Windows XP下USB转串口驱动编码实现分析

USB转COM驱动的编写实现有很多中方法,最近在网络上看到一个最常用的方式,即是虚拟一个COM口,在COM初试的时候进行打开一个USB设备.

当USB串COM口驱动处理Write和Read 等IRQ的时候,其实是去读写USB 设备驱动,读写方式是直接采用ZwReadFile和ZwWriteFile函数.但是真正的实现也不是这么简单,真实的实现是开辟一个线程和一段较大的缓冲区,线程用于适时读取USB设备的数据并保存在缓冲区当中.当上层应用程序向这个虚拟的COM口发送READ IRP(IRP_MJ_READ)的时候,则直接从这段缓冲区中读取数据返回即可.当然还不要忘记处理超时的问题处理.

因此USB转COM驱动在INF文件加载的时候实际上需要加载两个驱动程序,一个是USB驱动,对应与USB设备的VID和PID,另外一个就是COM虚拟驱动程序,需要对应设置COM口号.如下将对关键代码列出;

• 创建COM口设备对象

 status = IoCreateDevice(DriverObject,
        sizeof(VCP4USB_DEVICE_EXTENSION),
        &ntDeviceName,
        FILE_DEVICE_SERIAL_PORT,
        0,
        TRUE, // exclusive
        &fdo);

• 获取USB设备名称用于打开设备

  status = IoGetDeviceInterfaces(pGuid, NULL, 0, &pSymLink);
    DPrint(DBG_OTHER, ("IoGetDeviceInterface return %d/n", status));
    
    if ((status == STATUS_INVALID_DEVICE_REQUEST) || (*pSymLink == NULL))
        return STATUS_UNSUCCESSFUL;
    
    pCur = pSymLink;
    instanceCur = 0;
    status = STATUS_INVALID_INFO_CLASS;
    while (*pCur != NULL)
    {
        p = pCur;
        for (size = 0; *p != NULL; size ++)
            p ++;
        DPrint(DBG_OTHER, ("No.%d: size=%d/n", instanceCur, size));
        DPrint(DBG_OTHER, ("name:%ws/n", pCur));
        if (instance == instanceCur)
        {
            if (RtlCompareMemory(pCur, pPrefix, prefixLength) == prefixLength)
            {
                DPrint(DBG_OTHER, ("Find OK/n"));
                devName->MaximumLength = size * sizeof(WCHAR);
                devName->Buffer = (PWSTR)ExAllocatePool(NonPagedPool, (size + 1) * sizeof(WCHAR));
                if (devName->Buffer == NULL)
                {
                    DPrint(DBG_OTHER, ("Allocate devName error./n"));
                    status = STATUS_INSUFFICIENT_RESOURCES;
                    break;
                }
                RtlCopyMemory(devName->Buffer, pCur, (size+1) * sizeof(WCHAR));
                RtlInitUnicodeString(devName, devName->Buffer);
                status = STATUS_SUCCESS;
                break; // find ok and break
            }
        }
        pCur += size + 1; // skip last NULL
        instanceCur ++;
        if (instanceCur >= 2) // for debug
            break;
    }
    
    ExFreePool(pSymLink);
    return status;

• 打开USB设备端口

  curStatus = GetUsbDeviceName(&usbDeviceName, &GUID_CLASS_XXXX_BULK,
                &XXXX_SYMLINK_CMPSTR, XXXX_SYMLINK_STRLEN, 0);
  if (!hRead)
      rdStatus = createFile(&hRead, &usbDeviceName, L"//PIPE0", 6, GENERIC_READ);
  if (!hWrite)
      wtStatus = createFile(&hWrite, &usbDeviceName, L"//PIPE1", 6, GENERIC_WRITE);

其他的驱动程序处理都是常规处理,在这里就不多做介绍!