Serial Communication C++ ReadFile() - c++

Question

I created 2 functions to read and write across a serial port, I am coding in c++ with visual studios 2012, windows 7, 64 bit operating system, and using RS-232 serial cord. The board I'm connecting to is supposed to send 5 characters, TRG 1, upon pressing a button, the code works, however the output isn't always the correct values.
char serialRead()
{
char input[5];
DCB dcBus;
HANDLE hSerial;
DWORD bytesRead, eventMask;
COMMTIMEOUTS timeouts;
hSerial = CreateFile (L"\\\\.\\COM13", GENERIC_READ, 0, NULL, OPEN_EXISTING, 0, NULL);
if (hSerial == INVALID_HANDLE_VALUE)
{
cout << "error opening handle\n";
}
else
{
cout << "port opened\n";
}
dcBus.DCBlength = sizeof(dcBus);
if ((GetCommState(hSerial, &dcBus) == 0))
{
cout << "error getting comm state\n";
}
dcBus.BaudRate = CBR_9600;
dcBus.ByteSize = DATABITS_8;
dcBus.Parity = NOPARITY;
dcBus.StopBits = ONESTOPBIT;
if ((GetCommState(hSerial, &dcBus) == 0))
{
cout << "error setting comm state\n";
}
if ((GetCommTimeouts(hSerial, &timeouts) == 0))
{
cout << "error getting timeouts\n";
}
timeouts.ReadIntervalTimeout = 10;
timeouts.ReadTotalTimeoutMultiplier = 1;
timeouts.ReadTotalTimeoutConstant = 500;
timeouts.WriteTotalTimeoutMultiplier = 1;
timeouts.WriteTotalTimeoutConstant = 500;
if (SetCommTimeouts(hSerial, &timeouts) == 0)
{
cout << "error setting timeouts\n";
}
if (SetCommMask(hSerial, EV_RXCHAR) == 0)
{
cout << "error setting comm mask\n";
}
if (WaitCommEvent(hSerial, &eventMask, NULL))
{
if (ReadFile(hSerial, &input, 5, &bytesRead, NULL) !=0)
{
for (int i = 0; i < sizeof(input); i++)
{
cout << input[i];
}
cout << endl;
}
else
{
cout << "error reading file\n";
}
}
else
{
cout << "error waiting for comm event\n";
}
switch (input[4])
{
case '1' :
CloseHandle(hSerial);
return '1';
break;
case '2' :
CloseHandle(hSerial);
return '2';
break;
case '3' :
CloseHandle(hSerial);
return '3';
break;
case '4' :
CloseHandle(hSerial);
return '4';
break;
case '5':
CloseHandle(hSerial);
return '5';
break;
default :
CloseHandle(hSerial);
return '9';
break;
}
}
The code runs successfully in the sense that the port is configured correctly and data is being transmitted. The output varies, most of the time the output will print the whole "TRG 1", but randomly (it seems), the output will be "TRG|}|}" or "T|}|}|}|}", i.e. it will be part of the string and every character missing will be replaced with a "|}" instead of the correct characters. This is a problem because I want to be able to send it different values for trigger and run the switch of that variable.
I'm relatively new to serial communication and not an expert programmer so I'm wondering what's going on?

Answer 1

Serial communication is not packet-based. The information doesn't come to you in packages where the entire message can necessarily be read in one go; instead, it's a stream, so you could read half a message, a whole message, more than one message, etc.
As zdan said in the comments, you need to check the number bytes read from ReadFile and use that to compose 5-character packages which are your messages.
Specifically, only the first couple of characters up to the returned number of bytes read are valid; the rest are garbage.