should consult the
+ * detailed API documentation pages for the details:
+ * - \ref syncio
+ * - \ref asyncio
+ *
+ * \section theory Transfers at a logical level
+ *
+ * At a logical level, USB transfers typically happen in two parts. For
+ * example, when reading data from a endpoint:
+ * -# A request for data is sent to the device
+ * -# Some time later, the incoming data is received by the host
+ *
+ * or when writing data to an endpoint:
+ *
+ * -# The data is sent to the device
+ * -# Some time later, the host receives acknowledgement from the device that
+ *    the data has been transferred.
+ *
+ * There may be an indefinite delay between the two steps. Consider a
+ * fictional USB input device with a button that the user can press. In order
+ * to determine when the button is pressed, you would likely submit a request
+ * to read data on a bulk or interrupt endpoint and wait for data to arrive.
+ * Data will arrive when the button is pressed by the user, which is
+ * potentially hours later.
+ *
+ * libusb offers both a synchronous and an asynchronous interface to performing
+ * USB transfers. The main difference is that the synchronous interface
+ * combines both steps indicated above into a single function call, whereas
+ * the asynchronous interface separates them.
+ *
+ * \section sync The synchronous interface
+ *
+ * The synchronous I/O interface allows you to perform a USB transfer with
+ * a single function call. When the function call returns, the transfer has
+ * completed and you can parse the results.
+ *
+ * If you have used the libusb-0.1 before, this I/O style will seem familar to
+ * you. libusb-0.1 only offered a synchronous interface.
+ *
+ * In our input device example, to read button presses you might write code
+ * in the following style:
+\code
+unsigned char data[4];
+int actual_length,
+int r = libusb_bulk_transfer(handle, EP_IN, data, sizeof(data), &actual_length, 0);
+if (r == 0 && actual_length == sizeof(data)) {
+	// results of the transaction can now be found in the data buffer
+	// parse them here and report button press
+} else {
+	error();
+}
+\endcode
+ *
+ * The main advantage of this model is simplicity: you did everything with
+ * a single simple function call.
+ *
+ * However, this interface has its limitations. Your application will sleep
+ * inside libusb_bulk_transfer() until the transaction has completed. If it
+ * takes the user 3 hours to press the button, your application will be
+ * sleeping for that long. Execution will be tied up inside the library -
+ * the entire thread will be useless for that duration.
+ *
+ * Another issue is that by tieing up the thread with that single transaction
+ * there is no possibility of performing I/O with multiple endpoints and/or
+ * multiple devices simultaneously, unless you resort to creating one thread
+ * per transaction.
+ *
+ * Additionally, there is no opportunity to cancel the transfer after the
+ * request has been submitted.
+ *
+ * For details on how to use the synchronous API, see the
+ * \ref syncio "synchronous I/O API documentation" pages.
+ *
+ * \section async The asynchronous interface
+ *
+ * Asynchronous I/O is the most significant new feature in libusb-1.0.
+ * Although it is a more complex interface, it solves all the issues detailed
+ * above.
+ *
+ * Instead of providing which functions that block until the I/O has complete,
+ * libusb's asynchronous interface presents non-blocking functions which
+ * begin a transfer and then return immediately. Your application passes a
+ * callback function pointer to this non-blocking function, which libusb will
+ * call with the results of the transaction when it has completed.
+ *
+ * Transfers which have been submitted through the non-blocking functions
+ * can be cancelled with a separate function call.
+ *
+ * The non-blocking nature of this interface allows you to be simultaneously
+ * performing I/O to multiple endpoints on multiple devices, without having
+ * to use threads.
+ *
+ * This added flexibility does come with some complications though:
+ * - In the interest of being a lightweight library, libusb does not create
+ * threads and can only operate when your application is calling into it. Your
+ * application must call into libusb from it's main loop when events are ready
+ * to be handled, or you must use some other scheme to allow libusb to
+ * undertake whatever work needs to be done.
+ * - libusb also needs to be called into at certain fixed points in time in
+ * order to accurately handle transfer timeouts.
+ * - Memory handling becomes more complex. You cannot use stack memory unless
+ * the function with that stack is guaranteed not to return until the transfer
+ * callback has finished executing.
+ * - You generally lose some linearity from your code flow because submitting
+ * the transfer request is done in a separate function from where the transfer
+ * results are handled. This becomes particularly obvious when you want to
+ * submit a second transfer based on the results of an earlier transfer.
+ *
+ * Internally, libusb's synchronous interface is expressed in terms of function
+ * calls to the asynchronous interface.
+ *
+ * For details on how to use the asynchronous API, see the
+ * \ref asyncio "asynchronous I/O API" documentation pages.
+ */
+
+
+/**
+ * \page packetoverflow Packets and overflows
+ *
+ * \section packets Packet abstraction
+ *
+ * The USB specifications describe how data is transmitted in packets, with
+ * constraints on packet size defined by endpoint descriptors. The host must
+ * not send data payloads larger than the endpoint's maximum packet size.
+ *
+ * libusb and the underlying OS abstract out the packet concept, allowing you
+ * to request transfers of any size. Internally, the request will be divided
+ * up into correctly-sized packets. You do not have to be concerned with
+ * packet sizes, but there is one exception when considering overflows.
+ *
+ * \section overflow Bulk/interrupt transfer overflows
+ *
+ * When requesting data on a bulk endpoint, libusb requires you to supply a
+ * buffer and the maximum number of bytes of data that libusb can put in that
+ * buffer. However, the size of the buffer is not communicated to the device -
+ * the device is just asked to send any amount of data.
+ *
+ * There is no problem if the device sends an amount of data that is less than
+ * or equal to the buffer size. libusb reports this condition to you through
+ * the \ref libusb_transfer::actual_length "libusb_transfer.actual_length"
+ * field.
+ *
+ * Problems may occur if the device attempts to send more data than can fit in
+ * the buffer. libusb reports LIBUSB_TRANSFER_OVERFLOW for this condition but
+ * other behaviour is largely undefined: actual_length may or may not be
+ * accurate, the chunk of data that can fit in the buffer (before overflow)
+ * may or may not have been transferred.
+ *
+ * Overflows are nasty, but can be avoided. Even though you were told to
+ * ignore packets above, think about the lower level details: each transfer is
+ * split into packets (typically small, with a maximum size of 512 bytes).
+ * Overflows can only happen if the final packet in an incoming data transfer
+ * is smaller than the actual packet that the device wants to transfer.
+ * Therefore, you will never see an overflow if your transfer buffer size is a
+ * multiple of the endpoint's packet size: the final packet will either
+ * fill up completely or will be only partially filled.
+ */
+
+/**
+ * @defgroup asyncio Asynchronous device I/O
+ *
+ * This page details libusb's asynchronous (non-blocking) API for USB device
+ * I/O. This interface is very powerful but is also quite complex - you will
+ * need to read this page carefully to understand the necessary considerations
+ * and issues surrounding use of this interface. Simplistic applications
+ * may wish to consider the \ref syncio "synchronous I/O API" instead.
+ *
+ * The asynchronous interface is built around the idea of separating transfer
+ * submission and handling of transfer completion (the synchronous model
+ * combines both of these into one). There may be a long delay between
+ * submission and completion, however the asynchronous submission function
+ * is non-blocking so will return control to your application during that
+ * potentially long delay.
+ *
+ * \section asyncabstraction Transfer abstraction
+ *
+ * For the asynchronous I/O, libusb implements the concept of a generic
+ * transfer entity for all types of I/O (control, bulk, interrupt,
+ * isochronous). The generic transfer object must be treated slightly
+ * differently depending on which type of I/O you are performing with it.
+ *
+ * This is represented by the public libusb_transfer structure type.
+ *
+ * \section asynctrf Asynchronous transfers
+ *
+ * We can view asynchronous I/O as a 5 step process:
+ * -# <b>Allocation</b>: allocate a libusb_transfer
+ * -# <b>Filling</b>: populate the libusb_transfer instance with information
+ *    about the transfer you wish to perform
+ * -# <b>Submission</b>: ask libusb to submit the transfer
+ * -# <b>Completion handling</b>: examine transfer results in the
+ *    libusb_transfer structure
+ * -# <b>Deallocation</b>: clean up resources
+ *
+ *
+ * \subsection asyncalloc Allocation
+ *
+ * This step involves allocating memory for a USB transfer. This is the
+ * generic transfer object mentioned above. At this stage, the transfer
+ * is "blank" with no details about what type of I/O it will be used for.
+ *
+ * Allocation is done with the libusb_alloc_transfer() function. You must use
+ * this function rather than allocating your own transfers.
+ *
+ * \subsection asyncfill Filling
+ *
+ * This step is where you take a previously allocated transfer and fill it
+ * with information to determine the message type and direction, data buffer,
+ * callback function, etc.
+ *
+ * You can either fill the required fields yourself or you can use the
+ * helper functions: libusb_fill_control_transfer(), libusb_fill_bulk_transfer()
+ * and libusb_fill_interrupt_transfer().
+ *
+ * \subsection asyncsubmit Submission
+ *
+ * When you have allocated a transfer and filled it, you can submit it using
+ * libusb_submit_transfer(). This function returns immediately but can be
+ * regarded as firing off the I/O request in the background.
+ *
+ * \subsection asynccomplete Completion handling
+ *
+ * After a transfer has been submitted, one of four things can happen to it:
+ *
+ * - The transfer completes (i.e. some data was transferred)
+ * - The transfer has a timeout and the timeout expires before all data is
+ * transferred
+ * - The transfer fails due to an error
+ * - The transfer is cancelled
+ *
+ * Each of these will cause the user-specified transfer callback function to
+ * be invoked. It is up to the callback function to determine which of the
+ * above actually happened and to act accordingly.
+ *
+ * The user-specified callback is passed a pointer to the libusb_transfer
+ * structure which was used to setup and submit the transfer. At completion
+ * time, libusb has populated this structure with results of the transfer:
+ * success or failure reason, number of bytes of data transferred, etc. See
+ * the libusb_transfer structure documentation for more information.
+ *
+ * \subsection Deallocation
+ *
+ * When a transfer has completed (i.e. the callback function has been invoked),
+ * you are advised to free the transfer (unless you wish to resubmit it, see
+ * below). Transfers are deallocated with libusb_free_transfer().
+ *
+ * It is undefined behaviour to free a transfer which has not completed.
+ *
+ * \section asyncresubmit Resubmission
+ *
+ * You may be wondering why allocation, filling, and submission are all
+ * separated above where they could reasonably be combined into a single
+ * operation.
+ *
+ * The reason for separation is to allow you to resubmit transfers without
+ * having to allocate new ones every time. This is especially useful for
+ * common situations dealing with interrupt endpoints - you allocate one
+ * transfer, fill and submit it, and when it returns with results you just
+ * resubmit it for the next interrupt.
+ *
+ * \section asynccancel Cancellation
+ *
+ * Another advantage of using the asynchronous interface is that you have
+ * the ability to cancel transfers which have not yet completed. This is
+ * done by calling the libusb_cancel_transfer() function.
+ *
+ * libusb_cancel_transfer() is asynchronous/non-blocking in itself. When the
+ * cancellation actually completes, the transfer's callback function will
+ * be invoked, and the callback function should check the transfer status to
+ * determine that it was cancelled.
+ *
+ * Freeing the transfer after it has been cancelled but before cancellation
+ * has completed will result in undefined behaviour.
+ *
+ * When a transfer is cancelled, some of the data may have been transferred.
+ * libusb will communicate this to you in the transfer callback. Do not assume
+ * that no data was transferred.
+ *
+ * \section bulk_overflows Overflows on device-to-host bulk/interrupt endpoints
+ *
+ * If your device does not have predictable transfer sizes (or it misbehaves),
+ * your application may submit a request for data on an IN endpoint which is
+ * smaller than the data that the device wishes to send. In some circumstances
+ * this will cause an overflow, which is a nasty condition to deal with. See
+ * the \ref packetoverflow page for discussion.
+ *
+ * \section asyncctrl Considerations for control transfers
+ *
+ * The <tt>libusb_transfer</tt> structure is generic and hence does not
+ * include specific fields for the control-specific setup packet structure.
+ *
+ * In order to perform a control transfer, you must place the 8-byte setup
+ * packet at the start of the data buffer. To simplify this, you could
+ * cast the buffer pointer to type struct libusb_control_setup, or you can
+ * use the helper function libusb_fill_control_setup().
+ *
+ * The wLength field placed in the setup packet must be the length you would
+ * expect to be sent in the setup packet: the length of the payload that
+ * follows (or the expected maximum number of bytes to receive). However,
+ * the length field of the libusb_transfer object must be the length of
+ * the data buffer - i.e. it should be wLength <em>plus</em> the size of
+ * the setup packet (LIBUSB_CONTROL_SETUP_SIZE).
+ *
+ * If you use the helper functions, this is simplified for you:
+ * -# Allocate a buffer of size LIBUSB_CONTROL_SETUP_SIZE plus the size of the
+ * data you are sending/requesting.
+ * -# Call libusb_fill_control_setup() on the data buffer, using the transfer
+ * request size as the wLength value (i.e. do not include the extra space you
+ * allocated for the control setup).
+ * -# If this is a host-to-device transfer, place the data to be transferred
+ * in the data buffer, starting at offset LIBUSB_CONTROL_SETUP_SIZE.
+ * -# Call libusb_fill_control_transfer() to associate the data buffer with
+ * the transfer (and to set the remaining details such as callback and timeout).
+ *   - Note that there is no parameter to set the length field of the transfer.
+ *     The length is automatically inferred from the wLength field of the setup
+ *     packet.
+ * -# Submit the transfer.
+ *
+ * The multi-byte control setup fields (wValue, wIndex and wLength) must
+ * be given in little-endian byte order (the endianness of the USB bus).
+ * Endianness conversion is transparently handled by
+ * libusb_fill_control_setup() which is documented to accept host-endian
+ * values.
+ *
+ * Further considerations are needed when handling transfer completion in
+ * your callback function:
+ * - As you might expect, the setup packet will still be sitting at the start
+ * of the data buffer.
+ * - If this was a device-to-host transfer, the received data will be sitting
+ * at offset LIBUSB_CONTROL_SETUP_SIZE into the buffer.
+ * - The actual_length field of the transfer structure is relative to the
+ * wLength of the setup packet, rather than the size of the data buffer. So,
+ * if your wLength was 4, your transfer's <tt>length</tt> was 12, then you
+ * should expect an <tt>actual_length</tt> of 4 to indicate that the data was
+ * transferred in entirity.
+ *
+ * To simplify parsing of setup packets and obtaining the data from the
+ * correct offset, you may wish to use the libusb_control_transfer_get_data()
+ * and libusb_control_transfer_get_setup() functions within your transfer
+ * callback.
+ *
+ * Even though control endpoints do not halt, a completed control transfer
+ * may have a LIBUSB_TRANSFER_STALL status code. This indicates the control
+ * request was not supported.
+ *
+ * \section asyncintr Considerations for interrupt transfers
+ *
+ * All interrupt transfers are performed using the polling interval presented
+ * by the bInterval value of the endpoint descriptor.
+ *
+ * \section asynciso Considerations for isochronous transfers
+ *
+ * Isochronous transfers are more complicated than transfers to
+ * non-isochronous endpoints.
+ *
+ * To perform I/O to an isochronous endpoint, allocate the transfer by calling
+ * libusb_alloc_transfer() with an appropriate number of isochronous packets.
+ *
+ * During filling, set \ref libusb_transfer::type "type" to
+ * \ref libusb_transfer_type::LIBUSB_TRANSFER_TYPE_ISOCHRONOUS
+ * "LIBUSB_TRANSFER_TYPE_ISOCHRONOUS", and set
+ * \ref libusb_transfer::num_iso_packets "num_iso_packets" to a value less than
+ * or equal to the number of packets you requested during allocation.
+ * libusb_alloc_transfer() does not set either of these fields for you, given
+ * that you might not even use the transfer on an isochronous endpoint.
+ *
+ * Next, populate the length field for the first num_iso_packets entries in
+ * the \ref libusb_transfer::iso_packet_desc "iso_packet_desc" array. Section
+ * 5.6.3 of the USB2 specifications describe how the maximum isochronous
+ * packet length is determined by the wMaxPacketSize field in the endpoint
+ * descriptor.
+ * Two functions can help you here:
+ *
+ * - libusb_get_max_iso_packet_size() is an easy way to determine the max
+ *   packet size for an isochronous endpoint. Note that the maximum packet
+ *   size is actually the maximum number of bytes that can be transmitted in
+ *   a single microframe, therefore this function multiplies the maximum number
+ *   of bytes per transaction by the number of transaction opportunities per
+ *   microframe.
+ * - libusb_set_iso_packet_lengths() assigns the same length to all packets
+ *   within a transfer, which is usually what you want.
+ *
+ * For outgoing transfers, you'll obviously fill the buffer and populate the
+ * packet descriptors in hope that all the data gets transferred. For incoming
+ * transfers, you must ensure the buffer has sufficient capacity for
+ * the situation where all packets transfer the full amount of requested data.
+ *
+ * Completion handling requires some extra consideration. The
+ * \ref libusb_transfer::actual_length "actual_length" field of the transfer
+ * is meaningless and should not be examined; instead you must refer to the
+ * \ref libusb_iso_packet_descriptor::actual_length "actual_length" field of
+ * each individual packet.
+ *
+ * The \ref libusb_transfer::status "status" field of the transfer is also a
+ * little misleading:
+ *  - If the packets were submitted and the isochronous data microframes
+ *    completed normally, status will have value
+ *    \ref libusb_transfer_status::LIBUSB_TRANSFER_COMPLETED
+ *    "LIBUSB_TRANSFER_COMPLETED". Note that bus errors and software-incurred
+ *    delays are not counted as transfer errors; the transfer.status field may
+ *    indicate COMPLETED even if some or all of the packets failed. Refer to
+ *    the \ref libusb_iso_packet_descriptor::status "status" field of each
+ *    individual packet to determine packet failures.
+ *  - The status field will have value
+ *    \ref libusb_transfer_status::LIBUSB_TRANSFER_ERROR
+ *    "LIBUSB_TRANSFER_ERROR" only when serious errors were encountered.
+ *  - Other transfer status codes occur with normal behaviour.
+ *
+ * The data for each packet will be found at an offset into the buffer that
+ * can be calculated as if each prior packet completed in full. The
+ * libusb_get_iso_packet_buffer() and libusb_get_iso_packet_buffer_simple()
+ * functions may help you here.
+ *
+ * \section asyncmem Memory caveats
+ *
+ * In most circumstances, it is not safe to use stack memory for transfer
+ * buffers. This is because the function that fired off the asynchronous
+ * transfer may return before libusb has finished using the buffer, and when
+ * the function returns it's stack gets destroyed. This is true for both
+ * host-to-device and device-to-host transfers.
+ *
+ * The only case in which it is safe to use stack memory is where you can
+ * guarantee that the function owning the stack space for the buffer does not
+ * return until after the transfer's callback function has completed. In every
+ * other case, you need to use heap memory instead.
+ *
+ * \section asyncflags Fine control
+ *
+ * Through using this asynchronous interface, you may find yourself repeating
+ * a few simple operations many times. You can apply a bitwise OR of certain
+ * flags to a transfer to simplify certain things:
+ * - \ref libusb_transfer_flags::LIBUSB_TRANSFER_SHORT_NOT_OK
+ *   "LIBUSB_TRANSFER_SHORT_NOT_OK" results in transfers which transferred
+ *   less than the requested amount of data being marked with status
+ *   \ref libusb_transfer_status::LIBUSB_TRANSFER_ERROR "LIBUSB_TRANSFER_ERROR"
+ *   (they would normally be regarded as COMPLETED)
+ * - \ref libusb_transfer_flags::LIBUSB_TRANSFER_FREE_BUFFER
+ *   "LIBUSB_TRANSFER_FREE_BUFFER" allows you to ask libusb to free the transfer
+ *   buffer when freeing the transfer.
+ * - \ref libusb_transfer_flags::LIBUSB_TRANSFER_FREE_TRANSFER
+ *   "LIBUSB_TRANSFER_FREE_TRANSFER" causes libusb to automatically free the
+ *   transfer after the transfer callback returns.
+ *
+ * \section asyncevent Event handling
+ *
+ * In accordance of the aim of being a lightweight library, libusb does not
+ * create threads internally. This means that libusb code does not execute
+ * at any time other than when your application is calling a libusb function.
+ * However, an asynchronous model requires that libusb perform work at various
+ * points in time - namely processing the results of previously-submitted
+ * transfers and invoking the user-supplied callback function.
+ *
+ * This gives rise to the libusb_handle_events() function which your
+ * application must call into when libusb has work do to. This gives libusb
+ * the opportunity to reap pending transfers, invoke callbacks, etc.
+ *
+ * The first issue to discuss here is how your application can figure out
+ * when libusb has work to do. In fact, there are two naive options which
+ * do not actually require your application to know this:
+ * -# Periodically call libusb_handle_events() in non-blocking mode at fixed
+ *    short intervals from your main loop
+ * -# Repeatedly call libusb_handle_events() in blocking mode from a dedicated
+ *    thread.
+ *
+ * The first option is plainly not very nice, and will cause unnecessary
+ * CPU wakeups leading to increased power usage and decreased battery life.
+ * The second option is not very nice either, but may be the nicest option
+ * available to you if the "proper" approach can not be applied to your
+ * application (read on...).
+ *
+ * The recommended option is to integrate libusb with your application main
+ * event loop. libusb exposes a set of file descriptors which allow you to do
+ * this. Your main loop is probably already calling poll() or select() or a
+ * variant on a set of file descriptors for other event sources (e.g. keyboard
+ * button presses, mouse movements, network sockets, etc). You then add
+ * libusb's file descriptors to your poll()/select() calls, and when activity
+ * is detected on such descriptors you know it is time to call
+ * libusb_handle_events().
+ *
+ * There is one final event handling complication. libusb supports
+ * asynchronous transfers which time out after a specified time period, and
+ * this requires that libusb is called into at or after the timeout so that
+ * the timeout can be handled. So, in addition to considering libusb's file
+ * descriptors in your main event loop, you must also consider that libusb
+ * sometimes needs to be called into at fixed points in time even when there
+ * is no file descriptor activity.
+ *
+ * For the details on retrieving the set of file descriptors and determining
+ * the next timeout, see the \ref poll "polling and timing" API documentation.
+ */
+
+/**
+ * @defgroup poll Polling and timing
+ *
+ * This page documents libusb's functions for polling events and timing.
+ * These functions are only necessary for users of the
+ * \ref asyncio "asynchronous API". If you are only using the simpler
+ * \ref syncio "synchronous API" then you do not need to ever call these
+ * functions.
+ *
+ * The justification for the functionality described here has already been
+ * discussed in the \ref asyncevent "event handling" section of the
+ * asynchronous API documentation. In summary, libusb does not create internal
+ * threads for event processing and hence relies on your application calling
+ * into libusb at certain points in time so that pending events can be handled.
+ * In order to know precisely when libusb needs to be called into, libusb
+ * offers you a set of pollable file descriptors and information about when
+ * the next timeout expires.
+ *
+ * If you are using the asynchronous I/O API, you must take one of the two
+ * following options, otherwise your I/O will not complete.
+ *
+ * \section pollsimple The simple option
+ *
+ * If your application revolves solely around libusb and does not need to
+ * handle other event sources, you can have a program structure as follows:
+\code
+// initialize libusb
+// find and open device
+// maybe fire off some initial async I/O
+
+while (user_has_not_requested_exit)
+	libusb_handle_events(ctx);
+
+// clean up and exit
+\endcode
+ *
+ * With such a simple main loop, you do not have to worry about managing
+ * sets of file descriptors or handling timeouts. libusb_handle_events() will
+ * handle those details internally.
+ *
+ * \section pollmain The more advanced option
+ *
+ * \note This functionality is currently only available on Unix-like platforms.
+ * On Windows, libusb_get_pollfds() simply returns NULL. Exposing event sources
+ * on Windows will require some further thought and design.
+ *
+ * In more advanced applications, you will already have a main loop which
+ * is monitoring other event sources: network sockets, X11 events, mouse
+ * movements, etc. Through exposing a set of file descriptors, libusb is
+ * designed to cleanly integrate into such main loops.
+ *
+ * In addition to polling file descriptors for the other event sources, you
+ * take a set of file descriptors from libusb and monitor those too. When you
+ * detect activity on libusb's file descriptors, you call
+ * libusb_handle_events_timeout() in non-blocking mode.
+ *
+ * What's more, libusb may also need to handle events at specific moments in
+ * time. No file descriptor activity is generated at these times, so your
+ * own application needs to be continually aware of when the next one of these
+ * moments occurs (through calling libusb_get_next_timeout()), and then it
+ * needs to call libusb_handle_events_timeout() in non-blocking mode when
+ * these moments occur. This means that you need to adjust your
+ * poll()/select() timeout accordingly.
+ *
+ * libusb provides you with a set of file descriptors to poll and expects you
+ * to poll all of them, treating them as a single entity. The meaning of each
+ * file descriptor in the set is an internal implementation detail,
+ * platform-dependent and may vary from release to release. Don't try and
+ * interpret the meaning of the file descriptors, just do as libusb indicates,
+ * polling all of them at once.
+ *
+ * In pseudo-code, you want something that looks like:
+\code
+// initialise libusb
+
+libusb_get_pollfds(ctx)
+while (user has not requested application exit) {
+	libusb_get_next_timeout(ctx);
+	poll(on libusb file descriptors plus any other event sources of interest,
+		using a timeout no larger than the value libusb just suggested)
+	if (poll() indicated activity on libusb file descriptors)
+		libusb_handle_events_timeout(ctx, 0);
+	if (time has elapsed to or beyond the libusb timeout)
+		libusb_handle_events_timeout(ctx, 0);
+	// handle events from other sources here
+}
+
+// clean up and exit
+\endcode
+ *
+ * \subsection polltime Notes on time-based events
+ *
+ * The above complication with having to track time and call into libusb at
+ * specific moments is a bit of a headache. For maximum compatibility, you do
+ * need to write your main loop as above, but you may decide that you can
+ * restrict the supported platforms of your application and get away with
+ * a more simplistic scheme.
+ *
+ * These time-based event complications are \b not required on the following
+ * platforms:
+ *  - Darwin
+ *  - Linux, provided that the following version requirements are satisfied:
+ *   - Linux v2.6.27 or newer, compiled with timerfd support
+ *   - glibc v2.9 or newer
+ *   - libusb v1.0.5 or newer
+ *
+ * Under these configurations, libusb_get_next_timeout() will \em always return
+ * 0, so your main loop can be simplified to:
+\code
+// initialise libusb
+
+libusb_get_pollfds(ctx)
+while (user has not requested application exit) {
+	poll(on libusb file descriptors plus any other event sources of interest,
+		using any timeout that you like)
+	if (poll() indicated activity on libusb file descriptors)
+		libusb_handle_events_timeout(ctx, 0);
+	// handle events from other sources here
+}
+
+// clean up and exit
+\endcode
+ *
+ * Do remember that if you simplify your main loop to the above, you will
+ * lose compatibility with some platforms (including legacy Linux platforms,
+ * and <em>any future platforms supported by libusb which may have time-based
+ * event requirements</em>). The resultant problems will likely appear as
+ * strange bugs in your application.
+ *
+ * You can use the libusb_pollfds_handle_timeouts() function to do a runtime
+ * check to see if it is safe to ignore the time-based event complications.
+ * If your application has taken the shortcut of ignoring libusb's next timeout
+ * in your main loop, then you are advised to check the return value of
+ * libusb_pollfds_handle_timeouts() during application startup, and to abort
+ * if the platform does suffer from these timing complications.
+ *
+ * \subsection fdsetchange Changes in the file descriptor set
+ *
+ * The set of file descriptors that libusb uses as event sources may change
+ * during the life of your application. Rather than having to repeatedly
+ * call libusb_get_pollfds(), you can set up notification functions for when
+ * the file descriptor set changes using libusb_set_pollfd_notifiers().
+ *
+ * \subsection mtissues Multi-threaded considerations
+ *
+ * Unfortunately, the situation is complicated further when multiple threads
+ * come into play. If two threads are monitoring the same file descriptors,
+ * the fact that only one thread will be woken up when an event occurs causes
+ * some headaches.
+ *
+ * The events lock, event waiters lock, and libusb_handle_events_locked()
+ * entities are added to solve these problems. You do not need to be concerned
+ * with these entities otherwise.
+ *
+ * See the extra documentation: \ref mtasync
+ */
+
+/** \page mtasync Multi-threaded applications and asynchronous I/O
+ *
+ * libusb is a thread-safe library, but extra considerations must be applied
+ * to applications which interact with libusb from multiple threads.
+ *
+ * The underlying issue that must be addressed is that all libusb I/O
+ * revolves around monitoring file descriptors through the poll()/select()
+ * system calls. This is directly exposed at the
+ * \ref asyncio "asynchronous interface" but it is important to note that the
+ * \ref syncio "synchronous interface" is implemented on top of the
+ * asynchonrous interface, therefore the same considerations apply.
+ *
+ * The issue is that if two or more threads are concurrently calling poll()
+ * or select() on libusb's file descriptors then only one of those threads
+ * will be woken up when an event arrives. The others will be completely
+ * oblivious that anything has happened.
+ *
+ * Consider the following pseudo-code, which submits an asynchronous transfer
+ * then waits for its completion. This style is one way you could implement a
+ * synchronous interface on top of the asynchronous interface (and libusb
+ * does something similar, albeit more advanced due to the complications
+ * explained on this page).
+ *
+\code
+void cb(struct libusb_transfer *transfer)
+{
+	int *completed = transfer->user_data;
+	*completed = 1;
+}
+
+void myfunc() {
+	struct libusb_transfer *transfer;
+	unsigned char buffer[LIBUSB_CONTROL_SETUP_SIZE];
+	int completed = 0;
+
+	transfer = libusb_alloc_transfer(0);
+	libusb_fill_control_setup(buffer,
+		LIBUSB_REQUEST_TYPE_VENDOR | LIBUSB_ENDPOINT_OUT, 0x04, 0x01, 0, 0);
+	libusb_fill_control_transfer(transfer, dev, buffer, cb, &completed, 1000);
+	libusb_submit_transfer(transfer);
+
+	while (!completed) {
+		poll(libusb file descriptors, 120*1000);
+		if (poll indicates activity)
+			libusb_handle_events_timeout(ctx, 0);
+	}
+	printf("completed!");
+	// other code here
+}
+\endcode
+ *
+ * Here we are <em>serializing</em> completion of an asynchronous event
+ * against a condition - the condition being completion of a specific transfer.
+ * The poll() loop has a long timeout to minimize CPU usage during situations
+ * when nothing is happening (it could reasonably be unlimited).
+ *
+ * If this is the only thread that is polling libusb's file descriptors, there
+ * is no problem: there is no danger that another thread will swallow up the
+ * event that we are interested in. On the other hand, if there is another
+ * thread polling the same descriptors, there is a chance that it will receive
+ * the event that we were interested in. In this situation, <tt>myfunc()</tt>
+ * will only realise that the transfer has completed on the next iteration of
+ * the loop, <em>up to 120 seconds later.</em> Clearly a two-minute delay is
+ * undesirable, and don't even think about using short timeouts to circumvent
+ * this issue!
+ *
+ * The solution here is to ensure that no two threads are ever polling the
+ * file descriptors at the same time. A naive implementation of this would
+ * impact the capabilities of the library, so libusb offers the scheme
+ * documented below to ensure no loss of functionality.
+ *
+ * Before we go any further, it is worth mentioning that all libusb-wrapped
+ * event handling procedures fully adhere to the scheme documented below.
+ * This includes libusb_handle_events() and all the synchronous I/O functions -
+ * libusb hides this headache from you. You do not need to worry about any
+ * of these issues if you stick to that level.
+ *
+ * The problem is when we consider the fact that libusb exposes file
+ * descriptors to allow for you to integrate asynchronous USB I/O into
+ * existing main loops, effectively allowing you to do some work behind
+ * libusb's back. If you do take libusb's file descriptors and pass them to
+ * poll()/select() yourself, you need to be aware of the associated issues.
+ *
+ * \section eventlock The events lock
+ *
+ * The first concept to be introduced is the events lock. The events lock
+ * is used to serialize threads that want to handle events, such that only
+ * one thread is handling events at any one time.
+ *
+ * You must take the events lock before polling libusb file descriptors,
+ * using libusb_lock_events(). You must release the lock as soon as you have
+ * aborted your poll()/select() loop, using libusb_unlock_events().
+ *
+ * \section threadwait Letting other threads do the work for you
+ *
+ * Although the events lock is a critical part of the solution, it is not
+ * enough on it's own. You might wonder if the following is sufficient...
+\code
+	libusb_lock_events(ctx);
+	while (!completed) {
+		poll(libusb file descriptors, 120*1000);
+		if (poll indicates activity)
+			libusb_handle_events_timeout(ctx, 0);
+	}
+	libusb_unlock_events(ctx);
+\endcode
+ * ...and the answer is that it is not. This is because the transfer in the
+ * code shown above may take a long time (say 30 seconds) to complete, and
+ * the lock is not released until the transfer is completed.
+ *
+ * Another thread with similar code that wants to do event handling may be
+ * working with a transfer that completes after a few milliseconds. Despite
+ * having such a quick completion time, the other thread cannot check that
+ * status of its transfer until the code above has finished (30 seconds later)
+ * due to contention on the lock.
+ *
+ * To solve this, libusb offers you a mechanism to determine when another
+ * thread is handling events. It also offers a mechanism to block your thread
+ * until the event handling thread has completed an event (and this mechanism
+ * does not involve polling of file descriptors).
+ *
+ * After determining that another thread is currently handling events, you
+ * obtain the <em>event waiters</em> lock using libusb_lock_event_waiters().
+ * You then re-check that some other thread is still handling events, and if
+ * so, you call libusb_wait_for_event().
+ *
+ * libusb_wait_for_event() puts your application to sleep until an event
+ * occurs, or until a thread releases the events lock. When either of these
+ * things happen, your thread is woken up, and should re-check the condition
+ * it was waiting on. It should also re-check that another thread is handling
+ * events, and if not, it should start handling events itself.
+ *
+ * This looks like the following, as pseudo-code:
+\code
+retry:
+if (libusb_try_lock_events(ctx) == 0) {
+	// we obtained the event lock: do our own event handling
+	while (!completed) {
+		if (!libusb_event_handling_ok(ctx)) {
+			libusb_unlock_events(ctx);
+			goto retry;
+		}
+		poll(libusb file descriptors, 120*1000);
+		if (poll indicates activity)
+			libusb_handle_events_locked(ctx, 0);
+	}
+	libusb_unlock_events(ctx);
+} else {
+	// another thread is doing event handling. wait for it to signal us that
+	// an event has completed
+	libusb_lock_event_waiters(ctx);
+
+	while (!completed) {
+		// now that we have the event waiters lock, double check that another
+		// thread is still handling events for us. (it may have ceased handling
+		// events in the time it took us to reach this point)
+		if (!libusb_event_handler_active(ctx)) {
+			// whoever was handling events is no longer doing so, try again
+			libusb_unlock_event_waiters(ctx);
+			goto retry;
+		}
+
+		libusb_wait_for_event(ctx, NULL);
+	}
+	libusb_unlock_event_waiters(ctx);
+}
+printf("completed!\n");
+\endcode
+ *
+ * A naive look at the above code may suggest that this can only support
+ * one event waiter (hence a total of 2 competing threads, the other doing
+ * event handling), because the event waiter seems to have taken the event
+ * waiters lock while waiting for an event. However, the system does support
+ * multiple event waiters, because libusb_wait_for_event() actually drops
+ * the lock while waiting, and reaquires it before continuing.
+ *
+ * We have now implemented code which can dynamically handle situations where
+ * nobody is handling events (so we should do it ourselves), and it can also
+ * handle situations where another thread is doing event handling (so we can
+ * piggyback onto them). It is also equipped to handle a combination of
+ * the two, for example, another thread is doing event handling, but for
+ * whatever reason it stops doing so before our condition is met, so we take
+ * over the event handling.
+ *
+ * Four functions were introduced in the above pseudo-code. Their importance
+ * should be apparent from the code shown above.
+ * -# libusb_try_lock_events() is a non-blocking function which attempts
+ *    to acquire the events lock but returns a failure code if it is contended.
+ * -# libusb_event_handling_ok() checks that libusb is still happy for your
+ *    thread to be performing event handling. Sometimes, libusb needs to
+ *    interrupt the event handler, and this is how you can check if you have
+ *    been interrupted. If this function returns 0, the correct behaviour is
+ *    for you to give up the event handling lock, and then to repeat the cycle.
+ *    The following libusb_try_lock_events() will fail, so you will become an
+ *    events waiter. For more information on this, read \ref fullstory below.
+ * -# libusb_handle_events_locked() is a variant of
+ *    libusb_handle_events_timeout() that you can call while holding the
+ *    events lock. libusb_handle_events_timeout() itself implements similar
+ *    logic to the above, so be sure not to call it when you are
+ *    "working behind libusb's back", as is the case here.
+ * -# libusb_event_handler_active() determines if someone is currently
+ *    holding the events lock
+ *
+ * You might be wondering why there is no function to wake up all threads
+ * blocked on libusb_wait_for_event(). This is because libusb can do this
+ * internally: it will wake up all such threads when someone calls
+ * libusb_unlock_events() or when a transfer completes (at the point after its
+ * callback has returned).
+ *
+ * \subsection fullstory The full story
+ *
+ * The above explanation should be enough to get you going, but if you're
+ * really thinking through the issues then you may be left with some more
+ * questions regarding libusb's internals. If you're curious, read on, and if
+ * not, skip to the next section to avoid confusing yourself!
+ *
+ * The immediate question that may spring to mind is: what if one thread
+ * modifies the set of file descriptors that need to be polled while another
+ * thread is doing event handling?
+ *
+ * There are 2 situations in which this may happen.
+ * -# libusb_open() will add another file descriptor to the poll set,
+ *    therefore it is desirable to interrupt the event handler so that it
+ *    restarts, picking up the new descriptor.
+ * -# libusb_close() will remove a file descriptor from the poll set. There
+ *    are all kinds of race conditions that could arise here, so it is
+ *    important that nobody is doing event handling at this time.
+ *
+ * libusb handles these issues internally, so application developers do not
+ * have to stop their event handlers while opening/closing devices. Here's how
+ * it works, focusing on the libusb_close() situation first:
+ *
+ * -# During initialization, libusb opens an internal pipe, and it adds the read
+ *    end of this pipe to the set of file descriptors to be polled.
+ * -# During libusb_close(), libusb writes some dummy data on this control pipe.
+ *    This immediately interrupts the event handler. libusb also records
+ *    internally that it is trying to interrupt event handlers for this
+ *    high-priority event.
+ * -# At this point, some of the functions described above start behaving
+ *    differently:
+ *   - libusb_event_handling_ok() starts returning 1, indicating that it is NOT
+ *     OK for event handling to continue.
+ *   - libusb_try_lock_events() starts returning 1, indicating that another
+ *     thread holds the event handling lock, even if the lock is uncontended.
+ *   - libusb_event_handler_active() starts returning 1, indicating that
+ *     another thread is doing event handling, even if that is not true.
+ * -# The above changes in behaviour result in the event handler stopping and
+ *    giving up the events lock very quickly, giving the high-priority
+ *    libusb_close() operation a "free ride" to acquire the events lock. All
+ *    threads that are competing to do event handling become event waiters.
+ * -# With the events lock held inside libusb_close(), libusb can safely remove
+ *    a file descriptor from the poll set, in the safety of knowledge that
+ *    nobody is polling those descriptors or trying to access the poll set.
+ * -# After obtaining the events lock, the close operation completes very
+ *    quickly (usually a matter of milliseconds) and then immediately releases
+ *    the events lock.
+ * -# At the same time, the behaviour of libusb_event_handling_ok() and friends
+ *    reverts to the original, documented behaviour.
+ * -# The release of the events lock causes the threads that are waiting for
+ *    events to be woken up and to start competing to become event handlers
+ *    again. One of them will succeed; it will then re-obtain the list of poll
+ *    descriptors, and USB I/O will then continue as normal.
+ *
+ * libusb_open() is similar, and is actually a more simplistic case. Upon a
+ * call to libusb_open():
+ *
+ * -# The device is opened and a file descriptor is added to the poll set.
+ * -# libusb sends some dummy data on the control pipe, and records that it
+ *    is trying to modify the poll descriptor set.
+ * -# The event handler is interrupted, and the same behaviour change as for
+ *    libusb_close() takes effect, causing all event handling threads to become
+ *    event waiters.
+ * -# The libusb_open() implementation takes its free ride to the events lock.
+ * -# Happy that it has successfully paused the events handler, libusb_open()
+ *    releases the events lock.
+ * -# The event waiter threads are all woken up and compete to become event
+ *    handlers again. The one that succeeds will obtain the list of poll
+ *    descriptors again, which will include the addition of the new device.
+ *
+ * \subsection concl Closing remarks
+ *
+ * The above may seem a little complicated, but hopefully I have made it clear
+ * why such complications are necessary. Also, do not forget that this only
+ * applies to applications that take libusb's file descriptors and integrate
+ * them into their own polling loops.
+ *
+ * You may decide that it is OK for your multi-threaded application to ignore
+ * some of the rules and locks detailed above, because you don't think that
+ * two threads can ever be polling the descriptors at the same time. If that
+ * is the case, then that's good news for you because you don't have to worry.
+ * But be careful here; remember that the synchronous I/O functions do event
+ * handling internally. If you have one thread doing event handling in a loop
+ * (without implementing the rules and locking semantics documented above)
+ * and another trying to send a synchronous USB transfer, you will end up with
+ * two threads monitoring the same descriptors, and the above-described
+ * undesirable behaviour occuring. The solution is for your polling thread to
+ * play by the rules; the synchronous I/O functions do so, and this will result
+ * in them getting along in perfect harmony.
+ *
+ * If you do have a dedicated thread doing event handling, it is perfectly
+ * legal for it to take the event handling lock for long periods of time. Any
+ * synchronous I/O functions you call from other threads will transparently
+ * fall back to the "event waiters" mechanism detailed above. The only
+ * consideration that your event handling thread must apply is the one related
+ * to libusb_event_handling_ok(): you must call this before every poll(), and
+ * give up the events lock if instructed.
+ */
+
+int usbi_io_init(struct libusb_context *ctx)
+{
+	int r;
+
+	usbi_mutex_init(&ctx->flying_transfers_lock, NULL);
+	usbi_mutex_init(&ctx->pollfds_lock, NULL);
+	usbi_mutex_init(&ctx->pollfd_modify_lock, NULL);
+	usbi_mutex_init(&ctx->events_lock, NULL);
+	usbi_mutex_init(&ctx->event_waiters_lock, NULL);
+	usbi_cond_init(&ctx->event_waiters_cond, NULL);
+	list_init(&ctx->flying_transfers);
+	list_init(&ctx->pollfds);
+
+	/* FIXME should use an eventfd on kernels that support it */
+	r = usbi_pipe(ctx->ctrl_pipe);
+	if (r < 0) {
+		r = LIBUSB_ERROR_OTHER;
+		goto err;
+	}
+
+	r = usbi_add_pollfd(ctx, ctx->ctrl_pipe[0], POLLIN);
+	if (r < 0)
+		goto err_close_pipe;
+
+#ifdef USBI_TIMERFD_AVAILABLE
+	ctx->timerfd = timerfd_create(usbi_backend->get_timerfd_clockid(),
+		TFD_NONBLOCK);
+	if (ctx->timerfd >= 0) {
+		usbi_dbg("using timerfd for timeouts");
+		r = usbi_add_pollfd(ctx, ctx->timerfd, POLLIN);
+		if (r < 0) {
+			usbi_remove_pollfd(ctx, ctx->ctrl_pipe[0]);
+			close(ctx->timerfd);
+			goto err_close_pipe;
+		}
+	} else {
+		usbi_dbg("timerfd not available (code %d error %d)", ctx->timerfd, errno);
+		ctx->timerfd = -1;
+	}
+#endif
+
+	return 0;
+
+err_close_pipe:
+	usbi_close(ctx->ctrl_pipe[0]);
+	usbi_close(ctx->ctrl_pipe[1]);
+err:
+	usbi_mutex_destroy(&ctx->flying_transfers_lock);
+	usbi_mutex_destroy(&ctx->pollfds_lock);
+	usbi_mutex_destroy(&ctx->pollfd_modify_lock);
+	usbi_mutex_destroy(&ctx->events_lock);
+	usbi_mutex_destroy(&ctx->event_waiters_lock);
+	usbi_cond_destroy(&ctx->event_waiters_cond);
+	return r;
+}
+
+void usbi_io_exit(struct libusb_context *ctx)
+{
+	usbi_remove_pollfd(ctx, ctx->ctrl_pipe[0]);
+	usbi_close(ctx->ctrl_pipe[0]);
+	usbi_close(ctx->ctrl_pipe[1]);
+#ifdef USBI_TIMERFD_AVAILABLE
+	if (usbi_using_timerfd(ctx)) {
+		usbi_remove_pollfd(ctx, ctx->timerfd);
+		close(ctx->timerfd);
+	}
+#endif
+	usbi_mutex_destroy(&ctx->flying_transfers_lock);
+	usbi_mutex_destroy(&ctx->pollfds_lock);
+	usbi_mutex_destroy(&ctx->pollfd_modify_lock);
+	usbi_mutex_destroy(&ctx->events_lock);
+	usbi_mutex_destroy(&ctx->event_waiters_lock);
+	usbi_cond_destroy(&ctx->event_waiters_cond);
+}
+
+static int calculate_timeout(struct usbi_transfer *transfer)
+{
+	int r;
+	struct timespec current_time;
+	unsigned int timeout =
+		__USBI_TRANSFER_TO_LIBUSB_TRANSFER(transfer)->timeout;
+
+	if (!timeout)
+		return 0;
+
+	r = usbi_backend->clock_gettime(USBI_CLOCK_MONOTONIC, &current_time);
+	if (r < 0) {
+		usbi_err(ITRANSFER_CTX(transfer),
+			"failed to read monotonic clock, errno=%d", errno);
+		return r;
+	}
+
+	current_time.tv_sec += timeout / 1000;
+	current_time.tv_nsec += (timeout % 1000) * 1000000;
+
+	if (current_time.tv_nsec > 1000000000) {
+		current_time.tv_nsec -= 1000000000;
+		current_time.tv_sec++;
+	}
+
+	TIMESPEC_TO_TIMEVAL(&transfer->timeout, &current_time);
+	return 0;
+}
+
+/* add a transfer to the (timeout-sorted) active transfers list.
+ * returns 1 if the transfer has a timeout and it is the timeout next to
+ * expire */
+static int add_to_flying_list(struct usbi_transfer *transfer)
+{
+	struct usbi_transfer *cur;
+	struct timeval *timeout = &transfer->timeout;
+	struct libusb_context *ctx = ITRANSFER_CTX(transfer);
+	int r = 0;
+	int first = 1;
+
+	usbi_mutex_lock(&ctx->flying_transfers_lock);
+
+	/* if we have no other flying transfers, start the list with this one */
+	if (list_empty(&ctx->flying_transfers)) {
+		list_add(&transfer->list, &ctx->flying_transfers);
+		if (timerisset(timeout))
+			r = 1;
+		goto out;
+	}
+
+	/* if we have infinite timeout, append to end of list */
+	if (!timerisset(timeout)) {
+		list_add_tail(&transfer->list, &ctx->flying_transfers);
+		goto out;
+	}
+
+	/* otherwise, find appropriate place in list */
+	list_for_each_entry(cur, &ctx->flying_transfers, list, struct usbi_transfer) {
+		/* find first timeout that occurs after the transfer in question */
+		struct timeval *cur_tv = &cur->timeout;
+
+		if (!timerisset(cur_tv) || (cur_tv->tv_sec > timeout->tv_sec) ||
+				(cur_tv->tv_sec == timeout->tv_sec &&
+					cur_tv->tv_usec > timeout->tv_usec)) {
+			list_add_tail(&transfer->list, &cur->list);
+			r = first;
+			goto out;
+		}
+		first = 0;
+	}
+
+	/* otherwise we need to be inserted at the end */
+	list_add_tail(&transfer->list, &ctx->flying_transfers);
+out:
+	usbi_mutex_unlock(&ctx->flying_transfers_lock);
+	return r;
+}
+
+/** \ingroup asyncio
+ * Allocate a libusb transfer with a specified number of isochronous packet
+ * descriptors. The returned transfer is pre-initialized for you. When the new
+ * transfer is no longer needed, it should be freed with
+ * libusb_free_transfer().
+ *
+ * Transfers intended for non-isochronous endpoints (e.g. control, bulk,
+ * interrupt) should specify an iso_packets count of zero.
+ *
+ * For transfers intended for isochronous endpoints, specify an appropriate
+ * number of packet descriptors to be allocated as part of the transfer.
+ * The returned transfer is not specially initialized for isochronous I/O;
+ * you are still required to set the
+ * \ref libusb_transfer::num_iso_packets "num_iso_packets" and
+ * \ref libusb_transfer::type "type" fields accordingly.
+ *
+ * It is safe to allocate a transfer with some isochronous packets and then
+ * use it on a non-isochronous endpoint. If you do this, ensure that at time
+ * of submission, num_iso_packets is 0 and that type is set appropriately.
+ *
+ * \param iso_packets number of isochronous packet descriptors to allocate
+ * \returns a newly allocated transfer, or NULL on error
+ */
+DEFAULT_VISIBILITY
+struct libusb_transfer * LIBUSB_CALL libusb_alloc_transfer(
+	int iso_packets)
+{
+	size_t os_alloc_size = usbi_backend->transfer_priv_size
+		+ (usbi_backend->add_iso_packet_size * iso_packets);
+	size_t alloc_size = sizeof(struct usbi_transfer)
+		+ sizeof(struct libusb_transfer)
+		+ (sizeof(struct libusb_iso_packet_descriptor) * iso_packets)
+		+ os_alloc_size;
+	struct usbi_transfer *itransfer = malloc(alloc_size);
+	if (!itransfer)
+		return NULL;
+
+	memset(itransfer, 0, alloc_size);
+	itransfer->num_iso_packets = iso_packets;
+	usbi_mutex_init(&itransfer->lock, NULL);
+	return __USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+}
+
+/** \ingroup asyncio
+ * Free a transfer structure. This should be called for all transfers
+ * allocated with libusb_alloc_transfer().
+ *
+ * If the \ref libusb_transfer_flags::LIBUSB_TRANSFER_FREE_BUFFER
+ * "LIBUSB_TRANSFER_FREE_BUFFER" flag is set and the transfer buffer is
+ * non-NULL, this function will also free the transfer buffer using the
+ * standard system memory allocator (e.g. free()).
+ *
+ * It is legal to call this function with a NULL transfer. In this case,
+ * the function will simply return safely.
+ *
+ * It is not legal to free an active transfer (one which has been submitted
+ * and has not yet completed).
+ *
+ * \param transfer the transfer to free
+ */
+void API_EXPORTED libusb_free_transfer(struct libusb_transfer *transfer)
+{
+	struct usbi_transfer *itransfer;
+	if (!transfer)
+		return;
+
+	if (transfer->flags & LIBUSB_TRANSFER_FREE_BUFFER && transfer->buffer)
+		free(transfer->buffer);
+
+	itransfer = __LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);
+	usbi_mutex_destroy(&itransfer->lock);
+	free(itransfer);
+}
+
+/** \ingroup asyncio
+ * Submit a transfer. This function will fire off the USB transfer and then
+ * return immediately.
+ *
+ * \param transfer the transfer to submit
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns LIBUSB_ERROR_BUSY if the transfer has already been submitted.
+ * \returns another LIBUSB_ERROR code on other failure
+ */
+int API_EXPORTED libusb_submit_transfer(struct libusb_transfer *transfer)
+{
+	struct libusb_context *ctx = TRANSFER_CTX(transfer);
+	struct usbi_transfer *itransfer =
+		__LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);
+	int r;
+	int first;
+
+	usbi_mutex_lock(&itransfer->lock);
+	itransfer->transferred = 0;
+	itransfer->flags = 0;
+	r = calculate_timeout(itransfer);
+	if (r < 0) {
+		r = LIBUSB_ERROR_OTHER;
+		goto out;
+	}
+
+	first = add_to_flying_list(itransfer);
+	r = usbi_backend->submit_transfer(itransfer);
+	if (r) {
+		usbi_mutex_lock(&ctx->flying_transfers_lock);
+		list_del(&itransfer->list);
+		usbi_mutex_unlock(&ctx->flying_transfers_lock);
+	}
+#ifdef USBI_TIMERFD_AVAILABLE
+	else if (first && usbi_using_timerfd(ctx)) {
+		/* if this transfer has the lowest timeout of all active transfers,
+		 * rearm the timerfd with this transfer's timeout */
+		const struct itimerspec it = { {0, 0},
+			{ itransfer->timeout.tv_sec, itransfer->timeout.tv_usec * 1000 } };
+		usbi_dbg("arm timerfd for timeout in %dms (first in line)", transfer->timeout);
+		r = timerfd_settime(ctx->timerfd, TFD_TIMER_ABSTIME, &it, NULL);
+		if (r < 0)
+			r = LIBUSB_ERROR_OTHER;
+	}
+#endif
+
+out:
+	usbi_mutex_unlock(&itransfer->lock);
+	return r;
+}
+
+/** \ingroup asyncio
+ * Asynchronously cancel a previously submitted transfer.
+ * This function returns immediately, but this does not indicate cancellation
+ * is complete. Your callback function will be invoked at some later time
+ * with a transfer status of
+ * \ref libusb_transfer_status::LIBUSB_TRANSFER_CANCELLED
+ * "LIBUSB_TRANSFER_CANCELLED."
+ *
+ * \param transfer the transfer to cancel
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if the transfer is already complete or
+ * cancelled.
+ * \returns a LIBUSB_ERROR code on failure
+ */
+int API_EXPORTED libusb_cancel_transfer(struct libusb_transfer *transfer)
+{
+	struct usbi_transfer *itransfer =
+		__LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);
+	int r;
+
+	usbi_dbg("");
+	usbi_mutex_lock(&itransfer->lock);
+	r = usbi_backend->cancel_transfer(itransfer);
+	if (r < 0)
+		usbi_err(TRANSFER_CTX(transfer),
+			"cancel transfer failed error %d", r);
+	usbi_mutex_unlock(&itransfer->lock);
+	return r;
+}
+
+#ifdef USBI_TIMERFD_AVAILABLE
+static int disarm_timerfd(struct libusb_context *ctx)
+{
+	const struct itimerspec disarm_timer = { { 0, 0 }, { 0, 0 } };
+	int r;
+
+	usbi_dbg("");
+	r = timerfd_settime(ctx->timerfd, 0, &disarm_timer, NULL);
+	if (r < 0)
+		return LIBUSB_ERROR_OTHER;
+	else
+		return 0;
+}
+
+/* iterates through the flying transfers, and rearms the timerfd based on the
+ * next upcoming timeout.
+ * must be called with flying_list locked.
+ * returns 0 if there was no timeout to arm, 1 if the next timeout was armed,
+ * or a LIBUSB_ERROR code on failure.
+ */
+static int arm_timerfd_for_next_timeout(struct libusb_context *ctx)
+{
+	struct usbi_transfer *transfer;
+
+	list_for_each_entry(transfer, &ctx->flying_transfers, list, struct usbi_transfer) {
+		struct timeval *cur_tv = &transfer->timeout;
+
+		/* if we've reached transfers of infinite timeout, then we have no
+		 * arming to do */
+		if (!timerisset(cur_tv))
+			return 0;
+
+		/* act on first transfer that is not already cancelled */
+		if (!(transfer->flags & USBI_TRANSFER_TIMED_OUT)) {
+			int r;
+			const struct itimerspec it = { {0, 0},
+				{ cur_tv->tv_sec, cur_tv->tv_usec * 1000 } };
+			usbi_dbg("next timeout originally %dms", __USBI_TRANSFER_TO_LIBUSB_TRANSFER(transfer)->timeout);
+			r = timerfd_settime(ctx->timerfd, TFD_TIMER_ABSTIME, &it, NULL);
+			if (r < 0)
+				return LIBUSB_ERROR_OTHER;
+			return 1;
+		}
+	}
+
+	return 0;
+}
+#else
+static int disarm_timerfd(struct libusb_context *ctx)
+{
+	return 0;
+}
+static int arm_timerfd_for_next_timeout(struct libusb_context *ctx)
+{
+	return 0;
+}
+#endif
+
+/* Handle completion of a transfer (completion might be an error condition).
+ * This will invoke the user-supplied callback function, which may end up
+ * freeing the transfer. Therefore you cannot use the transfer structure
+ * after calling this function, and you should free all backend-specific
+ * data before calling it.
+ * Do not call this function with the usbi_transfer lock held. User-specified
+ * callback functions may attempt to directly resubmit the transfer, which
+ * will attempt to take the lock. */
+int usbi_handle_transfer_completion(struct usbi_transfer *itransfer,
+	enum libusb_transfer_status status)
+{
+	struct libusb_transfer *transfer =
+		__USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	struct libusb_context *ctx = TRANSFER_CTX(transfer);
+	uint8_t flags;
+	int r;
+
+	/* FIXME: could be more intelligent with the timerfd here. we don't need
+	 * to disarm the timerfd if there was no timer running, and we only need
+	 * to rearm the timerfd if the transfer that expired was the one with
+	 * the shortest timeout. */
+
+	usbi_mutex_lock(&ctx->flying_transfers_lock);
+	list_del(&itransfer->list);
+	r = arm_timerfd_for_next_timeout(ctx);
+	usbi_mutex_unlock(&ctx->flying_transfers_lock);
+
+	if (r < 0) {
+		return r;
+	} else if (r == 0) {
+		r = disarm_timerfd(ctx);
+		if (r < 0)
+			return r;
+	}
+
+	if (status == LIBUSB_TRANSFER_COMPLETED
+			&& transfer->flags & LIBUSB_TRANSFER_SHORT_NOT_OK) {
+		int rqlen = transfer->length;
+		if (transfer->type == LIBUSB_TRANSFER_TYPE_CONTROL)
+			rqlen -= LIBUSB_CONTROL_SETUP_SIZE;
+		if (rqlen != itransfer->transferred) {
+			usbi_dbg("interpreting short transfer as error");
+			status = LIBUSB_TRANSFER_ERROR;
+		}
+	}
+
+	flags = transfer->flags;
+	transfer->status = status;
+	transfer->actual_length = itransfer->transferred;
+	if (transfer->callback)
+		transfer->callback(transfer);
+	/* transfer might have been freed by the above call, do not use from
+	 * this point. */
+	if (flags & LIBUSB_TRANSFER_FREE_TRANSFER)
+		libusb_free_transfer(transfer);
+	usbi_mutex_lock(&ctx->event_waiters_lock);
+	usbi_cond_broadcast(&ctx->event_waiters_cond);
+	usbi_mutex_unlock(&ctx->event_waiters_lock);
+	return 0;
+}
+
+/* Similar to usbi_handle_transfer_completion() but exclusively for transfers
+ * that were asynchronously cancelled. The same concerns w.r.t. freeing of
+ * transfers exist here.
+ * Do not call this function with the usbi_transfer lock held. User-specified
+ * callback functions may attempt to directly resubmit the transfer, which
+ * will attempt to take the lock. */
+int usbi_handle_transfer_cancellation(struct usbi_transfer *transfer)
+{
+	/* if the URB was cancelled due to timeout, report timeout to the user */
+	if (transfer->flags & USBI_TRANSFER_TIMED_OUT) {
+		usbi_dbg("detected timeout cancellation");
+		return usbi_handle_transfer_completion(transfer, LIBUSB_TRANSFER_TIMED_OUT);
+	}
+
+	/* otherwise its a normal async cancel */
+	return usbi_handle_transfer_completion(transfer, LIBUSB_TRANSFER_CANCELLED);
+}
+
+/** \ingroup poll
+ * Attempt to acquire the event handling lock. This lock is used to ensure that
+ * only one thread is monitoring libusb event sources at any one time.
+ *
+ * You only need to use this lock if you are developing an application
+ * which calls poll() or select() on libusb's file descriptors directly.
+ * If you stick to libusb's event handling loop functions (e.g.
+ * libusb_handle_events()) then you do not need to be concerned with this
+ * locking.
+ *
+ * While holding this lock, you are trusted to actually be handling events.
+ * If you are no longer handling events, you must call libusb_unlock_events()
+ * as soon as possible.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \returns 0 if the lock was obtained successfully
+ * \returns 1 if the lock was not obtained (i.e. another thread holds the lock)
+ * \see \ref mtasync
+ */
+int API_EXPORTED libusb_try_lock_events(libusb_context *ctx)
+{
+	int r;
+	USBI_GET_CONTEXT(ctx);
+
+	/* is someone else waiting to modify poll fds? if so, don't let this thread
+	 * start event handling */
+	usbi_mutex_lock(&ctx->pollfd_modify_lock);
+	r = ctx->pollfd_modify;
+	usbi_mutex_unlock(&ctx->pollfd_modify_lock);
+	if (r) {
+		usbi_dbg("someone else is modifying poll fds");
+		return 1;
+	}
+
+	r = usbi_mutex_trylock(&ctx->events_lock);
+	if (r)
+		return 1;
+
+	ctx->event_handler_active = 1;
+	return 0;
+}
+
+/** \ingroup poll
+ * Acquire the event handling lock, blocking until successful acquisition if
+ * it is contended. This lock is used to ensure that only one thread is
+ * monitoring libusb event sources at any one time.
+ *
+ * You only need to use this lock if you are developing an application
+ * which calls poll() or select() on libusb's file descriptors directly.
+ * If you stick to libusb's event handling loop functions (e.g.
+ * libusb_handle_events()) then you do not need to be concerned with this
+ * locking.
+ *
+ * While holding this lock, you are trusted to actually be handling events.
+ * If you are no longer handling events, you must call libusb_unlock_events()
+ * as soon as possible.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \see \ref mtasync
+ */
+void API_EXPORTED libusb_lock_events(libusb_context *ctx)
+{
+	USBI_GET_CONTEXT(ctx);
+	usbi_mutex_lock(&ctx->events_lock);
+	ctx->event_handler_active = 1;
+}
+
+/** \ingroup poll
+ * Release the lock previously acquired with libusb_try_lock_events() or
+ * libusb_lock_events(). Releasing this lock will wake up any threads blocked
+ * on libusb_wait_for_event().
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \see \ref mtasync
+ */
+void API_EXPORTED libusb_unlock_events(libusb_context *ctx)
+{
+	USBI_GET_CONTEXT(ctx);
+	ctx->event_handler_active = 0;
+	usbi_mutex_unlock(&ctx->events_lock);
+
+	/* FIXME: perhaps we should be a bit more efficient by not broadcasting
+	 * the availability of the events lock when we are modifying pollfds
+	 * (check ctx->pollfd_modify)? */
+	usbi_mutex_lock(&ctx->event_waiters_lock);
+	usbi_cond_broadcast(&ctx->event_waiters_cond);
+	usbi_mutex_unlock(&ctx->event_waiters_lock);
+}
+
+/** \ingroup poll
+ * Determine if it is still OK for this thread to be doing event handling.
+ *
+ * Sometimes, libusb needs to temporarily pause all event handlers, and this
+ * is the function you should use before polling file descriptors to see if
+ * this is the case.
+ *
+ * If this function instructs your thread to give up the events lock, you
+ * should just continue the usual logic that is documented in \ref mtasync.
+ * On the next iteration, your thread will fail to obtain the events lock,
+ * and will hence become an event waiter.
+ *
+ * This function should be called while the events lock is held: you don't
+ * need to worry about the results of this function if your thread is not
+ * the current event handler.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \returns 1 if event handling can start or continue
+ * \returns 0 if this thread must give up the events lock
+ * \see \ref fullstory "Multi-threaded I/O: the full story"
+ */
+int API_EXPORTED libusb_event_handling_ok(libusb_context *ctx)
+{
+	int r;
+	USBI_GET_CONTEXT(ctx);
+
+	/* is someone else waiting to modify poll fds? if so, don't let this thread
+	 * continue event handling */
+	usbi_mutex_lock(&ctx->pollfd_modify_lock);
+	r = ctx->pollfd_modify;
+	usbi_mutex_unlock(&ctx->pollfd_modify_lock);
+	if (r) {
+		usbi_dbg("someone else is modifying poll fds");
+		return 0;
+	}
+
+	return 1;
+}
+
+
+/** \ingroup poll
+ * Determine if an active thread is handling events (i.e. if anyone is holding
+ * the event handling lock).
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \returns 1 if a thread is handling events
+ * \returns 0 if there are no threads currently handling events
+ * \see \ref mtasync
+ */
+int API_EXPORTED libusb_event_handler_active(libusb_context *ctx)
+{
+	int r;
+	USBI_GET_CONTEXT(ctx);
+
+	/* is someone else waiting to modify poll fds? if so, don't let this thread
+	 * start event handling -- indicate that event handling is happening */
+	usbi_mutex_lock(&ctx->pollfd_modify_lock);
+	r = ctx->pollfd_modify;
+	usbi_mutex_unlock(&ctx->pollfd_modify_lock);
+	if (r) {
+		usbi_dbg("someone else is modifying poll fds");
+		return 1;
+	}
+
+	return ctx->event_handler_active;
+}
+
+/** \ingroup poll
+ * Acquire the event waiters lock. This lock is designed to be obtained under
+ * the situation where you want to be aware when events are completed, but
+ * some other thread is event handling so calling libusb_handle_events() is not
+ * allowed.
+ *
+ * You then obtain this lock, re-check that another thread is still handling
+ * events, then call libusb_wait_for_event().
+ *
+ * You only need to use this lock if you are developing an application
+ * which calls poll() or select() on libusb's file descriptors directly,
+ * <b>and</b> may potentially be handling events from 2 threads simultaenously.
+ * If you stick to libusb's event handling loop functions (e.g.
+ * libusb_handle_events()) then you do not need to be concerned with this
+ * locking.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \see \ref mtasync
+ */
+void API_EXPORTED libusb_lock_event_waiters(libusb_context *ctx)
+{
+	USBI_GET_CONTEXT(ctx);
+	usbi_mutex_lock(&ctx->event_waiters_lock);
+}
+
+/** \ingroup poll
+ * Release the event waiters lock.
+ * \param ctx the context to operate on, or NULL for the default context
+ * \see \ref mtasync
+ */
+void API_EXPORTED libusb_unlock_event_waiters(libusb_context *ctx)
+{
+	USBI_GET_CONTEXT(ctx);
+	usbi_mutex_unlock(&ctx->event_waiters_lock);
+}
+
+/** \ingroup poll
+ * Wait for another thread to signal completion of an event. Must be called
+ * with the event waiters lock held, see libusb_lock_event_waiters().
+ *
+ * This function will block until any of the following conditions are met:
+ * -# The timeout expires
+ * -# A transfer completes
+ * -# A thread releases the event handling lock through libusb_unlock_events()
+ *
+ * Condition 1 is obvious. Condition 2 unblocks your thread <em>after</em>
+ * the callback for the transfer has completed. Condition 3 is important
+ * because it means that the thread that was previously handling events is no
+ * longer doing so, so if any events are to complete, another thread needs to
+ * step up and start event handling.
+ *
+ * This function releases the event waiters lock before putting your thread
+ * to sleep, and reacquires the lock as it is being woken up.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param tv maximum timeout for this blocking function. A NULL value
+ * indicates unlimited timeout.
+ * \returns 0 after a transfer completes or another thread stops event handling
+ * \returns 1 if the timeout expired
+ * \see \ref mtasync
+ */
+int API_EXPORTED libusb_wait_for_event(libusb_context *ctx, struct timeval *tv)
+{
+	struct timespec timeout;
+	int r;
+
+	USBI_GET_CONTEXT(ctx);
+	if (tv == NULL) {
+		usbi_cond_wait(&ctx->event_waiters_cond, &ctx->event_waiters_lock);
+		return 0;
+	}
+
+	r = usbi_backend->clock_gettime(USBI_CLOCK_REALTIME, &timeout);
+	if (r < 0) {
+		usbi_err(ctx, "failed to read realtime clock, error %d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	timeout.tv_sec += tv->tv_sec;
+	timeout.tv_nsec += tv->tv_usec * 1000;
+	if (timeout.tv_nsec > 1000000000) {
+		timeout.tv_nsec -= 1000000000;
+		timeout.tv_sec++;
+	}
+
+	r = usbi_cond_timedwait(&ctx->event_waiters_cond,
+		&ctx->event_waiters_lock, &timeout);
+	return (r == ETIMEDOUT);
+}
+
+static void handle_timeout(struct usbi_transfer *itransfer)
+{
+	struct libusb_transfer *transfer =
+		__USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	int r;
+
+	itransfer->flags |= USBI_TRANSFER_TIMED_OUT;
+	r = libusb_cancel_transfer(transfer);
+	if (r < 0)
+		usbi_warn(TRANSFER_CTX(transfer),
+			"async cancel failed %d errno=%d", r, errno);
+}
+
+#ifdef USBI_OS_HANDLES_TIMEOUT
+static int handle_timeouts_locked(struct libusb_context *ctx)
+{
+	return 0;
+}
+static int handle_timeouts(struct libusb_context *ctx)
+{
+	return 0;
+}
+#else
+static int handle_timeouts_locked(struct libusb_context *ctx)
+{
+	int r;
+	struct timespec systime_ts;
+	struct timeval systime;
+	struct usbi_transfer *transfer;
+
+	if (list_empty(&ctx->flying_transfers))
+		return 0;
+
+	/* get current time */
+	r = usbi_backend->clock_gettime(USBI_CLOCK_MONOTONIC, &systime_ts);
+	if (r < 0)
+		return r;
+
+	TIMESPEC_TO_TIMEVAL(&systime, &systime_ts);
+
+	/* iterate through flying transfers list, finding all transfers that
+	 * have expired timeouts */
+	list_for_each_entry(transfer, &ctx->flying_transfers, list, struct usbi_transfer) {
+		struct timeval *cur_tv = &transfer->timeout;
+
+		/* if we've reached transfers of infinite timeout, we're all done */
+		if (!timerisset(cur_tv))
+			return 0;
+
+		/* ignore timeouts we've already handled */
+		if (transfer->flags & USBI_TRANSFER_TIMED_OUT)
+			continue;
+
+		/* if transfer has non-expired timeout, nothing more to do */
+		if ((cur_tv->tv_sec > systime.tv_sec) ||
+				(cur_tv->tv_sec == systime.tv_sec &&
+					cur_tv->tv_usec > systime.tv_usec))
+			return 0;
+
+		/* otherwise, we've got an expired timeout to handle */
+		handle_timeout(transfer);
+	}
+	return 0;
+}
+
+static int handle_timeouts(struct libusb_context *ctx)
+{
+	int r;
+	USBI_GET_CONTEXT(ctx);
+	usbi_mutex_lock(&ctx->flying_transfers_lock);
+	r = handle_timeouts_locked(ctx);
+	usbi_mutex_unlock(&ctx->flying_transfers_lock);
+	return r;
+}
+#endif
+
+#ifdef USBI_TIMERFD_AVAILABLE
+static int handle_timerfd_trigger(struct libusb_context *ctx)
+{
+	int r;
+
+	r = disarm_timerfd(ctx);
+	if (r < 0)
+		return r;
+
+	usbi_mutex_lock(&ctx->flying_transfers_lock);
+
+	/* process the timeout that just happened */
+	r = handle_timeouts_locked(ctx);
+	if (r < 0)
+		goto out;
+
+	/* arm for next timeout*/
+	r = arm_timerfd_for_next_timeout(ctx);
+
+out:
+	usbi_mutex_unlock(&ctx->flying_transfers_lock);
+	return r;
+}
+#endif
+
+/* do the actual event handling. assumes that no other thread is concurrently
+ * doing the same thing. */
+static int handle_events(struct libusb_context *ctx, struct timeval *tv)
+{
+	int r;
+	struct usbi_pollfd *ipollfd;
+	nfds_t nfds = 0;
+	struct pollfd *fds;
+	int i = -1;
+	int timeout_ms;
+
+	usbi_mutex_lock(&ctx->pollfds_lock);
+	list_for_each_entry(ipollfd, &ctx->pollfds, list, struct usbi_pollfd)
+		nfds++;
+
+	/* TODO: malloc when number of fd's changes, not on every poll */
+	fds = malloc(sizeof(*fds) * nfds);
+	if (!fds) {
+		usbi_mutex_unlock(&ctx->pollfds_lock);
+		return LIBUSB_ERROR_NO_MEM;
+	}
+
+	list_for_each_entry(ipollfd, &ctx->pollfds, list, struct usbi_pollfd) {
+		struct libusb_pollfd *pollfd = &ipollfd->pollfd;
+		int fd = pollfd->fd;
+		i++;
+		fds[i].fd = fd;
+		fds[i].events = pollfd->events;
+		fds[i].revents = 0;
+	}
+	usbi_mutex_unlock(&ctx->pollfds_lock);
+
+	timeout_ms = (tv->tv_sec * 1000) + (tv->tv_usec / 1000);
+
+	/* round up to next millisecond */
+	if (tv->tv_usec % 1000)
+		timeout_ms++;
+
+	usbi_dbg("poll() %d fds with timeout in %dms", nfds, timeout_ms);
+	r = usbi_poll(fds, nfds, timeout_ms);
+	usbi_dbg("poll() returned %d", r);
+	if (r == 0) {
+		free(fds);
+		return handle_timeouts(ctx);
+	} else if (r == -1 && errno == EINTR) {
+		free(fds);
+		return LIBUSB_ERROR_INTERRUPTED;
+	} else if (r < 0) {
+		free(fds);
+		usbi_err(ctx, "poll failed %d err=%d\n", r, errno);
+		return LIBUSB_ERROR_IO;
+	}
+
+	/* fd[0] is always the ctrl pipe */
+	if (fds[0].revents) {
+		/* another thread wanted to interrupt event handling, and it succeeded!
+		 * handle any other events that cropped up at the same time, and
+		 * simply return */
+		usbi_dbg("caught a fish on the control pipe");
+
+		if (r == 1) {
+			r = 0;
+			goto handled;
+		} else {
+			/* prevent OS backend from trying to handle events on ctrl pipe */
+			fds[0].revents = 0;
+			r--;
+		}
+	}
+
+#ifdef USBI_TIMERFD_AVAILABLE
+	/* on timerfd configurations, fds[1] is the timerfd */
+	if (usbi_using_timerfd(ctx) && fds[1].revents) {
+		/* timerfd indicates that a timeout has expired */
+		int ret;
+		usbi_dbg("timerfd triggered");
+
+		ret = handle_timerfd_trigger(ctx);
+		if (ret < 0) {
+			/* return error code */
+			r = ret;
+			goto handled;
+		} else if (r == 1) {
+			/* no more active file descriptors, nothing more to do */
+			r = 0;
+			goto handled;
+		} else {
+			/* more events pending...
+			 * prevent OS backend from trying to handle events on timerfd */
+			fds[1].revents = 0;
+			r--;
+		}
+	}
+#endif
+
+	r = usbi_backend->handle_events(ctx, fds, nfds, r);
+	if (r)
+		usbi_err(ctx, "backend handle_events failed with error %d", r);
+
+handled:
+	free(fds);
+	return r;
+}
+
+/* returns the smallest of:
+ *  1. timeout of next URB
+ *  2. user-supplied timeout
+ * returns 1 if there is an already-expired timeout, otherwise returns 0
+ * and populates out
+ */
+static int get_next_timeout(libusb_context *ctx, struct timeval *tv,
+	struct timeval *out)
+{
+	struct timeval timeout;
+	int r = libusb_get_next_timeout(ctx, &timeout);
+	if (r) {
+		/* timeout already expired? */
+		if (!timerisset(&timeout))
+			return 1;
+
+		/* choose the smallest of next URB timeout or user specified timeout */
+		if (timercmp(&timeout, tv, <))
+			*out = timeout;
+		else
+			*out = *tv;
+	} else {
+		*out = *tv;
+	}
+	return 0;
+}
+
+/** \ingroup poll
+ * Handle any pending events.
+ *
+ * libusb determines "pending events" by checking if any timeouts have expired
+ * and by checking the set of file descriptors for activity.
+ *
+ * If a zero timeval is passed, this function will handle any already-pending
+ * events and then immediately return in non-blocking style.
+ *
+ * If a non-zero timeval is passed and no events are currently pending, this
+ * function will block waiting for events to handle up until the specified
+ * timeout. If an event arrives or a signal is raised, this function will
+ * return early.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param tv the maximum time to block waiting for events, or zero for
+ * non-blocking mode
+ * \returns 0 on success, or a LIBUSB_ERROR code on failure
+ */
+int API_EXPORTED libusb_handle_events_timeout_check(libusb_context *ctx,
+	struct timeval *tv, int *completed)
+{
+	int r;
+	struct timeval poll_timeout;
+
+	USBI_GET_CONTEXT(ctx);
+	r = get_next_timeout(ctx, tv, &poll_timeout);
+	if (r) {
+		/* timeout already expired */
+		return handle_timeouts(ctx);
+	}
+
+retry:
+	if (libusb_try_lock_events(ctx) == 0) {
+		r = 0;
+		if (completed == NULL || !*completed) {
+			/* we obtained the event lock: do our own event handling */
+			usbi_dbg("doing our own event handling");
+			r = handle_events(ctx, &poll_timeout);
+		}
+		libusb_unlock_events(ctx);
+		return r;
+	}
+
+	/* another thread is doing event handling. wait for pthread events that
+	 * notify event completion. */
+	libusb_lock_event_waiters(ctx);
+
+	if (completed == NULL || !*completed) {
+		if (!libusb_event_handler_active(ctx)) {
+			/* we hit a race: whoever was event handling earlier finished in the
+			 * time it took us to reach this point. try the cycle again. */
+			libusb_unlock_event_waiters(ctx);
+			usbi_dbg("event handler was active but went away, retrying");
+			goto retry;
+		}
+
+		usbi_dbg("another thread is doing event handling, wait for notification");
+		r = libusb_wait_for_event(ctx, &poll_timeout);
+	}
+	libusb_unlock_event_waiters(ctx);
+
+	if (r < 0)
+		return r;
+	else if (r == 1)
+		return handle_timeouts(ctx);
+	else
+		return 0;
+}
+
+int API_EXPORTED libusb_handle_events_timeout(libusb_context *ctx,
+	struct timeval *tv)
+{
+	return libusb_handle_events_timeout_check(ctx, tv, NULL);
+}
+
+int API_EXPORTED libusb_handle_events_check(libusb_context *ctx,
+	int *completed)
+{
+	struct timeval tv;
+	tv.tv_sec = 60;
+	tv.tv_usec = 0;
+	return libusb_handle_events_timeout_check(ctx, &tv, completed);
+}
+
+/** \ingroup poll
+ * Handle any pending events in blocking mode. There is currently a timeout
+ * hardcoded at 60 seconds but we plan to make it unlimited in future. For
+ * finer control over whether this function is blocking or non-blocking, or
+ * for control over the timeout, use libusb_handle_events_timeout() instead.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \returns 0 on success, or a LIBUSB_ERROR code on failure
+ */
+int API_EXPORTED libusb_handle_events(libusb_context *ctx)
+{
+	struct timeval tv;
+	tv.tv_sec = 60;
+	tv.tv_usec = 0;
+	return libusb_handle_events_timeout_check(ctx, &tv, NULL);
+}
+
+/** \ingroup poll
+ * Handle any pending events by polling file descriptors, without checking if
+ * any other threads are already doing so. Must be called with the event lock
+ * held, see libusb_lock_events().
+ *
+ * This function is designed to be called under the situation where you have
+ * taken the event lock and are calling poll()/select() directly on libusb's
+ * file descriptors (as opposed to using libusb_handle_events() or similar).
+ * You detect events on libusb's descriptors, so you then call this function
+ * with a zero timeout value (while still holding the event lock).
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param tv the maximum time to block waiting for events, or zero for
+ * non-blocking mode
+ * \returns 0 on success, or a LIBUSB_ERROR code on failure
+ * \see \ref mtasync
+ */
+int API_EXPORTED libusb_handle_events_locked(libusb_context *ctx,
+	struct timeval *tv)
+{
+	int r;
+	struct timeval poll_timeout;
+
+	USBI_GET_CONTEXT(ctx);
+	r = get_next_timeout(ctx, tv, &poll_timeout);
+	if (r) {
+		/* timeout already expired */
+		return handle_timeouts(ctx);
+	}
+
+	return handle_events(ctx, &poll_timeout);
+}
+
+/** \ingroup poll
+ * Determines whether your application must apply special timing considerations
+ * when monitoring libusb's file descriptors.
+ *
+ * This function is only useful for applications which retrieve and poll
+ * libusb's file descriptors in their own main loop (\ref pollmain).
+ *
+ * Ordinarily, libusb's event handler needs to be called into at specific
+ * moments in time (in addition to times when there is activity on the file
+ * descriptor set). The usual approach is to use libusb_get_next_timeout()
+ * to learn about when the next timeout occurs, and to adjust your
+ * poll()/select() timeout accordingly so that you can make a call into the
+ * library at that time.
+ *
+ * Some platforms supported by libusb do not come with this baggage - any
+ * events relevant to timing will be represented by activity on the file
+ * descriptor set, and libusb_get_next_timeout() will always return 0.
+ * This function allows you to detect whether you are running on such a
+ * platform.
+ *
+ * Since v1.0.5.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \returns 0 if you must call into libusb at times determined by
+ * libusb_get_next_timeout(), or 1 if all timeout events are handled internally
+ * or through regular activity on the file descriptors.
+ * \see \ref pollmain "Polling libusb file descriptors for event handling"
+ */
+int API_EXPORTED libusb_pollfds_handle_timeouts(libusb_context *ctx)
+{
+#if defined(USBI_OS_HANDLES_TIMEOUT)
+	return 1;
+#elif defined(USBI_TIMERFD_AVAILABLE)
+	USBI_GET_CONTEXT(ctx);
+	return usbi_using_timerfd(ctx);
+#else
+	return 0;
+#endif
+}
+
+/** \ingroup poll
+ * Determine the next internal timeout that libusb needs to handle. You only
+ * need to use this function if you are calling poll() or select() or similar
+ * on libusb's file descriptors yourself - you do not need to use it if you
+ * are calling libusb_handle_events() or a variant directly.
+ *
+ * You should call this function in your main loop in order to determine how
+ * long to wait for select() or poll() to return results. libusb needs to be
+ * called into at this timeout, so you should use it as an upper bound on
+ * your select() or poll() call.
+ *
+ * When the timeout has expired, call into libusb_handle_events_timeout()
+ * (perhaps in non-blocking mode) so that libusb can handle the timeout.
+ *
+ * This function may return 1 (success) and an all-zero timeval. If this is
+ * the case, it indicates that libusb has a timeout that has already expired
+ * so you should call libusb_handle_events_timeout() or similar immediately.
+ * A return code of 0 indicates that there are no pending timeouts.
+ *
+ * On some platforms, this function will always returns 0 (no pending
+ * timeouts). See \ref polltime.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param tv output location for a relative time against the current
+ * clock in which libusb must be called into in order to process timeout events
+ * \returns 0 if there are no pending timeouts, 1 if a timeout was returned,
+ * or LIBUSB_ERROR_OTHER on failure
+ */
+int API_EXPORTED libusb_get_next_timeout(libusb_context *ctx,
+	struct timeval *tv)
+{
+#ifndef USBI_OS_HANDLES_TIMEOUT
+	struct usbi_transfer *transfer;
+	struct timespec cur_ts;
+	struct timeval cur_tv;
+	struct timeval *next_timeout;
+	int r;
+	int found = 0;
+
+	USBI_GET_CONTEXT(ctx);
+	if (usbi_using_timerfd(ctx))
+		return 0;
+
+	usbi_mutex_lock(&ctx->flying_transfers_lock);
+	if (list_empty(&ctx->flying_transfers)) {
+		usbi_mutex_unlock(&ctx->flying_transfers_lock);
+		usbi_dbg("no URBs, no timeout!");
+		return 0;
+	}
+
+	/* find next transfer which hasn't already been processed as timed out */
+	list_for_each_entry(transfer, &ctx->flying_transfers, list, struct usbi_transfer) {
+		if (!(transfer->flags & USBI_TRANSFER_TIMED_OUT)) {
+			found = 1;
+			break;
+		}
+	}
+	usbi_mutex_unlock(&ctx->flying_transfers_lock);
+
+	if (!found) {
+		usbi_dbg("all URBs have already been processed for timeouts");
+		return 0;
+	}
+
+	next_timeout = &transfer->timeout;
+
+	/* no timeout for next transfer */
+	if (!timerisset(next_timeout)) {
+		usbi_dbg("no URBs with timeouts, no timeout!");
+		return 0;
+	}
+
+	r = usbi_backend->clock_gettime(USBI_CLOCK_MONOTONIC, &cur_ts);
+	if (r < 0) {
+		usbi_err(ctx, "failed to read monotonic clock, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+	TIMESPEC_TO_TIMEVAL(&cur_tv, &cur_ts);
+
+	if (!timercmp(&cur_tv, next_timeout, <)) {
+		usbi_dbg("first timeout already expired");
+		timerclear(tv);
+	} else {
+		timersub(next_timeout, &cur_tv, tv);
+		usbi_dbg("next timeout in %d.%06ds", tv->tv_sec, tv->tv_usec);
+	}
+
+	return 1;
+#else
+	return 0;
+#endif
+}
+
+/** \ingroup poll
+ * Register notification functions for file descriptor additions/removals.
+ * These functions will be invoked for every new or removed file descriptor
+ * that libusb uses as an event source.
+ *
+ * To remove notifiers, pass NULL values for the function pointers.
+ *
+ * Note that file descriptors may have been added even before you register
+ * these notifiers (e.g. at libusb_init() time).
+ *
+ * Additionally, note that the removal notifier may be called during
+ * libusb_exit() (e.g. when it is closing file descriptors that were opened
+ * and added to the poll set at libusb_init() time). If you don't want this,
+ * remove the notifiers immediately before calling libusb_exit().
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param added_cb pointer to function for addition notifications
+ * \param removed_cb pointer to function for removal notifications
+ * \param user_data User data to be passed back to callbacks (useful for
+ * passing context information)
+ */
+void API_EXPORTED libusb_set_pollfd_notifiers(libusb_context *ctx,
+	libusb_pollfd_added_cb added_cb, libusb_pollfd_removed_cb removed_cb,
+	void *user_data)
+{
+	USBI_GET_CONTEXT(ctx);
+	ctx->fd_added_cb = added_cb;
+	ctx->fd_removed_cb = removed_cb;
+	ctx->fd_cb_user_data = user_data;
+}
+
+/* Add a file descriptor to the list of file descriptors to be monitored.
+ * events should be specified as a bitmask of events passed to poll(), e.g.
+ * POLLIN and/or POLLOUT. */
+int usbi_add_pollfd(struct libusb_context *ctx, int fd, short events)
+{
+	struct usbi_pollfd *ipollfd = malloc(sizeof(*ipollfd));
+	if (!ipollfd)
+		return LIBUSB_ERROR_NO_MEM;
+
+	usbi_dbg("add fd %d events %d", fd, events);
+	ipollfd->pollfd.fd = fd;
+	ipollfd->pollfd.events = events;
+	usbi_mutex_lock(&ctx->pollfds_lock);
+	list_add_tail(&ipollfd->list, &ctx->pollfds);
+	usbi_mutex_unlock(&ctx->pollfds_lock);
+
+	if (ctx->fd_added_cb)
+		ctx->fd_added_cb(fd, events, ctx->fd_cb_user_data);
+	return 0;
+}
+
+/* Remove a file descriptor from the list of file descriptors to be polled. */
+void usbi_remove_pollfd(struct libusb_context *ctx, int fd)
+{
+	struct usbi_pollfd *ipollfd;
+	int found = 0;
+
+	usbi_dbg("remove fd %d", fd);
+	usbi_mutex_lock(&ctx->pollfds_lock);
+	list_for_each_entry(ipollfd, &ctx->pollfds, list, struct usbi_pollfd)
+		if (ipollfd->pollfd.fd == fd) {
+			found = 1;
+			break;
+		}
+
+	if (!found) {
+		usbi_dbg("couldn't find fd %d to remove", fd);
+		usbi_mutex_unlock(&ctx->pollfds_lock);
+		return;
+	}
+
+	list_del(&ipollfd->list);
+	usbi_mutex_unlock(&ctx->pollfds_lock);
+	free(ipollfd);
+	if (ctx->fd_removed_cb)
+		ctx->fd_removed_cb(fd, ctx->fd_cb_user_data);
+}
+
+/** \ingroup poll
+ * Retrieve a list of file descriptors that should be polled by your main loop
+ * as libusb event sources.
+ *
+ * The returned list is NULL-terminated and should be freed with free() when
+ * done. The actual list contents must not be touched.
+ *
+ * As file descriptors are a Unix-specific concept, this function is not
+ * available on Windows and will always return NULL.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \returns a NULL-terminated list of libusb_pollfd structures
+ * \returns NULL on error
+ * \returns NULL on platforms where the functionality is not available
+ */
+DEFAULT_VISIBILITY
+const struct libusb_pollfd ** LIBUSB_CALL libusb_get_pollfds(
+	libusb_context *ctx)
+{
+#ifndef OS_WINDOWS
+	struct libusb_pollfd **ret = NULL;
+	struct usbi_pollfd *ipollfd;
+	size_t i = 0;
+	size_t cnt = 0;
+	USBI_GET_CONTEXT(ctx);
+
+	usbi_mutex_lock(&ctx->pollfds_lock);
+	list_for_each_entry(ipollfd, &ctx->pollfds, list, struct usbi_pollfd)
+		cnt++;
+
+	ret = calloc(cnt + 1, sizeof(struct libusb_pollfd *));
+	if (!ret)
+		goto out;
+
+	list_for_each_entry(ipollfd, &ctx->pollfds, list, struct usbi_pollfd)
+		ret[i++] = (struct libusb_pollfd *) ipollfd;
+	ret[cnt] = NULL;
+
+out:
+	usbi_mutex_unlock(&ctx->pollfds_lock);
+	return (const struct libusb_pollfd **) ret;
+#else
+	return NULL;
+#endif
+}
+
+/* Backends call this from handle_events to report disconnection of a device.
+ * The transfers get cancelled appropriately.
+ */
+void usbi_handle_disconnect(struct libusb_device_handle *handle)
+{
+	struct usbi_transfer *cur;
+	struct usbi_transfer *to_cancel;
+
+	usbi_dbg("device %d.%d",
+		handle->dev->bus_number, handle->dev->device_address);
+
+	/* terminate all pending transfers with the LIBUSB_TRANSFER_NO_DEVICE
+	 * status code.
+	 *
+	 * this is a bit tricky because:
+	 * 1. we can't do transfer completion while holding flying_transfers_lock
+	 * 2. the transfers list can change underneath us - if we were to build a
+	 *    list of transfers to complete (while holding look), the situation
+	 *    might be different by the time we come to free them
+	 *
+	 * so we resort to a loop-based approach as below
+	 * FIXME: is this still potentially racy?
+	 */
+
+	while (1) {
+		usbi_mutex_lock(&HANDLE_CTX(handle)->flying_transfers_lock);
+		to_cancel = NULL;
+		list_for_each_entry(cur, &HANDLE_CTX(handle)->flying_transfers, list, struct usbi_transfer)
+			if (__USBI_TRANSFER_TO_LIBUSB_TRANSFER(cur)->dev_handle == handle) {
+				to_cancel = cur;
+				break;
+			}
+		usbi_mutex_unlock(&HANDLE_CTX(handle)->flying_transfers_lock);
+
+		if (!to_cancel)
+			break;
+
+		usbi_backend->clear_transfer_priv(to_cancel);
+		usbi_handle_transfer_completion(to_cancel, LIBUSB_TRANSFER_NO_DEVICE);
+	}
+
+}