Windows XP下USB轉串口驅動編碼實現分析

       USB轉COM驅動的編寫實現有很多中方法,最近在網絡上看到一個最常用的方式,即是虛擬一個COM口,在COM初試的時候進行打開一個USB設備.

       當USB串COM口驅動處理Write和Read 等IRQ的時候,其實是去讀寫USB 設備驅動,讀寫方式是直接採用ZwReadFile和ZwWriteFile函數.但是真正的實現也不是這麼簡單,真實的實現是開闢一個線程和一段較大的緩衝區,線程用於適時讀取USB設備的數據並保存在緩衝區當中.當上層應用程序向這個虛擬的COM口發送READ IRP(IRP_MJ_READ)的時候,則直接從這段緩衝區中讀取數據返回即可.當然還不要忘記處理超時的問題處理.

       因此USB轉COM驅動在INF文件加載的時候實際上需要加載兩個驅動程序,一個是USB驅動,對應與USB設備的VID和PID,另外一個就是COM虛擬驅動程序,需要對應設置COM口號.如下將對關鍵代碼列出;

• 創建COM口設備對象

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

• 獲取USB設備名稱用於打開設備

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;

•     打開USB設備端口 

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);

    其他的驅動程序處理都是常規處理,在這裏就不多做介紹!