Is it normal for winapi to take around 70ms to read from serial port? - winapi

Question

The true time it takes from when I send the first bit to a serial port to when I receive the last bit it pings back I measured to be 6ms but ReadFile takes around 70-80ms. I'm wondering if this is expected, is this just Windows or is it my code at fault? Here's the function to send and read from the serial port, in my main I have declared and initialized the HANDLE and called that function.
int sendBytes(char* command, char* COM, HANDLE hSerial, int read) {
BOOL Write_Status;
DCB dcbSerialParams = { 0 }; // Initializing DCB structure
dcbSerialParams.DCBlength = sizeof(dcbSerialParams);
Write_Status = GetCommState(hSerial, &dcbSerialParams); //retreives the current settings
if (Write_Status == FALSE) {
printf("\n Error! in GetCommState()");
CloseHandle(hSerial);
return 1;
}
dcbSerialParams.BaudRate = CBR_57600;
dcbSerialParams.ByteSize = 8;
dcbSerialParams.StopBits = ONESTOPBIT;
dcbSerialParams.Parity = NOPARITY;
Write_Status = SetCommState(hSerial, &dcbSerialParams); //Configuring the port according to settings in DCB
if (Write_Status == FALSE)
{
CloseHandle(hSerial);
return 1;
}
///*----------------------------- Writing a Character to Serial Port----------------------------------------*/
int length = strlen(command);
char send[20];
strcpy(send, command);
send[length + 1] = 13;
send[length + 2] = 10;
DWORD dNoOFBytestoWrite; // No of bytes to write into the port
DWORD dNoOfBytesWritten = 0; // No of bytes written to the port
dNoOFBytestoWrite = length + 2; // Calculating the no of bytes to write into the port
if (!WriteFile(hSerial, send, dNoOFBytestoWrite, &dNoOfBytesWritten, NULL))
printf("Error writing text to %s\n", COM);
if (read) {
int maxChars = 100;
BOOL Read_Status; // Status of the various operations
DWORD dwEventMask; // Event mask to trigger
char SerialBuffer[100]; // Buffer Containing Rxed Data
DWORD NoBytesRead; // Bytes read by ReadFile()
///*------------------------------------ Setting Receive Mask ----------------------------------------------*/
Read_Status = SetCommMask(hSerial, EV_RXCHAR); //Configure Windows to Monitor the serial device for Character Reception
if (Read_Status == FALSE)
printf("\n\n Error! in Setting CommMask");
// else
// printf("\n\n Setting CommMask successfull");
///*------------------------------------ Setting WaitComm() Event ----------------------------------------*/
// printf("\n\n Waiting for Data Reception");
Read_Status = WaitCommEvent(hSerial, &dwEventMask, NULL); //Wait for the character to be received
// /*-------------------------- Program will Wait here till a Character is received ------------------------*/
if (Read_Status == FALSE)
{
printf("\n Error! in Setting WaitCommEvent()");
}
else //If WaitCommEvent()==True Read the RXed data using ReadFile();
{
// printf("\n\n Characters Received \t");
clock_t begin = clock();
if (!ReadFile(hSerial, SerialBuffer, 24, &NoBytesRead, NULL))
{
printf("wrong character");
return 1;
}
clock_t end = clock();
double time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
printf("time : %f\n",time_spent);
}
}
}

Answer 1

This is not how you measure timing with sub-second precision:
clock_t begin = clock();
// stuff
clock_t end = clock();
double time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
This is how you measure timing:
LARGE_INTEGER before, after, frequency;
QueryPerformanceCounter(&before);
// stuff
QueryPerformanceCounter(&after);
QueryPerformanceFrequency(&frequency);
double time_spent = (after.QuadPart - before.QuadPart) / (double)frequency.QuadPart;
CLOCKS_PER_SEC is imprecise, and then clock() can be even worse, often as bad as the scheduler quantum which is typically 10ms or 15ms.