Microsoft Windows API Serial ReadFile Producing Unexpected Output - c++

Question

I am currently trying to write a program that will read Bluetooth output from an Arduino HC-05 module on a Serial Communications Port.
http://cdn.makezine.com/uploads/2014/03/hc_hc-05-user-instructions-bluetooth.pdf
When I open a Putty terminal and tell it to listen to COM4, I am able to see the output that the program running on the Arduino is printing.
However, when I run the following program to try to process incoming data on the serial port programatically, I get the output shown.
#include <Windows.h>
#include <string>
#include <atltrace.h>
#include <iostream>
int main(int argc, char** argv[]) {
HANDLE hComm = CreateFile(
L"COM4",
GENERIC_READ | GENERIC_WRITE,
0,
0,
OPEN_EXISTING,
NULL,
0
);
if (hComm == INVALID_HANDLE_VALUE) {
std::cout << "Error opening COM4" << std::endl;
return 1;
}
DWORD dwRead;
BOOL fWaitingOnRead = false;
OVERLAPPED osReader = { 0 };
char message[100];
osReader.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if (osReader.hEvent == NULL) {
std::cout << "Error creating overlapping event" << std::endl;
return 2;
}
while (1) {
if (!fWaitingOnRead) {
if (!ReadFile(
hComm,
&message,
sizeof(message),
&dwRead,
NULL
)) {
if (GetLastError() != ERROR_IO_PENDING) {
std::cout << "Communications error" << std::endl;
return 3;
}
}
else {
message[100] = '\0';
std::cout << message << std::endl;
}
}
}
return 0;
}
I have made changes to the handle and the ReadFile function call so that it will be making the calls synchronously in an infinite loop. However, Visual Studio pops up a warning saying that the program has stopped working then asks to debug or close program. My assumption is that it must be stalling somewhere or failing to execute some WindowsAPI function somewhere up the stack.
Any help, pointers, greatly appreciated.

Answer 1

At least IMO, using overlapped I/O for this job is pretty severe overkill. You could make it work, but it would take a lot of extra effort on your part, and probably accomplish very little.
The big thing with using comm ports under Windows is to set the timeouts to at least halfway meaningful values. When I first did this, I started by setting all of the values to 1, with the expectation that this would sort of work, but probably consume excessive CPU time, so I'd want to experiment with higher values to retain fast enough response, while reducing CPU usage.
So, I wrote some code that just set all the values in the COMMTIMEOUTS structure to 1, and setup the comm port to send/read data.
I've never gotten around to experimenting with longer timeouts to try to reduce CPU usage, because even on the machine I was using when I first wrote this (probably a Pentium II, or thereabouts), it was functional, and consumed too little CPU time to care about--I couldn't really see the difference between the machine completely idle, and this transferring data. There might be circumstances that would justify more work, but at least for any need I've had, it seems to be adequate as it is.

Answer 2

That's because message has the wrong type.
To contain a string, it should be an array of characters, not an array of pointers to characters.
Additionally, to treat it as a string, you need to set the array element after the last character to '\0'. ReadFile will put the number of characters it reads into dwRead.
Also, it appears that you are not using overlapped I/O correctly. This simple program has no need for overlapped I/O - remove it. (As pointed out by #EJP, you are checking for ERROR_IO_PENDING incorrectly. Remove that too.)

Answer 3

See comments below, in your program:
if (!fWaitingOnRead) {
if (!ReadFile( // here you make a non-blocking read.
hComm,
message,
sizeof(*message),
&dwRead,
&osReader
)) {
// Windows reports you should wait for input.
//
if (GetLastError() != ERROR_IO_PENDING) {
std::cout << "Communications error" << std::endl;
return 3;
}
else { // <-- remove this.
// insert call to GetOverlappedcResult here.
std::cout << message << std::endl;
}
}
}
return 0; // instead of waiting for input, you exit.
}
After you call ReadFile() you have to insert a call for GetOverlappedResult(hComm, &osReader, &dwBytesRceived, TRUE) to wait for the read operation to complete and have some bytes in your buffer.
You will also need to have a loop in your program if you don't want to exit prematurely.
If you do not want to do overlapped i/o (which is a wise decision) , do not pass an OVERLAPPED pointer to ReadFile. ReadFile will block until it has some data to give you. You will then obviously not need to call GetOverlappedresult()
For the serial port, you also need to fill in a DCB structure. https://msdn.microsoft.com/en-us/library/windows/desktop/aa363214(v=vs.85).aspx
You can use BuildCommDCB()to initialize it. There is a link to it in the MS doc, CallGetCommState(hComm, &dcb) to initialize the serial port hardware. The serial port needs to know which baud rate etc. you need for your app.