usbser.sys: ReadFile success with 0 bytes read

Hello everybody,

I am having trouble making Windows API ReadFile function working
properly with USB CDC (Communication Device Class) driver.
Specifically, I am using CDC drivers supplied with Windows XP:
usbser.sys 5.1.2600.5512
serenum.sys 5.1.2600.5512

In the sample code below, the first ReadFile in the loop (marked with (*))
sometimes exit with success and 0 bytes read (only after timeout elapses)
and then the subsequent ReadFile gets all the bytes correctly (9 bytes in
my test).
So this piece of software is working correctly and getting all responses to
sent serial commands, but sometimes there are timeout delays for the first
ReadFile going wrong and I want to avoid these.

Note that the second ReadFile is always working fine (no "(2) Read"
error).

I tried to tune the delay in the answering device between the first
character received and the response transmission, founding that there is
a peak of faulty ReadFiles at about 70 microseconds of delay (460 every
1,000,000 tests), while this number is going to about 50 per million at
minimum delay, and to zero per million (all OK) above 250 microseconds.

If I try to use some CDC demo driver found in the net (alternative to
usbser.sys), the faulty ReadFiles number is dramatically reduced, however
not null, being 1 per million with a 70 microseconds delay.

The device is using bulk mode with 512 bytes buffer (transmission is forced
after the last byte if the number of bytes is less than 512, like in my
test).

Have anyone already dealt with this problem?
Any solution other than waiting more than 250 microseconds before
answering?
Other ideas?

Thank you very much in advance.

//---------------------------------------------------------------------------

#include <windows.h>
#include <iostream.h>
#include <iomanip.h>
#include <string.h>

#pragma hdrstop

//---------------------------------------------------------------------------

int main()
{ HANDLE hSerial; DWORD dwError; DCB dcbSerialParams = {0}; COMMTIMEOUTS timeouts = {0}; char szRead[101] = {0}; char szWrite[101] = {0}; DWORD dwBytesToRead = 0; DWORD dwBytesRead = 0; DWORD dwBytesLeft = 0; DWORD dwBytesToWrite = 0; DWORD dwBytesWritten = 0; unsigned long uTestIdx; unsigned long uTestNum; unsigned long uErrorNum; SYSTEMTIME lt; char sCOMport[7] = {0}; char sCOMportIdx[4] = {0};

cout << "--- Test ReadFile ---" << endl;

cout << "Enter COM port index: "; cin >> sCOMportIdx; if (sCOMportIdx == NULL) { cout << "Please specify a COM port." << endl; return -1; } sprintf(sCOMport, "COM%s", sCOMportIdx);

cout << "Enter string to send (<CR> will be appended): "; cin >> szWrite; strcat(szWrite, "\015"); dwBytesToWrite = strlen(szWrite);

cout << "Enter bytes to read: "; cin >> dwBytesToRead;

cout << "Enter number of tests: "; cin >> uTestNum;

hSerial = CreateFile ( sCOMport, GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0 );

if (hSerial == INVALID_HANDLE_VALUE) { dwError = GetLastError(); if (dwError == ERROR_FILE_NOT_FOUND) cout << "Serial port " << sCOMport << " does not exist." << endl; else cout << "Error " << dwError; return -1; }

cout << "Port open." << endl;

dcbSerialParams.DCBlength = sizeof(dcbSerialParams); if (!GetCommState(hSerial, &dcbSerialParams)) cout << "Error getting serial port state." << endl; dcbSerialParams.BaudRate = CBR_115200; dcbSerialParams.ByteSize = 8; dcbSerialParams.StopBits = ONESTOPBIT; dcbSerialParams.Parity = EVENPARITY; if (!SetCommState(hSerial, &dcbSerialParams)) cout << "Error setting serial port state." << endl;

timeouts.ReadIntervalTimeout = 0; timeouts.ReadTotalTimeoutMultiplier = 1; timeouts.ReadTotalTimeoutConstant = 350; timeouts.WriteTotalTimeoutConstant = 0; timeouts.WriteTotalTimeoutMultiplier = 0; if (!SetCommTimeouts(hSerial, &timeouts)) cout << "Error setting serial port timeouts." << endl;

GetLocalTime(<); cout << "Time = " << setw(2) << setfill('0') << lt.wHour << ":" << setw(2) << setfill('0') << lt.wMinute << ":" << setw(2) << setfill('0') << lt.wSecond << endl;

for (uTestIdx = 0, uErrorNum = 0; uTestIdx < uTestNum; uTestIdx++) { if (!WriteFile(hSerial, szWrite, dwBytesToWrite, &dwBytesWritten, NULL)) cout << "WriteFile error " << GetLastError() << endl;

if (!ReadFile(hSerial, szRead, dwBytesToRead, &dwBytesRead, NULL)) // (*) ReadFile success with 0 bytes read cout << "ReadFile error " << GetLastError() << endl; else { if (dwBytesRead < dwBytesToRead) { cout << "Error " << setw(10) << setfill(' ') << (++uErrorNum) << " - Read " << dwBytesRead << " bytes = " << ((dwBytesRead != 0) ? szRead : "") << endl; dwBytesLeft = dwBytesToRead - dwBytesRead; if (!ReadFile(hSerial, szRead, dwBytesLeft, &dwBytesRead, NULL)) cout << "ReadFile error " << GetLastError() << endl; if (dwBytesRead != dwBytesLeft) cout << "Error " << setw(10) << setfill(' ') << (++uErrorNum) << " - (2) Read " << dwBytesRead << " bytes = " << ((dwBytesRead != 0) ? szRead : "") << endl; } } }

GetLocalTime(<); cout << "Time = " << setw(2) << setfill('0') << lt.wHour << ":" << setw(2) << setfill('0') << lt.wMinute << ":" << setw(2) << setfill('0') << lt.wSecond << endl;

CloseHandle(hSerial);

cout << "Port closed." << endl;

return 0;
} //---------------------------------------------------------------------------