Explain DWORD use of DWORD in Serial Communication - c

Question

I am studing serial communication with win32 using C now. reading from serial port is given as below.
DWORD dwEventMask;
DWORD dwSize;
if(!SetCommMask(hSerial, EV_RXCHAR)){
//Error handling
printf("Error Setting Comm Mask \n");
}
if(WaitCommEvent(hSerial, &dwEventMask, NULL))
{
unsigned char szBuf[1024];
DWORD dwIncommingReadSize;
do
{
if(ReadFile(hSerial, &szBuf, 1, &dwIncommingReadSize, NULL) != 0) {
//Handle Error Condition
}
if(dwIncommingReadSize > 0)
{
dwSize += dwIncommingReadSize;
sb.sputn(&szBuf, dwIncommingReadSize);
printf("Reading from port \n");
}
else{
//Handle Error Condition
}
printf("Reading data from port \n");
} while(dwIncommingReadSize > 0);
}
else
{
//Handle Error Condition
}
They have used DWORD dwIncommingReadSize for while condition (while(dwIncommingReadSize > 0);.
Please explain how this condition is satifisfied. No modification can be seen for that.
Again please explain following part.
if(dwIncommingReadSize > 0)
{
dwSize += dwIncommingReadSize;
sb.sputn(&szBuf, dwIncommingReadSize);
printf("Reading from port \n");
}

Answer 1

This line:
if(ReadFile(hSerial, &szBuf, 1, &dwIncommingReadSize, NULL)
passes the address of dwIncommingReadSize (however badly spelt it may be) to the function so it can change it to whatever it wants.
It's similar to:
void fn (int *x) { *x = 42; }
:
int xyzzy = 1;
fn (&xyzzy);
// Here, xyzzy is now 42.
In terms of your second question, it's a little hard to tell without seeing more of the code, but it looks like it's simply increasing a "total size" variable for each block of data read in (plus whatever sb.sputn is supposed to do).
This is typical where a single read may not get all the data you want - you simply store what you got and then go back for more.