Re: Bluetooth communication

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>
Tue, 29 Apr 2008 17:36:51 -0500
<#Hb8NkkqIHA.2520@TK2MSFTNGP02.phx.gbl> wrote:

Hi everyone,

I'm have two iPac Devices and I'm trying send some bytes from one to
another via Bluetooth communication.
I use the CreateFile command to send the data and the ReadFile command
to read the data.
In booth commands I use the appropriate COM port.
When I try to send the bytes, it appears to work (the return is 1).
When I try to read the bytes, it also appears to work (the return is

But it always gives me the return 1 when I read the bytes, even when
nothing is sent. So I don't know if my process is working. Can anyone
help me?


The code below I use to read te data:
CString PortRead(HANDLE comPort, DWORD dwBytesToRead)
BOOL bResult;
BYTE * totalBytes; //total de bytes

unused variable, delete

BYTE * readBytes; //bytes parcialmente lidos
DWORD dwBytesRemain; //bytes restantes a serem lidos

no logic for partial reads, this is also useless

DWORD dwBytesRead; //bytes lidos
DWORD dwBytesPacket; //tamanho do pacote

unused variable, delete

int nCounter;

unused variable, delete

dwBytesPacket = PACKET_SIZE;
dwBytesRemain = dwBytesToRead;
readBytes = 0;

Now your pointer is NULL.

bResult = ReadFile(comPort, &readBytes, dwBytesToRead, &dwBytesRead,

You are reading into the pointer variable, not into a buffer. So you can
read at most (sizeof (BYTE*)) bytes, probably 4. Bad idea. You need to
allocate a buffer and pass a pointer to the buffer, not the address of a

You need to check bResult here, before using the data.

CString s = (LPCTSTR)readBytes;

Since the pointer got overwritten, it is garbage. This should cause your
program to crash.
Once you use a buffer properly, you have another problem because ReadFile
doesn't NUL-terminate the data.
Also you should never use TCHAR with sockets, the data must be either
Unicode or not (on WinCE it is always Unicode).

if (s == "")
return FALSE;

return s;

if (bResult == FALSE)
return FALSE;

and this code down here never runs, because you already returned. End
result is you never even checked bResult.



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