1 Opening the serial port
Opening the serial port is very easy, especially if you’ve ever done Windows file I/O before. First, make sure you include windows.h. You can then use the following code to open it:
HANDLE hSerial;
hSerial = CreateFile("COM1",
GENERIC_READ | GENERIC_WRITE,
0,
0,
OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL,
0);
if(hSerial==INVALID_HANDLE_VALUE){
if(GetLastError()==ERROR_FILE_NOT_FOUND){
//serial port does not exist. Inform user.
}
//some other error occurred. Inform user.
}

So let’s walk through this. First, we declare a variable of type HANDLE and initialize it with a call to
CreateFile. The first argument to CreateFile is simply the name of the file you want to open. In this
case, we want to open a serial port, so we use "COM1", "COM2", etc. The next argument tells Windows whether you want to read or write to the serial port. If you don’t need to do one of these, just leave it out. Arguments 3 and 4 should pretty much always be 0. The next argument tells us that Windows should only open an existing file, and since serial ports already exist, this is what we want. After this comes FILE ATTRIBUTE NORMAL, which just tells Windows we don’t want anything fancy here. The final argument should also be zero.

2 Setting Parameters
So now that we have a HANDLE to the serial port, we need to set the parameters for it (baud rate, stop bits, etc).
Windows does this through a struct called DCB:

Code:
DCB dcbSerialParams = {0};
dcbSerial.DCBlength=sizeof(dcbSerialParams);
if (!GetCommState(hSerial, &dcbSerialParams)) {
//error getting state
}
dcbSerialParams.BaudRate=CBR_19200;
dcbSerialParams.ByteSize=8;
dcbSerialParams.StopBits=ONESTOPBIT;
dcbSerialParams.Parity=NOPARITY;
if(!SetCommState(hSerial, &dcbSerialParams)){
//error setting serial port state
}
Once again, we’ll walk through this line-by-line. First, we create a variable of type DCB, clear all its
fields, and set its size parameter (this is required due to a strange quirk of Windows). We then use the
GetCommState function, which takes in our serial port handle and the DCB struct, to fill in the parameters currently in use by the serial port.
Anyway, once we have this, we just need to set the parameters we care about. Notably, baud rate, byte size, stop bits, and parity. For whatever reason, Windows requires that we use special constants to specify the baud rate. These constants are pretty straightforward: CBR 19200 for 19200 baud, CBR 9600 for 9600 baud, CBR 57600 for 57600 baud, etc.
Byte size we can just specify directly, but stop bits and parity once again require special constants. The options for StopBits are ONESTOPBIT, ONE5STOPBITS, TWOSTOPBITS. Similarly, the most commonly used options for Parity are EVENPARITY, NOPARITY, ODDPARITY. There are others, but these are quite obscure.

See the MSDN library entry (do search for DCB) for full details.
Similarly, there are loads of other fields in a DCB struct that you can use for some other more obscure
serial parameters.
After we’ve set up the DCB struct how we want it, we need to apply these setting to the serial port.
This is accomplished with the SetCommState function.

3 Setting timeouts
One of the big problems with serial port communication is that if there isn’t any data coming into the serial port (if the serial port device gets disconnected or turned off, for example) then attempting to read from the port can cause your application to hang while waiting for data to show up. There are two ways of going about fixing this. First, you can use multithreading in your application with one thread dealing with the serial port stuff and the other doing the actual processing. This can get very messy and complex and really isn’t necessary. The other way is much simpler: just tell Windows not to wait for data to show up! This is accomplished quite simply:
Code:
COMMTIMEOUTS timeouts={0};
timeouts.ReadIntervalTimeout=50;
timeouts.ReadTotalTimeoutConstant=50;
timeouts.ReadTotalTimeoutMultiplier=10;
timeouts.WriteTotalTimeoutConstant=50;
timeouts.WriteTotalTimeoutMultiplier=10;
if(!SetCommTimeouts(hSerial, &timeouts)){
//error occureed. Inform user
}
The COMMTIMEOUTS structure is pretty straightforward, and the above fields are the only ones it has.
A quick rundown:
• ReadIntervalTimeout specifies how long (in milliseconds) to wait between receiving characters before
timing out.
• ReadTotalTimeoutConstant specifies how long to wait (in milliseconds) before returning.
• ReadTotalTimeoutMultiplier specifies how much additional time to wait (in milliseconds) before
returning for each byte that was requested in the read operation.
• WriteTotalTimeoutConstant and WriteTotalTimeoutMultiplier do the same thing, just for writes
instead of reads.

One special case that comes up pretty often: Setting ReadIntervalTimeout to MAXDWORD and both
ReadTotalTimeoutConstant and ReadTotalTimeoutMultiplier to zero will cause any read operations to
return immediately with whatever characters are in the buffer (ie, have already been received), even if there aren’t any.

After we’ve setup the COMMTIMEOUTS structure how we want it, we need to apply the settings to the serial port using the SetCommTimeouts function.

4 Reading/Writing data
Alright, so once you have an open serial port with the correct parameters and timeouts, you can start doing the actual reads. These are extremely simple. Suppose we want to read n bytes from the serial port. Then we just do:

Code:
char szBuff[n + 1] = {0};
DWORD dwBytesRead = 0;
if(!ReadFile(hSerial, szBuff, n, &dwBytesRead, NULL)){
//error occurred. Report to user.
}
ReadFile takes in a HANDLE to a file (our serial port in this case), a buffer to store the data in, the
number of bytes to read, a pointer to an integer that will be set to the number of bytes actually read, and NULL.
Note that dwBytesRead will contain the number of bytes actually read by the ReadFile operation.
Writing data is exactly the same, except the function is called WriteFile.

5 Closing down
Once you’re done using the serial port, make sure you close the handle. If you don’t weird things can happen, like nobody else being able to access the serial port until you reboot. In any event, it’s really simple to do, so just remember to do it:

Code:
CloseHandle(hSerial);