Reading from a COM port destroys lines - c

Question

I'm trying to read data from a COM port line-by-line in Windows. In PuTTY, the COM connection looks fine - my serial device (an MSP430 Launchpad) outputs the string "Data" once per second. However, when I use a simple C program to read the COM port and print out the number of bytes read, then the data itself, it gets completely mangled:
0
6 Data
2 Data
4 ta
6 Data
3 Data
3 a
a
6 Data
6 Data
2 Data
The lines saying 6 Data are correct (four characters, then \r\n), but what's happening to those lines that do not contain a complete message? According to the documentation, ReadFile should read an entire line by default. Is this incorrect - do I need to buffer it myself and wait for a linefeed character?
Note that not all those errors would occur in each run of the code; I did a few runs and compiled a variety of errors for your viewing pleasure. Here's the code I'm using:
#include <windows.h>
#include <stdio.h>
static DCB settings;
static HANDLE serial;
static char line[200];
static unsigned long read;
static unsigned int lineLength = sizeof(line) / sizeof(char);
int main(void) {
int i = 10;
serial = CreateFile("COM4",
GENERIC_READ | GENERIC_WRITE,
0, NULL,
OPEN_EXISTING,
0, NULL);
GetCommState(serial, &settings);
settings.BaudRate = CBR_9600;
settings.ByteSize = 8;
settings.Parity = NOPARITY;
settings.StopBits = ONESTOPBIT;
SetCommState(serial, &settings);
while(i) {
ReadFile(serial, &line, lineLength, &read, 0);
printf("%lu %s\n", read, line);
i--;
}
scanf("%c", &read);
return 0;
}
Compiled in Windows 7 64-bit using Visual Studio Express 2012.

Answer 1

What's happening is that the ReadFile is returning after it gets any data. Since data may come on a serial port at some point in the future, ReadFile will return when it gets some amount of data on the serial port. The same thing happens in Linux as well, if you attempt to read from a serial port. The data that you get back may or may not be an entire line, depending on how much information is in the buffer when your process gets dispatched again.
If you take another look at the documentation, notice that it will only return a line when the HANDLE is in console mode:
Characters can be read from the console input buffer by using ReadFile with a handle to console input. The console mode determines the exact behavior of the ReadFile function. By default, the console mode is ENABLE_LINE_INPUT, which indicates that ReadFile should read until it reaches a carriage return. If you press Ctrl+C, the call succeeds, but GetLastError returns ERROR_OPERATION_ABORTED. For more information, see CreateFile.