libusb: android上集成libusb库

1. 下载libusb库。

可以到libusb库的官网(https://libusb.info/)或者是其官方的github仓库(https://github.com/libusb/libusb/releases)上下载最新的libusb库源码。

2. 添加libusb库到android studio项目中。

这里以源码编译的方式添加，使用的仍然是ndk-build的方式，而非cmake，使用源码编译的好处在于，可以随时调试libusb库的源码，方便加log。

2.1 在app/src/main目录下新建一个jni目录，用于存放libusb库的源代码。

2.2 打开app目录下的build.gradle文件，指定JNI目录：

2.3 在jni目录下新建libusb-support目录和makefile文件，libusb-support目录用于存放libusb库的源代码。

这里单独新建一个libusb-support目录的目的在于，将来可能会有很多个第三方模块，比如libusb,libyuv, sdl，分目录存放可以划分得更清楚，可以直接把libusb的代码放到jni目录下，只要makefile能找到就行。
Android.mk

#
# Copyright (c) 2019 Realsil.Inc. All rights reserved.
#
include $(call all-subdir-makefiles)

Application.mk

#
# Copyright (c) 2019 SEP.Inc. All rights reserved.
#

APP_ABI := all

APP_LDFLAGS := -llog

最终目录结构如下：

2.3 拷贝libusb库的源代码到libusb-support目录

打开下载的libusb库源代码，找到libusb文件夹，将整个文件拷贝到libusb-support目录下，我下载的是1.0.23版本：

打开android文件夹，拷贝config.h文件到libusb-support目录下

打开jni文件夹，拷贝Android.mk、libusb.mk文件到libusb-support目录下：

最终目录结构如下：

接下来修改Android.mk和libusb.mk文件，默认编译出来的库会包含一些测试用例和示例代码，我们不需要它。
修改Android.mk, 注释掉example.mk和tests.mk

# Android build config for libusb, examples and tests
# Copyright © 2012-2013 RealVNC Ltd. <toby.gray@realvnc.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
#

LOCAL_PATH:= $(call my-dir)

include $(LOCAL_PATH)/libusb.mk

# include $(LOCAL_PATH)/examples.mk
# include $(LOCAL_PATH)/tests.mk

修改libusb.mk, 更新当前源代码的位置：

# Android build config for libusb
# Copyright © 2012-2013 RealVNC Ltd. <toby.gray@realvnc.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
#

LOCAL_PATH:= $(call my-dir)
LIBUSB_ROOT_REL:= .
LIBUSB_ROOT_ABS:= $(LOCAL_PATH)

# libusb

include $(CLEAR_VARS)

LIBUSB_ROOT_REL:= .
LIBUSB_ROOT_ABS:= $(LOCAL_PATH)

LOCAL_SRC_FILES := \
  $(LIBUSB_ROOT_REL)/libusb/core.c \
  $(LIBUSB_ROOT_REL)/libusb/descriptor.c \
  $(LIBUSB_ROOT_REL)/libusb/hotplug.c \
  $(LIBUSB_ROOT_REL)/libusb/io.c \
  $(LIBUSB_ROOT_REL)/libusb/sync.c \
  $(LIBUSB_ROOT_REL)/libusb/strerror.c \
  $(LIBUSB_ROOT_REL)/libusb/os/linux_usbfs.c \
  $(LIBUSB_ROOT_REL)/libusb/os/poll_posix.c \
  $(LIBUSB_ROOT_REL)/libusb/os/threads_posix.c \
  $(LIBUSB_ROOT_REL)/libusb/os/linux_netlink.c \
  $(LIBUSB_ROOT_REL)/libusb/realsil_usb_audio.c \
  $(LIBUSB_ROOT_REL)/libusb/jnihelp.cpp


LOCAL_C_INCLUDES += \
  $(LOCAL_PATH)/ \
  $(LIBUSB_ROOT_ABS)/libusb \
  $(LIBUSB_ROOT_ABS)/libusb/os

LOCAL_EXPORT_C_INCLUDES := \
  $(LIBUSB_ROOT_ABS)/libusb

LOCAL_LDLIBS := -llog

LOCAL_MODULE := usb

include $(BUILD_SHARED_LIBRARY)