Reading from serial ports using C++

Andres's Avatar, Join Date: Jul 2008
Go4Expert Member
Hi does anyone here know how to read from a serial port using c++?

I just got this got from the internet but it doesnt work:

CSerial serial;
if (serial.Open(2, 9600))
AfxMessageBox("Port opened successfully");
else
AfxMessageBox("Failed to open port!");
xpi0t0s's Avatar, Join Date: Aug 2004
Mentor
I've used the WinAPI directly; this was to open the com port for writing and I just had a terminal emulator on the other end listening and displaying debug messages from my program. Probably doesn't take a lot of adapting (in fact may be usable directly for reading from the port):
Code:
CommPortHandle = OpenComm("com1",CommBufferSize,CommBufferSize);
if (CommPortHandle >= 0)
{
DCB dcb;
BuildCommDCB("com1:9600,n,8,1", &dcb);
dcb.BaudRate = 19200;
SetCommState(&dcb);
}
Andres's Avatar, Join Date: Jul 2008
Go4Expert Member
thanks xpi0t0s...hey does the following code do the same thing?
Code:
#include "windows.h"
#include "time.h"
#include "string.h"
#include "stdio.h"

BOOL SetCommDefaults(HANDLE hSerial);

int main(int argc, char* argv[])
{
    HANDLE hSerial = CreateFile("COM2",GENERIC_READ | GENERIC_WRITE,0,NULL,OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED |FILE_FLAG_NO_BUFFERING,NULL);
    if (hSerial == INVALID_HANDLE_VALUE) return GetLastError();
    SetCommDefaults(hSerial);

    HANDLE hReadEvent = CreateEvent(NULL,TRUE,FALSE,"RxEvent");
    OVERLAPPED ovRead;
    OVERLAPPED ovWrite;
    memset(&ovRead,0,sizeof(ovRead));
    memset(&ovWrite,0,sizeof(ovWrite));

    ovRead.hEvent = hReadEvent;

    char szRxChar = 0;
    DWORD dwBytesRead = 0;
    DWORD dwBytesWritten = 0;

    while(szRxChar != 'q')
    {
        // Check if a read is outstanding
        if (HasOverlappedIoCompleted(&ovRead))
        {
            // Issue a serial port read
            if (!ReadFile(hSerial,&szRxChar,1,
                    &dwBytesRead,&ovRead))
            {
                DWORD dwErr = GetLastError();
                if (dwErr!=ERROR_IO_PENDING)
                    return dwErr;
            }
        }

        // Write the time out to the serial port
        time_t t_time = time(0);
        char buf[50];
        sprintf(buf,"Time is %s\n\r",ctime(&t_time));
        if (HasOverlappedIoCompleted(&ovWrite))
        {
            WriteFile(hSerial,buf,strlen(buf),
                    &dwBytesWritten,&ovWrite);
        }


        // ... Do some other processing

        // Wait 5 seconds for serial input
        if (!(HasOverlappedIoCompleted(&ovRead)))
            WaitForSingleObject(hReadEvent,5000);

        // Check if serial input has arrived
        if (GetOverlappedResult(hSerial,&ovRead,
                &dwBytesRead,FALSE))
        {
            // Wait for the write
            GetOverlappedResult(hSerial,&ovWrite,
                &dwBytesWritten,TRUE);
            // Display a response to input
            sprintf(buf,"You pressed the '%c' key\n\r",
                szRxChar);
            WriteFile(hSerial,buf,strlen(buf),
                    &dwBytesWritten,&ovWrite);
        }

    }

    CloseHandle(hSerial);
    CloseHandle(hReadEvent);
    return 0;
}

BOOL SetCommDefaults(HANDLE hSerial)
{
    DCB dcb;
    memset(&dcb,0,sizeof(dcb));
    dcb.DCBlength=sizeof(dcb);
    if (!GetCommState(hSerial,&dcb))
        return FALSE;
    dcb.BaudRate=9600;
    dcb.ByteSize=8;
    dcb.Parity=0;
    dcb.StopBits=ONESTOPBIT;
    if (!SetCommState(hSerial,&dcb))
        return FALSE;
    return TRUE;
}

Last edited by shabbir; 7Oct2008 at 10:13.. Reason: Code block
xpi0t0s's Avatar, Join Date: Aug 2004
Mentor
No. I vaguely remember (from about 1993) experimenting with CreateFile as the "correct" way to use COM ports, not getting anywhere (probably because it involved millions of cryptic parameters), then finding that OpenComm worked perfectly well for what I wanted.

Checking the help in VS2005 it seems OpenComm has disappeared, so ignore what I said earlier. That code was written back in the Windows 3.1 days and seems now to be completely obsolete. Sorry about that...