Commit 75183773 authored by Thiago Santini's avatar Thiago Santini

Small code changes, removes deps from repository

parent a1140503
all: libusb-1.0.la libusb-1.0.dll
AUTOMAKE_OPTIONS = subdir-objects
lib_LTLIBRARIES = libusb-1.0.la
POSIX_POLL_SRC = os/poll_posix.h os/poll_posix.c
POSIX_THREADS_SRC = os/threads_posix.h os/threads_posix.c
WINDOWS_POLL_SRC = os/poll_windows.h os/poll_windows.c
WINDOWS_THREADS_SRC = os/threads_windows.h os/threads_windows.c
LINUX_USBFS_SRC = os/linux_usbfs.h os/linux_usbfs.c
DARWIN_USB_SRC = os/darwin_usb.h os/darwin_usb.c
OPENBSD_USB_SRC = os/openbsd_usb.c
NETBSD_USB_SRC = os/netbsd_usb.c
SUNOS_USB_SRC = os/sunos_usb.c os/sunos_usb.h
WINDOWS_COMMON_SRC = os/windows_nt_common.h os/windows_nt_common.c \
os/windows_common.h libusb-1.0.rc libusb-1.0.def
WINDOWS_USB_SRC = os/windows_winusb.h os/windows_winusb.c
WINDOWS_USBDK_SRC = os/windows_usbdk.h os/windows_usbdk.c
WINCE_USB_SRC = os/wince_usb.h os/wince_usb.c
HAIKU_USB_SRC = os/haiku_usb.h os/haiku_usb_backend.cpp \
os/haiku_usb_raw.h os/haiku_usb_raw.cpp os/haiku_pollfs.cpp
EXTRA_DIST = $(POSIX_POLL_SRC) $(POSIX_THREADS_SRC) \
$(WINDOWS_POLL_SRC) $(WINDOWS_THREADS_SRC) \
$(LINUX_USBFS_SRC) $(DARWIN_USB_SRC) \
$(OPENBSD_USB_SRC) $(NETBSD_USB_SRC) \
$(WINDOWS_COMMON_SRC) $(WINDOWS_USB_SRC) $(WINDOWS_USBDK_SRC) \
$(WINCE_USB_SRC) $(HAIKU_USB_SRC) \
os/linux_udev.c os/linux_netlink.c
if OS_LINUX
if USE_UDEV
OS_SRC = $(LINUX_USBFS_SRC) os/linux_udev.c
else
OS_SRC = $(LINUX_USBFS_SRC) os/linux_netlink.c
endif
endif
if OS_DARWIN
OS_SRC = $(DARWIN_USB_SRC)
AM_CFLAGS_EXT = -no-cpp-precomp
endif
if OS_OPENBSD
OS_SRC = $(OPENBSD_USB_SRC)
endif
if OS_NETBSD
OS_SRC = $(NETBSD_USB_SRC)
endif
if OS_SUNOS
OS_SRC = $(SUNOS_USB_SRC)
endif
if OS_HAIKU
noinst_LTLIBRARIES = libusb_haiku.la
libusb_haiku_la_SOURCES = $(HAIKU_USB_SRC)
libusb_1_0_la_LIBADD = libusb_haiku.la
endif
if OS_WINDOWS
if USE_USBDK
OS_SRC = $(WINDOWS_USBDK_SRC) $(WINDOWS_COMMON_SRC)
else
OS_SRC = $(WINDOWS_USB_SRC) $(WINDOWS_COMMON_SRC)
endif
.rc.lo:
$(AM_V_GEN)$(LIBTOOL) $(AM_V_lt) $(AM_LIBTOOLFLAGS) $(LIBTOOLFLAGS) --tag=RC --mode=compile $(RC) $(RCFLAGS) -i $< -o $@
libusb-1.0.rc: version.h version_nano.h
endif
libusb-1.0.dll: libusb-1.0.def libusb-1.0.la
if CREATE_IMPORT_LIB
# Rebuild the import lib from the .def so that MS and MinGW DLLs can be interchanged
$(AM_V_GEN)$(DLLTOOL) $(DLLTOOLFLAGS) --kill-at --input-def $(srcdir)/libusb-1.0.def --dllname $@ --output-lib .libs/$@.a
endif
if OS_WINDOWS
POLL_SRC = $(WINDOWS_POLL_SRC)
else
POLL_SRC = $(POSIX_POLL_SRC)
endif
if THREADS_POSIX
THREADS_SRC = $(POSIX_THREADS_SRC)
else
THREADS_SRC = $(WINDOWS_THREADS_SRC)
endif
libusb_1_0_la_CFLAGS = $(AM_CFLAGS)
libusb_1_0_la_LDFLAGS = $(LTLDFLAGS)
libusb_1_0_la_SOURCES = libusbi.h libusb.h version.h version_nano.h \
core.c descriptor.c hotplug.h hotplug.c io.c strerror.c sync.c \
$(POLL_SRC) $(THREADS_SRC) $(OS_SRC)
hdrdir = $(includedir)/libusb-1.0
hdr_HEADERS = libusb.h
/* -*- Mode: C; indent-tabs-mode:t ; c-basic-offset:8 -*- */
/*
* Core functions for libusb
* Copyright © 2012-2013 Nathan Hjelm <hjelmn@cs.unm.edu>
* Copyright © 2007-2008 Daniel Drake <dsd@gentoo.org>
* Copyright © 2001 Johannes Erdfelt <johannes@erdfelt.com>
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
#include "config.h"
#include <errno.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#ifdef HAVE_SYS_TYPES_H
#include <sys/types.h>
#endif
#ifdef HAVE_SYS_TIME_H
#include <sys/time.h>
#endif
#ifdef HAVE_SYSLOG_H
#include <syslog.h>
#endif
#ifdef __ANDROID__
#include <android/log.h>
#endif
#include "libusbi.h"
#include "hotplug.h"
#if defined(OS_LINUX)
const struct usbi_os_backend * const usbi_backend = &linux_usbfs_backend;
#elif defined(OS_DARWIN)
const struct usbi_os_backend * const usbi_backend = &darwin_backend;
#elif defined(OS_OPENBSD)
const struct usbi_os_backend * const usbi_backend = &openbsd_backend;
#elif defined(OS_NETBSD)
const struct usbi_os_backend * const usbi_backend = &netbsd_backend;
#elif defined(OS_WINDOWS)
#if defined(USE_USBDK)
const struct usbi_os_backend * const usbi_backend = &usbdk_backend;
#else
const struct usbi_os_backend * const usbi_backend = &windows_backend;
#endif
#elif defined(OS_WINCE)
const struct usbi_os_backend * const usbi_backend = &wince_backend;
#elif defined(OS_HAIKU)
const struct usbi_os_backend * const usbi_backend = &haiku_usb_raw_backend;
#elif defined (OS_SUNOS)
const struct usbi_os_backend * const usbi_backend = &sunos_backend;
#else
#error "Unsupported OS"
#endif
struct libusb_context *usbi_default_context = NULL;
static const struct libusb_version libusb_version_internal =
{ LIBUSB_MAJOR, LIBUSB_MINOR, LIBUSB_MICRO, LIBUSB_NANO,
LIBUSB_RC, "http://libusb.info" };
static int default_context_refcnt = 0;
static usbi_mutex_static_t default_context_lock = USBI_MUTEX_INITIALIZER;
static struct timeval timestamp_origin = { 0, 0 };
usbi_mutex_static_t active_contexts_lock = USBI_MUTEX_INITIALIZER;
struct list_head active_contexts_list;
/**
* \mainpage libusb-1.0 API Reference
*
* \section intro Introduction
*
* libusb is an open source library that allows you to communicate with USB
* devices from userspace. For more info, see the
* <a href="http://libusb.info">libusb homepage</a>.
*
* This documentation is aimed at application developers wishing to
* communicate with USB peripherals from their own software. After reviewing
* this documentation, feedback and questions can be sent to the
* <a href="http://mailing-list.libusb.info">libusb-devel mailing list</a>.
*
* This documentation assumes knowledge of how to operate USB devices from
* a software standpoint (descriptors, configurations, interfaces, endpoints,
* control/bulk/interrupt/isochronous transfers, etc). Full information
* can be found in the <a href="http://www.usb.org/developers/docs/">USB 3.0
* Specification</a> which is available for free download. You can probably
* find less verbose introductions by searching the web.
*
* \section API Application Programming Interface (API)
*
* See the \ref libusb_api page for a complete list of the libusb functions.
*
* \section features Library features
*
* - All transfer types supported (control/bulk/interrupt/isochronous)
* - 2 transfer interfaces:
* -# Synchronous (simple)
* -# Asynchronous (more complicated, but more powerful)
* - Thread safe (although the asynchronous interface means that you
* usually won't need to thread)
* - Lightweight with lean API
* - Compatible with libusb-0.1 through the libusb-compat-0.1 translation layer
* - Hotplug support (on some platforms). See \ref libusb_hotplug.
*
* \section gettingstarted Getting Started
*
* To begin reading the API documentation, start with the Modules page which
* links to the different categories of libusb's functionality.
*
* One decision you will have to make is whether to use the synchronous
* or the asynchronous data transfer interface. The \ref libusb_io documentation
* provides some insight into this topic.
*
* Some example programs can be found in the libusb source distribution under
* the "examples" subdirectory. The libusb homepage includes a list of
* real-life project examples which use libusb.
*
* \section errorhandling Error handling
*
* libusb functions typically return 0 on success or a negative error code
* on failure. These negative error codes relate to LIBUSB_ERROR constants
* which are listed on the \ref libusb_misc "miscellaneous" documentation page.
*
* \section msglog Debug message logging
*
* libusb uses stderr for all logging. By default, logging is set to NONE,
* which means that no output will be produced. However, unless the library
* has been compiled with logging disabled, then any application calls to
* libusb_set_debug(), or the setting of the environmental variable
* LIBUSB_DEBUG outside of the application, can result in logging being
* produced. Your application should therefore not close stderr, but instead
* direct it to the null device if its output is undesirable.
*
* The libusb_set_debug() function can be used to enable logging of certain
* messages. Under standard configuration, libusb doesn't really log much
* so you are advised to use this function to enable all error/warning/
* informational messages. It will help debug problems with your software.
*
* The logged messages are unstructured. There is no one-to-one correspondence
* between messages being logged and success or failure return codes from
* libusb functions. There is no format to the messages, so you should not
* try to capture or parse them. They are not and will not be localized.
* These messages are not intended to being passed to your application user;
* instead, you should interpret the error codes returned from libusb functions
* and provide appropriate notification to the user. The messages are simply
* there to aid you as a programmer, and if you're confused because you're
* getting a strange error code from a libusb function, enabling message
* logging may give you a suitable explanation.
*
* The LIBUSB_DEBUG environment variable can be used to enable message logging
* at run-time. This environment variable should be set to a log level number,
* which is interpreted the same as the libusb_set_debug() parameter. When this
* environment variable is set, the message logging verbosity level is fixed
* and libusb_set_debug() effectively does nothing.
*
* libusb can be compiled without any logging functions, useful for embedded
* systems. In this case, libusb_set_debug() and the LIBUSB_DEBUG environment
* variable have no effects.
*
* libusb can also be compiled with verbose debugging messages always. When
* the library is compiled in this way, all messages of all verbosities are
* always logged. libusb_set_debug() and the LIBUSB_DEBUG environment variable
* have no effects.
*
* \section remarks Other remarks
*
* libusb does have imperfections. The \ref libusb_caveats "caveats" page attempts
* to document these.
*/
/**
* \page libusb_caveats Caveats
*
* \section devresets Device resets
*
* The libusb_reset_device() function allows you to reset a device. If your
* program has to call such a function, it should obviously be aware that
* the reset will cause device state to change (e.g. register values may be
* reset).
*
* The problem is that any other program could reset the device your program
* is working with, at any time. libusb does not offer a mechanism to inform
* you when this has happened, so if someone else resets your device it will
* not be clear to your own program why the device state has changed.
*
* Ultimately, this is a limitation of writing drivers in userspace.
* Separation from the USB stack in the underlying kernel makes it difficult
* for the operating system to deliver such notifications to your program.
* The Linux kernel USB stack allows such reset notifications to be delivered
* to in-kernel USB drivers, but it is not clear how such notifications could
* be delivered to second-class drivers that live in userspace.
*
* \section blockonly Blocking-only functionality
*
* The functionality listed below is only available through synchronous,
* blocking functions. There are no asynchronous/non-blocking alternatives,
* and no clear ways of implementing these.
*
* - Configuration activation (libusb_set_configuration())
* - Interface/alternate setting activation (libusb_set_interface_alt_setting())
* - Releasing of interfaces (libusb_release_interface())
* - Clearing of halt/stall condition (libusb_clear_halt())
* - Device resets (libusb_reset_device())
*
* \section configsel Configuration selection and handling
*
* When libusb presents a device handle to an application, there is a chance
* that the corresponding device may be in unconfigured state. For devices
* with multiple configurations, there is also a chance that the configuration
* currently selected is not the one that the application wants to use.
*
* The obvious solution is to add a call to libusb_set_configuration() early
* on during your device initialization routines, but there are caveats to
* be aware of:
* -# If the device is already in the desired configuration, calling
* libusb_set_configuration() using the same configuration value will cause
* a lightweight device reset. This may not be desirable behaviour.
* -# In the case where the desired configuration is already active, libusb
* may not even be able to perform a lightweight device reset. For example,
* take my USB keyboard with fingerprint reader: I'm interested in driving
* the fingerprint reader interface through libusb, but the kernel's
* USB-HID driver will almost always have claimed the keyboard interface.
* Because the kernel has claimed an interface, it is not even possible to
* perform the lightweight device reset, so libusb_set_configuration() will
* fail. (Luckily the device in question only has a single configuration.)
* -# libusb will be unable to set a configuration if other programs or
* drivers have claimed interfaces. In particular, this means that kernel
* drivers must be detached from all the interfaces before
* libusb_set_configuration() may succeed.
*
* One solution to some of the above problems is to consider the currently
* active configuration. If the configuration we want is already active, then
* we don't have to select any configuration:
\code
cfg = -1;
libusb_get_configuration(dev, &cfg);
if (cfg != desired)
libusb_set_configuration(dev, desired);
\endcode
*
* This is probably suitable for most scenarios, but is inherently racy:
* another application or driver may change the selected configuration
* <em>after</em> the libusb_get_configuration() call.
*
* Even in cases where libusb_set_configuration() succeeds, consider that other
* applications or drivers may change configuration after your application
* calls libusb_set_configuration().
*
* One possible way to lock your device into a specific configuration is as
* follows:
* -# Set the desired configuration (or use the logic above to realise that
* it is already in the desired configuration)
* -# Claim the interface that you wish to use
* -# Check that the currently active configuration is the one that you want
* to use.
*
* The above method works because once an interface is claimed, no application
* or driver is able to select another configuration.
*
* \section earlycomp Early transfer completion
*
* NOTE: This section is currently Linux-centric. I am not sure if any of these
* considerations apply to Darwin or other platforms.
*
* When a transfer completes early (i.e. when less data is received/sent in
* any one packet than the transfer buffer allows for) then libusb is designed
* to terminate the transfer immediately, not transferring or receiving any
* more data unless other transfers have been queued by the user.
*
* On legacy platforms, libusb is unable to do this in all situations. After
* the incomplete packet occurs, "surplus" data may be transferred. For recent
* versions of libusb, this information is kept (the data length of the
* transfer is updated) and, for device-to-host transfers, any surplus data was
* added to the buffer. Still, this is not a nice solution because it loses the
* information about the end of the short packet, and the user probably wanted
* that surplus data to arrive in the next logical transfer.
*
*
* \section zlp Zero length packets
*
* - libusb is able to send a packet of zero length to an endpoint simply by
* submitting a transfer of zero length.
* - The \ref libusb_transfer_flags::LIBUSB_TRANSFER_ADD_ZERO_PACKET
* "LIBUSB_TRANSFER_ADD_ZERO_PACKET" flag is currently only supported on Linux.
*/
/**
* \page libusb_contexts Contexts
*
* It is possible that libusb may be used simultaneously from two independent
* libraries linked into the same executable. For example, if your application
* has a plugin-like system which allows the user to dynamically load a range
* of modules into your program, it is feasible that two independently
* developed modules may both use libusb.
*
* libusb is written to allow for these multiple user scenarios. The two
* "instances" of libusb will not interfere: libusb_set_debug() calls
* from one user will not affect the same settings for other users, other
* users can continue using libusb after one of them calls libusb_exit(), etc.
*
* This is made possible through libusb's <em>context</em> concept. When you
* call libusb_init(), you are (optionally) given a context. You can then pass
* this context pointer back into future libusb functions.
*
* In order to keep things simple for more simplistic applications, it is
* legal to pass NULL to all functions requiring a context pointer (as long as
* you're sure no other code will attempt to use libusb from the same process).
* When you pass NULL, the default context will be used. The default context
* is created the first time a process calls libusb_init() when no other
* context is alive. Contexts are destroyed during libusb_exit().
*
* The default context is reference-counted and can be shared. That means that
* if libusb_init(NULL) is called twice within the same process, the two
* users end up sharing the same context. The deinitialization and freeing of
* the default context will only happen when the last user calls libusb_exit().
* In other words, the default context is created and initialized when its
* reference count goes from 0 to 1, and is deinitialized and destroyed when
* its reference count goes from 1 to 0.
*
* You may be wondering why only a subset of libusb functions require a
* context pointer in their function definition. Internally, libusb stores
* context pointers in other objects (e.g. libusb_device instances) and hence
* can infer the context from those objects.
*/
/**
* \page libusb_api Application Programming Interface
*
* This is the complete list of libusb functions, structures and
* enumerations in alphabetical order.
*
* \section Functions
* - libusb_alloc_streams()
* - libusb_alloc_transfer()
* - libusb_attach_kernel_driver()
* - libusb_bulk_transfer()
* - libusb_cancel_transfer()
* - libusb_claim_interface()
* - libusb_clear_halt()
* - libusb_close()
* - libusb_control_transfer()
* - libusb_control_transfer_get_data()
* - libusb_control_transfer_get_setup()
* - libusb_cpu_to_le16()
* - libusb_detach_kernel_driver()
* - libusb_dev_mem_alloc()
* - libusb_dev_mem_free()
* - libusb_error_name()
* - libusb_event_handler_active()
* - libusb_event_handling_ok()
* - libusb_exit()
* - libusb_fill_bulk_stream_transfer()
* - libusb_fill_bulk_transfer()
* - libusb_fill_control_setup()
* - libusb_fill_control_transfer()
* - libusb_fill_interrupt_transfer()
* - libusb_fill_iso_transfer()
* - libusb_free_bos_descriptor()
* - libusb_free_config_descriptor()
* - libusb_free_container_id_descriptor()
* - libusb_free_device_list()
* - libusb_free_pollfds()
* - libusb_free_ss_endpoint_companion_descriptor()
* - libusb_free_ss_usb_device_capability_descriptor()
* - libusb_free_streams()
* - libusb_free_transfer()
* - libusb_free_usb_2_0_extension_descriptor()
* - libusb_get_active_config_descriptor()
* - libusb_get_bos_descriptor()
* - libusb_get_bus_number()
* - libusb_get_config_descriptor()
* - libusb_get_config_descriptor_by_value()
* - libusb_get_configuration()
* - libusb_get_container_id_descriptor()
* - libusb_get_descriptor()
* - libusb_get_device()
* - libusb_get_device_address()
* - libusb_get_device_descriptor()
* - libusb_get_device_list()
* - libusb_get_device_speed()
* - libusb_get_iso_packet_buffer()
* - libusb_get_iso_packet_buffer_simple()
* - libusb_get_max_iso_packet_size()
* - libusb_get_max_packet_size()
* - libusb_get_next_timeout()
* - libusb_get_parent()
* - libusb_get_pollfds()
* - libusb_get_port_number()
* - libusb_get_port_numbers()
* - libusb_get_port_path()
* - libusb_get_ss_endpoint_companion_descriptor()
* - libusb_get_ss_usb_device_capability_descriptor()
* - libusb_get_string_descriptor()
* - libusb_get_string_descriptor_ascii()
* - libusb_get_usb_2_0_extension_descriptor()
* - libusb_get_version()
* - libusb_handle_events()
* - libusb_handle_events_completed()
* - libusb_handle_events_locked()
* - libusb_handle_events_timeout()
* - libusb_handle_events_timeout_completed()
* - libusb_has_capability()
* - libusb_hotplug_deregister_callback()
* - libusb_hotplug_register_callback()
* - libusb_init()
* - libusb_interrupt_event_handler()
* - libusb_interrupt_transfer()
* - libusb_kernel_driver_active()
* - libusb_lock_events()
* - libusb_lock_event_waiters()
* - libusb_open()
* - libusb_open_device_with_vid_pid()
* - libusb_pollfds_handle_timeouts()
* - libusb_ref_device()
* - libusb_release_interface()
* - libusb_reset_device()
* - libusb_set_auto_detach_kernel_driver()
* - libusb_set_configuration()
* - libusb_set_debug()
* - libusb_set_interface_alt_setting()
* - libusb_set_iso_packet_lengths()
* - libusb_setlocale()
* - libusb_set_pollfd_notifiers()
* - libusb_strerror()
* - libusb_submit_transfer()
* - libusb_transfer_get_stream_id()
* - libusb_transfer_set_stream_id()
* - libusb_try_lock_events()
* - libusb_unlock_events()
* - libusb_unlock_event_waiters()
* - libusb_unref_device()
* - libusb_wait_for_event()
*
* \section Structures
* - libusb_bos_descriptor
* - libusb_bos_dev_capability_descriptor
* - libusb_config_descriptor
* - libusb_container_id_descriptor
* - \ref libusb_context
* - libusb_control_setup
* - \ref libusb_device
* - libusb_device_descriptor
* - \ref libusb_device_handle
* - libusb_endpoint_descriptor
* - libusb_interface
* - libusb_interface_descriptor
* - libusb_iso_packet_descriptor
* - libusb_pollfd
* - libusb_ss_endpoint_companion_descriptor
* - libusb_ss_usb_device_capability_descriptor
* - libusb_transfer
* - libusb_usb_2_0_extension_descriptor
* - libusb_version
*
* \section Enums
* - \ref libusb_bos_type
* - \ref libusb_capability
* - \ref libusb_class_code
* - \ref libusb_descriptor_type
* - \ref libusb_endpoint_direction
* - \ref libusb_error
* - \ref libusb_iso_sync_type
* - \ref libusb_iso_usage_type
* - \ref libusb_log_level
* - \ref libusb_request_recipient
* - \ref libusb_request_type
* - \ref libusb_speed
* - \ref libusb_ss_usb_device_capability_attributes
* - \ref libusb_standard_request
* - \ref libusb_supported_speed
* - \ref libusb_transfer_flags
* - \ref libusb_transfer_status
* - \ref libusb_transfer_type
* - \ref libusb_usb_2_0_extension_attributes
*/
/**
* @defgroup libusb_lib Library initialization/deinitialization
* This page details how to initialize and deinitialize libusb. Initialization
* must be performed before using any libusb functionality, and similarly you
* must not call any libusb functions after deinitialization.
*/
/**
* @defgroup libusb_dev Device handling and enumeration
* The functionality documented below is designed to help with the following
* operations:
* - Enumerating the USB devices currently attached to the system
* - Choosing a device to operate from your software
* - Opening and closing the chosen device
*
* \section nutshell In a nutshell...
*
* The description below really makes things sound more complicated than they
* actually are. The following sequence of function calls will be suitable
* for almost all scenarios and does not require you to have such a deep
* understanding of the resource management issues:
* \code
// discover devices
libusb_device **list;
libusb_device *found = NULL;
ssize_t cnt = libusb_get_device_list(NULL, &list);
ssize_t i = 0;
int err = 0;
if (cnt < 0)
error();
for (i = 0; i < cnt; i++) {
libusb_device *device = list[i];
if (is_interesting(device)) {
found = device;
break;
}
}
if (found) {
libusb_device_handle *handle;
err = libusb_open(found, &handle);
if (err)
error();
// etc
}
libusb_free_device_list(list, 1);
\endcode
*
* The two important points:
* - You asked libusb_free_device_list() to unreference the devices (2nd
* parameter)
* - You opened the device before freeing the list and unreferencing the
* devices
*
* If you ended up with a handle, you can now proceed to perform I/O on the
* device.
*
* \section devshandles Devices and device handles
* libusb has a concept of a USB device, represented by the
* \ref libusb_device opaque type. A device represents a USB device that
* is currently or was previously connected to the system. Using a reference
* to a device, you can determine certain information about the device (e.g.
* you can read the descriptor data).
*
* The libusb_get_device_list() function can be used to obtain a list of
* devices currently connected to the system. This is known as device
* discovery.
*
* Just because you have a reference to a device does not mean it is
* necessarily usable. The device may have been unplugged, you may not have
* permission to operate such device, or another program or driver may be
* using the device.
*
* When you've found a device that you'd like to operate, you must ask
* libusb to open the device using the libusb_open() function. Assuming
* success, libusb then returns you a <em>device handle</em>
* (a \ref libusb_device_handle pointer). All "real" I/O operations then
* operate on the handle rather than the original device pointer.
*
* \section devref Device discovery and reference counting
*
* Device discovery (i.e. calling libusb_get_device_list()) returns a
* freshly-allocated list of devices. The list itself must be freed when
* you are done with it. libusb also needs to know when it is OK to free
* the contents of the list - the devices themselves.
*
* To handle these issues, libusb provides you with two separate items:
* - A function to free the list itself
* - A reference counting system for the devices inside
*
* New devices presented by the libusb_get_device_list() function all have a
* reference count of 1. You can increase and decrease reference count using
* libusb_ref_device() and libusb_unref_device(). A device is destroyed when
* its reference count reaches 0.
*
* With the above information in mind, the process of opening a device can