+/**
+ * \page io Synchronous and asynchronous device I/O
+ *
+ * \section intro Introduction
+ *
+ * If you're using libusbx in your application, you're probably wanting to
+ * perform I/O with devices - you want to perform USB data transfers.
+ *
+ * libusbx 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.
+ *
+ * libusbx 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, LIBUSB_ENDPOINT_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,
+ * libusbx'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 libusbx 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, libusbx does not create
+ * threads and can only operate when your application is calling into it. Your
+ * application must call into libusbx from it's main loop when events are ready
+ * to be handled, or you must use some other scheme to allow libusbx to
+ * undertake whatever work needs to be done.
+ * - libusbx 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, libusbx'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.
+ *
+ * libusbx 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, libusbx requires you to supply a
+ * buffer and the maximum number of bytes of data that libusbx 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. libusbx 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. libusbx 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 libusbx'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, libusbx 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 libusbx 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, libusbx 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.
+ * libusbx 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 libusbx 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 libusbx to free the transfer
+ * buffer when freeing the transfer.
+ * - \ref libusb_transfer_flags::LIBUSB_TRANSFER_FREE_TRANSFER
+ * "LIBUSB_TRANSFER_FREE_TRANSFER" causes libusbx to automatically free the
+ * transfer after the transfer callback returns.
+ *
+ * \section asyncevent Event handling
+ *
+ * In accordance of the aim of being a lightweight library, libusbx does not
+ * create threads internally. This means that libusbx code does not execute
+ * at any time other than when your application is calling a libusbx function.
+ * However, an asynchronous model requires that libusbx 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 libusbx has work do to. This gives libusbx
+ * the opportunity to reap pending transfers, invoke callbacks, etc.
+ *
+ * The first issue to discuss here is how your application can figure out
+ * when libusbx 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 libusbx with your application main
+ * event loop. libusbx 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
+ * libusbx'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. libusbx supports
+ * asynchronous transfers which time out after a specified time period, and
+ * this requires that libusbx is called into at or after the timeout so that
+ * the timeout can be handled. So, in addition to considering libusbx's file
+ * descriptors in your main event loop, you must also consider that libusbx
+ * 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 libusbx'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, libusbx does not create internal
+ * threads for event processing and hence relies on your application calling
+ * into libusbx at certain points in time so that pending events can be handled.
+ * In order to know precisely when libusbx needs to be called into, libusbx
+ * 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 libusbx and does not need to
+ * handle other event sources, you can have a program structure as follows:
+\code
+// initialize libusbx
+// 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, libusbx 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 libusbx and monitor those too. When you
+ * detect activity on libusbx's file descriptors, you call
+ * libusb_handle_events_timeout() in non-blocking mode.
+ *
+ * What's more, libusbx 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.
+ *
+ * libusbx 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 libusbx indicates,
+ * polling all of them at once.
+ *
+ * In pseudo-code, you want something that looks like:
+\code
+// initialise libusbx
+
+libusb_get_pollfds(ctx)
+while (user has not requested application exit) {
+ libusb_get_next_timeout(ctx);
+ poll(on libusbx file descriptors plus any other event sources of interest,
+ using a timeout no larger than the value libusbx just suggested)
+ if (poll() indicated activity on libusbx file descriptors)
+ libusb_handle_events_timeout(ctx, &zero_tv);
+ if (time has elapsed to or beyond the libusbx timeout)
+ libusb_handle_events_timeout(ctx, &zero_tv);
+ // 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 libusbx 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
+ * - libusbx 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 libusbx
+
+libusb_get_pollfds(ctx)
+while (user has not requested application exit) {
+ poll(on libusbx file descriptors plus any other event sources of interest,
+ using any timeout that you like)
+ if (poll() indicated activity on libusbx file descriptors)
+ libusb_handle_events_timeout(ctx, &zero_tv);
+ // 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 libusbx 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 libusbx'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 libusbx 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
+ *
+ * libusbx is a thread-safe library, but extra considerations must be applied
+ * to applications which interact with libusbx from multiple threads.
+ *
+ * The underlying issue that must be addressed is that all libusbx 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 libusbx'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 libusbx
+ * 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(libusbx file descriptors, 120*1000);
+ if (poll indicates activity)
+ libusb_handle_events_timeout(ctx, &zero_tv);
+ }
+ 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 libusbx'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 libusbx 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 its variants, and all the
+ * synchronous I/O functions - libusbx hides this headache from you.
+ *
+ * \section Using libusb_handle_events() from multiple threads
+ *
+ * Even when only using libusb_handle_events() and synchronous I/O functions,
+ * you can still have a race condition. You might be tempted to solve the
+ * above with libusb_handle_events() like so:
+ *
+\code
+ libusb_submit_transfer(transfer);
+
+ while (!completed) {
+ libusb_handle_events(ctx);
+ }
+ printf("completed!");
+\endcode
+ *
+ * This however has a race between the checking of completed and
+ * libusb_handle_events() acquiring the events lock, so another thread
+ * could have completed the transfer, resulting in this thread hanging
+ * until either a timeout or another event occurs. See also commit
+ * 6696512aade99bb15d6792af90ae329af270eba6 which fixes this in the
+ * synchronous API implementation of libusb.
+ *
+ * Fixing this race requires checking the variable completed only after
+ * taking the event lock, which defeats the concept of just calling
+ * libusb_handle_events() without worrying about locking. This is why
+ * libusb-1.0.9 introduces the new libusb_handle_events_timeout_completed()
+ * and libusb_handle_events_completed() functions, which handles doing the
+ * completion check for you after they have acquired the lock:
+ *
+\code
+ libusb_submit_transfer(transfer);
+
+ while (!completed) {
+ libusb_handle_events_completed(ctx, &completed);
+ }
+ printf("completed!");
+\endcode
+ *
+ * This nicely fixes the race in our example. Note that if all you want to
+ * do is submit a single transfer and wait for its completion, then using
+ * one of the synchronous I/O functions is much easier.
+ *
+ * \section eventlock The events lock
+ *
+ * The problem is when we consider the fact that libusbx 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
+ * libusbx's back. If you do take libusbx's file descriptors and pass them to
+ * poll()/select() yourself, you need to be aware of the associated issues.
+ *
+ * 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 libusbx 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(libusbx file descriptors, 120*1000);
+ if (poll indicates activity)
+ libusb_handle_events_timeout(ctx, &zero_tv);
+ }
+ 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, libusbx 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(libusbx 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 libusbx is still happy for your
+ * thread to be performing event handling. Sometimes, libusbx 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 libusbx'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 libusbx 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 libusbx'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.
+ *
+ * libusbx 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, libusbx 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(), libusbx writes some dummy data on this control pipe.
+ * This immediately interrupts the event handler. libusbx 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(), libusbx 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.
+ * -# libusbx 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 libusbx'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.
+ */