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初始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 
    1  status = IoCreateDevice(DriverObject,
2         sizeof(VCP4USB_DEVICE_EXTENSION),
3         &ntDeviceName,
4         FILE_DEVICE_SERIAL_PORT,
5         0,
6         TRUE, // exclusive
7         &fdo);

     

  • 获取USB设备名称用于打开设备

     1   status = IoGetDeviceInterfaces(pGuid, NULL, 0, &pSymLink);
 2     DPrint(DBG_OTHER, ("IoGetDeviceInterface return %d/n", status));
 3     
 4     if ((status == STATUS_INVALID_DEVICE_REQUEST) || (*pSymLink == NULL))
 5         return STATUS_UNSUCCESSFUL;
 6     
 7     pCur = pSymLink;
 8     instanceCur = 0;
 9     status = STATUS_INVALID_INFO_CLASS;
10     while (*pCur != NULL)
11     {
12         p = pCur;
13         for (size = 0; *p != NULL; size ++)
14             p ++;
15         DPrint(DBG_OTHER, ("No.%d: size=%d/n", instanceCur, size));
16         DPrint(DBG_OTHER, ("name:%ws/n", pCur));
17         if (instance == instanceCur)
18         {
19             if (RtlCompareMemory(pCur, pPrefix, prefixLength) == prefixLength)
20             {
21                 DPrint(DBG_OTHER, ("Find OK/n"));
22                 devName->MaximumLength = size * sizeof(WCHAR);
23                 devName->Buffer = (PWSTR)ExAllocatePool(NonPagedPool, (size + 1) * sizeof(WCHAR));
24                 if (devName->Buffer == NULL)
25                 {
26                     DPrint(DBG_OTHER, ("Allocate devName error./n"));
27                     status = STATUS_INSUFFICIENT_RESOURCES;
28                     break;
29                 }
30                 RtlCopyMemory(devName->Buffer, pCur, (size+1) * sizeof(WCHAR));
31                 RtlInitUnicodeString(devName, devName->Buffer);
32                 status = STATUS_SUCCESS;
33                 break; // find ok and break
34             }
35         }
36         pCur += size + 1; // skip last NULL
37         instanceCur ++;
38         if (instanceCur >= 2) // for debug
39             break;
40     }
41     
42     ExFreePool(pSymLink);
43     return status;

  • 打开USB设备端口

    1   curStatus = GetUsbDeviceName(&usbDeviceName, &GUID_CLASS_XXXX_BULK,
2                 &XXXX_SYMLINK_CMPSTR, XXXX_SYMLINK_STRLEN, 0);
3   if (!hRead)
4       rdStatus = createFile(&hRead, &usbDeviceName, L"//PIPE0", 6, GENERIC_READ);
5   if (!hWrite)
6       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!

posted on 2013-02-02 18:34  WoLF89  阅读(1474)  评论(0)  编辑  收藏  举报

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