USB学习3-USB描述符
本片博客是基于对韦东山老师的USB相关学习视频、《USB2.0》、《MindShare USB 3.0 Technology》、《vdocuments.mx_universal-serial-bus-system-architecture》的学习总结,主要是借鉴了韦东山老师的学习笔记。
USB学习3-USB描述符
- 1. USB设备状态切换图
- 2. 标准设备请求
- 2.1 SETUP事务的数据格式
- 2.2 标准设备请求
- 2.2 设备/配置/接口/端点
- 3. 描述符
- 3.1 设备描述符
- 3.2 设备限定符(Device_Qualifier)
- 3.3 配置描述符
- 3.4 Other_Speed_Configuration:其他速度配置
- 3.5 接口描述符
- 3.6 端点描述符
- 3.7 字符串描述符(String)
- 3.6 示例
- 4. 设备枚举过程示例
1. USB设备状态切换图
2. 标准设备请求
2.1 SETUP事务的数据格式
Host使用控制传输来识别设备、设置设备地址、启动设备的某些特性,对于控制传输,它首先发出"setup事务",如下:
在"setup事务"中,
- SETUP令牌包:用来通知设备,“要开始传输了”
- DATA0数据包:它含有固定的格式,用来告诉设备"是读还是写"、“读什么”、“写什么”
Hos通过DATA0数据包发送8字节数据给设备,它的格式如下图所示:
2.2 标准设备请求
控制传输的建立事务中,可以使用下列格式的数据:
上表中各个"宏"取值如下:
2.2 设备/配置/接口/端点
在SETUP事务的数据里,表示了要访问的是什么:Device?Interface?Endpoint?
对于一个USB设备,它可以多种配置(Configuration)。比如4G上网卡就有2种配置:U盘、上网卡。第1次把4G上网卡插入电脑时,它是一个U盘,可以按照里面的程序。装好程序后,把它再次插入电脑,它就是一个上网卡。驱动程序可以选择让它工作于哪种配置,同一时间只能有一种配置。大多数的USB设备只有一种配置。
一个配置下,可以有多个接口(Interface),接口等同于功能(Function)。比如USB耳机有两个接口(功能):声音收发、按键控制。
一个接口,可能有多个设置(Setting),比如默认设置下它使用较低的带宽,可以选择其他设置以使用更高带宽。
一个接口,由一个或多个端点(Endpoint)组成。端点0属于整个设备的,端点0是双向的。接口还可以有其他端点,这些端点是单向的,要么是批量(Bulk)端点、要么是中断(Interrupt)端点、要么是同步(Isochronous)端点。
3. 描述符
怎么描述设备、配置、接口、端点?使用描述符(Descriptors),有设备描述符、配置描述符、接口描述符、端点描述符。所谓描述符,就是一些格式化的数据,用来描述信息。
一个USB设备,
- 只有一个设备描述符:用来表示设备的ID、它有多少个配置、它的端点0一次最大能传输多少字节数据
- 可能有多个配置描述符:用来表示它有多少个接口、供电方式、最大电流
- 一个配置描述符下面,可能有多个接口描述符:用来表示它是哪类接口、有几个设置(Setting)、有几个端点
- 一个接口描述符符下面,可能有多个端点描述符:用来表示端点号、方向(IN/OUT)、类型(批量/中断/同步)
还有一些字符串描述符(String descriptors),它用可读的文字来描述设备,是可选的。
3.1 设备描述符
3.2 设备限定符(Device_Qualifier)
The device_qualifier descriptor describes information about a high-speed capable device that would change if the device were operating at the other speed. For example, if the device is currently operating at full-speed, the device_qualifier returns information about how it would operate at high-speed and vice-versa. Table 9-9 shows the fields of the device_qualifier descriptor.
device_限定符描述符描述了有关高速能力设备的信息,如果设备以其他速度运行,该设备将会发生变化。例如,如果设备当前全速运行,设备限定符返回有关高速运行的信息,反之亦然。表9-9显示了device_限定符描述符的字段。
3.3 配置描述符
3.4 Other_Speed_Configuration:其他速度配置
The other_speed_configuration descriptor shown in Table 9-11 describes a configuration of a highspeed capable device if it were operating at its other possible speed. The structure of the other_speed_configuration is identical to a configuration descriptor.
表9-11中所示的另一个速度配置描述符描述了一个高速能力设备的配置,如果它以其他可能的速度运行。Other_Speed_Configuration的结构与配置描述符相同。
3.5 接口描述符
3.6 端点描述符
3.7 字符串描述符(String)
String descriptors are optional. As noted previously, if a device does not support string descriptors, all references to string descriptors within device, configuration, and interface descriptors must be reset to zero.
字符串描述符是可选的。如前所述,如果设备不支持字符串描述符,则在设备、配置和接口描述符中对字符串描述符的所有引用都必须重置为零。
The strings in a USB device may support multiple languages. When requesting a string descriptor, the requester specifies the desired language using a sixteenbit language ID (LANGID) defined by the USB-IF. The list of currently defined USB LANGIDs can be found at http://www.usb.org/developers/docs.html. String index zero for all languages returns a string descriptor that contains an array of two-byte LANGID codes supported by the device. Table 9-15 shows the LANGID code array. A USB device may omit all string descriptors. USB devices that omit all string descriptors must not return an array of LANGID codes.
USB设备中的字符串可能支持多种语言。当请求字符串描述符时,请求者使用USB-IF定义的16位语言ID(LANGID)指定所需的语言。当前定义的USBlangid的列表可以在http://www.usb.org/developers/docs.html上找到。所有语言的字符串索引0返回一个字符串描述符,该描述符包含一个由设备支持的双字节LANGID代码数组。表9-15显示了LANGID代码数组。一个USB设备可能会省略所有的字符串描述符。省略所有字符串描述符的USB设备不能返回LANGID代码数组。
The array of LANGID codes is not NULL-terminated. The size of the array (in bytes) is computed by
subtracting two from the value of the first byte of the descriptor.
LANGID代码数组没有空终止。数组的大小(以字节为单位)是通过从描述符的第一个字节的值中减去2来计算的。
The UNICODE string descriptor (shown in Table 9-16) is not NULL-terminated. The string length is
computed by subtracting two from the value of the first byte of the descriptor.
UNICODE字符串描述符(如表9-16所示)并不以NULL结尾。字符串的长度是通过从描述符的第一个字节的值中减去两个来计算的。
3.6 示例
在Ubuntu中可以执行lsusb -v
查看USB设备的描述符信息:
xxx@jimmy:linux-5.10$ lsusb -v
Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Couldn't open device, some information will be missing
Device Descriptor:bLength 18bDescriptorType 1bcdUSB 3.00bDeviceClass 9 HubbDeviceSubClass 0 bDeviceProtocol 3 bMaxPacketSize0 9idVendor 0x1d6b Linux FoundationidProduct 0x0003 3.0 root hubbcdDevice 5.15iManufacturer 3 iProduct 2 iSerial 1 bNumConfigurations 1Configuration Descriptor:bLength 9bDescriptorType 2wTotalLength 0x001fbNumInterfaces 1bConfigurationValue 1iConfiguration 0 bmAttributes 0xe0Self PoweredRemote WakeupMaxPower 0mAInterface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 0bAlternateSetting 0bNumEndpoints 1bInterfaceClass 9 HubbInterfaceSubClass 0 bInterfaceProtocol 0 Full speed (or root) hubiInterface 0 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x81 EP 1 INbmAttributes 3Transfer Type InterruptSynch Type NoneUsage Type DatawMaxPacketSize 0x0004 1x 4 bytesbInterval 12bMaxBurst 0Bus 003 Device 004: ID 0e0f:0002 VMware, Inc. Virtual USB Hub
Couldn't open device, some information will be missing
Device Descriptor:bLength 18bDescriptorType 1bcdUSB 2.00bDeviceClass 9 HubbDeviceSubClass 0 bDeviceProtocol 0 Full speed (or root) hubbMaxPacketSize0 64idVendor 0x0e0f VMware, Inc.idProduct 0x0002 Virtual USB HubbcdDevice 1.00iManufacturer 1 iProduct 2 iSerial 0 bNumConfigurations 1Configuration Descriptor:bLength 9bDescriptorType 2wTotalLength 0x0019bNumInterfaces 1bConfigurationValue 1iConfiguration 1 bmAttributes 0xe0Self PoweredRemote WakeupMaxPower 0mAInterface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 0bAlternateSetting 0bNumEndpoints 1bInterfaceClass 9 HubbInterfaceSubClass 0 bInterfaceProtocol 0 Full speed (or root) hubiInterface 1 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x81 EP 1 INbmAttributes 3Transfer Type InterruptSynch Type NoneUsage Type DatawMaxPacketSize 0x0001 1x 1 bytesbInterval 4Bus 003 Device 003: ID 0e0f:0002 VMware, Inc. Virtual USB Hub
Couldn't open device, some information will be missing
Device Descriptor:bLength 18bDescriptorType 1bcdUSB 1.10bDeviceClass 9 HubbDeviceSubClass 0 bDeviceProtocol 0 Full speed (or root) hubbMaxPacketSize0 8idVendor 0x0e0f VMware, Inc.idProduct 0x0002 Virtual USB HubbcdDevice 1.00iManufacturer 1 iProduct 2 iSerial 0 bNumConfigurations 1Configuration Descriptor:bLength 9bDescriptorType 2wTotalLength 0x0019bNumInterfaces 1bConfigurationValue 1iConfiguration 1 bmAttributes 0xe0Self PoweredRemote WakeupMaxPower 0mAInterface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 0bAlternateSetting 0bNumEndpoints 1bInterfaceClass 9 HubbInterfaceSubClass 0 bInterfaceProtocol 0 Full speed (or root) hubiInterface 1 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x81 EP 1 INbmAttributes 3Transfer Type InterruptSynch Type NoneUsage Type DatawMaxPacketSize 0x0001 1x 1 bytesbInterval 255Bus 003 Device 002: ID 0e0f:0003 VMware, Inc. Virtual Mouse
Couldn't open device, some information will be missing
Device Descriptor:bLength 18bDescriptorType 1bcdUSB 1.10bDeviceClass 0 bDeviceSubClass 0 bDeviceProtocol 0 bMaxPacketSize0 8idVendor 0x0e0f VMware, Inc.idProduct 0x0003 Virtual MousebcdDevice 1.03iManufacturer 1 iProduct 2 iSerial 0 bNumConfigurations 1Configuration Descriptor:bLength 9bDescriptorType 2wTotalLength 0x0022bNumInterfaces 1bConfigurationValue 1iConfiguration 1 bmAttributes 0xc0Self PoweredMaxPower 0mAInterface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 0bAlternateSetting 0bNumEndpoints 1bInterfaceClass 3 Human Interface DevicebInterfaceSubClass 1 Boot Interface SubclassbInterfaceProtocol 2 MouseiInterface 1 HID Device Descriptor:bLength 9bDescriptorType 33bcdHID 1.10bCountryCode 0 Not supportedbNumDescriptors 1bDescriptorType 34 ReportwDescriptorLength 46Report Descriptors: ** UNAVAILABLE **Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x81 EP 1 INbmAttributes 3Transfer Type InterruptSynch Type NoneUsage Type DatawMaxPacketSize 0x0008 1x 8 bytesbInterval 1Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Couldn't open device, some information will be missing
Device Descriptor:bLength 18bDescriptorType 1bcdUSB 2.00bDeviceClass 9 HubbDeviceSubClass 0 bDeviceProtocol 1 Single TTbMaxPacketSize0 64idVendor 0x1d6b Linux FoundationidProduct 0x0002 2.0 root hubbcdDevice 5.15iManufacturer 3 iProduct 2 iSerial 1 bNumConfigurations 1Configuration Descriptor:bLength 9bDescriptorType 2wTotalLength 0x0019bNumInterfaces 1bConfigurationValue 1iConfiguration 0 bmAttributes 0xe0Self PoweredRemote WakeupMaxPower 0mAInterface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 0bAlternateSetting 0bNumEndpoints 1bInterfaceClass 9 HubbInterfaceSubClass 0 bInterfaceProtocol 0 Full speed (or root) hubiInterface 0 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x81 EP 1 INbmAttributes 3Transfer Type InterruptSynch Type NoneUsage Type DatawMaxPacketSize 0x0004 1x 4 bytesbInterval 12Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Couldn't open device, some information will be missing
Device Descriptor:bLength 18bDescriptorType 1bcdUSB 2.00bDeviceClass 9 HubbDeviceSubClass 0 bDeviceProtocol 0 Full speed (or root) hubbMaxPacketSize0 64idVendor 0x1d6b Linux FoundationidProduct 0x0002 2.0 root hubbcdDevice 5.15iManufacturer 3 iProduct 2 iSerial 1 bNumConfigurations 1Configuration Descriptor:bLength 9bDescriptorType 2wTotalLength 0x0019bNumInterfaces 1bConfigurationValue 1iConfiguration 0 bmAttributes 0xe0Self PoweredRemote WakeupMaxPower 0mAInterface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 0bAlternateSetting 0bNumEndpoints 1bInterfaceClass 9 HubbInterfaceSubClass 0 bInterfaceProtocol 0 Full speed (or root) hubiInterface 0 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x81 EP 1 INbmAttributes 3Transfer Type InterruptSynch Type NoneUsage Type DatawMaxPacketSize 0x0004 1x 4 bytesbInterval 12Bus 002 Device 003: ID 0e0f:0002 VMware, Inc. Virtual USB Hub
Couldn't open device, some information will be missing
Device Descriptor:bLength 18bDescriptorType 1bcdUSB 1.10bDeviceClass 9 HubbDeviceSubClass 0 bDeviceProtocol 0 Full speed (or root) hubbMaxPacketSize0 8idVendor 0x0e0f VMware, Inc.idProduct 0x0002 Virtual USB HubbcdDevice 1.00iManufacturer 1 iProduct 2 iSerial 0 bNumConfigurations 1Configuration Descriptor:bLength 9bDescriptorType 2wTotalLength 0x0019bNumInterfaces 1bConfigurationValue 1iConfiguration 1 bmAttributes 0xe0Self PoweredRemote WakeupMaxPower 0mAInterface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 0bAlternateSetting 0bNumEndpoints 1bInterfaceClass 9 HubbInterfaceSubClass 0 bInterfaceProtocol 0 Full speed (or root) hubiInterface 1 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x81 EP 1 INbmAttributes 3Transfer Type InterruptSynch Type NoneUsage Type DatawMaxPacketSize 0x0001 1x 1 bytesbInterval 255Bus 002 Device 002: ID 0e0f:0008 VMware, Inc. Virtual Bluetooth Adapter
Couldn't open device, some information will be missing
Device Descriptor:bLength 18bDescriptorType 1bcdUSB 2.00bDeviceClass 224 WirelessbDeviceSubClass 1 Radio FrequencybDeviceProtocol 1 BluetoothbMaxPacketSize0 64idVendor 0x0e0f VMware, Inc.idProduct 0x0008 bcdDevice 1.00iManufacturer 1 iProduct 2 iSerial 3 bNumConfigurations 1Configuration Descriptor:bLength 9bDescriptorType 2wTotalLength 0x00b1bNumInterfaces 2bConfigurationValue 1iConfiguration 0 bmAttributes 0xc0Self PoweredMaxPower 0mAInterface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 0bAlternateSetting 0bNumEndpoints 3bInterfaceClass 224 WirelessbInterfaceSubClass 1 Radio FrequencybInterfaceProtocol 1 BluetoothiInterface 0 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x81 EP 1 INbmAttributes 3Transfer Type InterruptSynch Type NoneUsage Type DatawMaxPacketSize 0x0010 1x 16 bytesbInterval 1Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x02 EP 2 OUTbmAttributes 2Transfer Type BulkSynch Type NoneUsage Type DatawMaxPacketSize 0x0040 1x 64 bytesbInterval 0Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x82 EP 2 INbmAttributes 2Transfer Type BulkSynch Type NoneUsage Type DatawMaxPacketSize 0x0040 1x 64 bytesbInterval 0Interface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 1bAlternateSetting 0bNumEndpoints 2bInterfaceClass 224 WirelessbInterfaceSubClass 1 Radio FrequencybInterfaceProtocol 1 BluetoothiInterface 0 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x03 EP 3 OUTbmAttributes 1Transfer Type IsochronousSynch Type NoneUsage Type DatawMaxPacketSize 0x0000 1x 0 bytesbInterval 1Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x83 EP 3 INbmAttributes 1Transfer Type IsochronousSynch Type NoneUsage Type DatawMaxPacketSize 0x0000 1x 0 bytesbInterval 1Interface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 1bAlternateSetting 1bNumEndpoints 2bInterfaceClass 224 WirelessbInterfaceSubClass 1 Radio FrequencybInterfaceProtocol 1 BluetoothiInterface 0 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x03 EP 3 OUTbmAttributes 1Transfer Type IsochronousSynch Type NoneUsage Type DatawMaxPacketSize 0x0009 1x 9 bytesbInterval 1Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x83 EP 3 INbmAttributes 1Transfer Type IsochronousSynch Type NoneUsage Type DatawMaxPacketSize 0x0009 1x 9 bytesbInterval 1Interface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 1bAlternateSetting 2bNumEndpoints 2bInterfaceClass 224 WirelessbInterfaceSubClass 1 Radio FrequencybInterfaceProtocol 1 BluetoothiInterface 0 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x03 EP 3 OUTbmAttributes 1Transfer Type IsochronousSynch Type NoneUsage Type DatawMaxPacketSize 0x0011 1x 17 bytesbInterval 1Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x83 EP 3 INbmAttributes 1Transfer Type IsochronousSynch Type NoneUsage Type DatawMaxPacketSize 0x0011 1x 17 bytesbInterval 1Interface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 1bAlternateSetting 3bNumEndpoints 2bInterfaceClass 224 WirelessbInterfaceSubClass 1 Radio FrequencybInterfaceProtocol 1 BluetoothiInterface 0 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x03 EP 3 OUTbmAttributes 1Transfer Type IsochronousSynch Type NoneUsage Type DatawMaxPacketSize 0x0019 1x 25 bytesbInterval 1Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x83 EP 3 INbmAttributes 1Transfer Type IsochronousSynch Type NoneUsage Type DatawMaxPacketSize 0x0019 1x 25 bytesbInterval 1Interface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 1bAlternateSetting 4bNumEndpoints 2bInterfaceClass 224 WirelessbInterfaceSubClass 1 Radio FrequencybInterfaceProtocol 1 BluetoothiInterface 0 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x03 EP 3 OUTbmAttributes 1Transfer Type IsochronousSynch Type NoneUsage Type DatawMaxPacketSize 0x0021 1x 33 bytesbInterval 1Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x83 EP 3 INbmAttributes 1Transfer Type IsochronousSynch Type NoneUsage Type DatawMaxPacketSize 0x0021 1x 33 bytesbInterval 1Interface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 1bAlternateSetting 5bNumEndpoints 2bInterfaceClass 224 WirelessbInterfaceSubClass 1 Radio FrequencybInterfaceProtocol 1 BluetoothiInterface 0 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x03 EP 3 OUTbmAttributes 1Transfer Type IsochronousSynch Type NoneUsage Type DatawMaxPacketSize 0x0031 1x 49 bytesbInterval 1Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x83 EP 3 INbmAttributes 1Transfer Type IsochronousSynch Type NoneUsage Type DatawMaxPacketSize 0x0031 1x 49 bytesbInterval 1Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Couldn't open device, some information will be missing
Device Descriptor:bLength 18bDescriptorType 1bcdUSB 1.10bDeviceClass 9 HubbDeviceSubClass 0 bDeviceProtocol 0 Full speed (or root) hubbMaxPacketSize0 64idVendor 0x1d6b Linux FoundationidProduct 0x0001 1.1 root hubbcdDevice 5.15iManufacturer 3 iProduct 2 iSerial 1 bNumConfigurations 1Configuration Descriptor:bLength 9bDescriptorType 2wTotalLength 0x0019bNumInterfaces 1bConfigurationValue 1iConfiguration 0 bmAttributes 0xe0Self PoweredRemote WakeupMaxPower 0mAInterface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 0bAlternateSetting 0bNumEndpoints 1bInterfaceClass 9 HubbDeviceSubClass 0 bDeviceProtocol 0 Full speed (or root) hubbMaxPacketSize0 64idVendor 0x1d6b Linux FoundationidProduct 0x0001 1.1 root hubbcdDevice 5.15iManufacturer 3 iProduct 2 iSerial 1 bNumConfigurations 1Configuration Descriptor:bLength 9bDescriptorType 2wTotalLength 0x0019bNumInterfaces 1bConfigurationValue 1iConfiguration 0 bmAttributes 0xe0Self PoweredRemote WakeupMaxPower 0mAInterface Descriptor:bLength 9bDescriptorType 4bInterfaceNumber 0bAlternateSetting 0bNumEndpoints 1bInterfaceClass 9 HubbInterfaceSubClass 0 bInterfaceProtocol 0 Full speed (or root) hubiInterface 0 Endpoint Descriptor:bLength 7bDescriptorType 5bEndpointAddress 0x81 EP 1 INbmAttributes 3Transfer Type InterruptSynch Type NoneUsage Type DatawMaxPacketSize 0x0002 1x 2 bytesbInterval 255
xxx@jimmy:linux-5.10$
4. 设备枚举过程示例
使用"usbprotocolsuite"打开,可以看到设备的枚举过程:
- 使用控制传输,读取设备信息(设备描述符):第一次读取时,它只需要得到8字节数据,因为第8个数据表示端点0能传输的最大数据长度。
- Host分配地址给设备,然后把新地址发给设备:
- 使用新地址,重新读取设备描述符,设备描述符长度是18:
- 读取配置描述符:它传入的长度是255,想一次性把当前配置描述符、它下面的接口描述符、端点描述符全部读出来
- 读取字符描述符
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