correctly reading or writing serial port Windows API

Question

I have read alot of issues with serial port reading and writing. None so far have helped me figure out what my code is missing. The msdn example for c++ has undefined variables and missing brackets so although i can add brackets it still does not function. Here's what I've got at this point. It appears I can open the port and do the configuration but I cannot read a byte/char of data. I really just want a simple asyncronous serial read/write for aprogram to read from an Arduino.

class MY_SERIAL
{

HANDLE serialinstance;

DWORD      dwStoredFlags;
DWORD      dwRes;
DWORD      dwCommEvent;
OVERLAPPED osStatus = {0};
BOOL       fWaitingOnStat;
//dwStoredFlags = EV_BREAK | EV_CTS   | EV_DSR | EV_ERR | EV_RING | EV_RLSD | EV_RXCHAR |      EV_RXFLAG | EV_TXEMPTY ;


DCB dcb;
COMMTIMEOUTS timeouts;

COMMCONFIG serialconfig;



public:
char inBuffer[1000];
char outBuffer[100];

PDWORD noBytes;

void close_serial()
{
    CloseHandle(serialinstance);
}
//----------------------------------------------------
bool open_serial(LPCSTR portNumber)   // serial port name use this format  "\\\\.\\COM10"
{

    serialinstance = CreateFile(portNumber, GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, FILE_FLAG_OVERLAPPED, NULL);
    if(serialinstance == INVALID_HANDLE_VALUE)
    {
        int error = GetLastError();
        printf("ERROR opening serial port  %s\r\n", portNumber);
        if(error == 0x2){printf("ERROR_FILE_NOT_FOUND\r\n");}
        if(error == 0x5){printf("ERROR_ACCESS_DENIED\r\n");}
        if(error == 0xC){printf("ERROR_INVALID_ACCESS\r\n");}
        if(error == 0x6){printf("ERROR_INVALID_HANDLE\r\n");}
        printf("error code %d\r\n", error);
        return false;
    }
    if(GetCommState(serialinstance, &dcb)!= true)
    {
        printf("ERROR getting current state of COM   %d \r\n", GetLastError());
        return false;
    }
    else{printf("debug   read current comstate\r\n");}


    FillMemory(&dcb, sizeof(dcb), 0); //zero initialize the structure
    dcb.DCBlength = sizeof(dcb);      //fill in length

    dcb.BaudRate = CBR_115200;     //  baud rate
    dcb.ByteSize = 8;             //  data size, xmit and rcv
    dcb.Parity   = NOPARITY;      //  parity bit
    dcb.StopBits = ONESTOPBIT;

    if(SetCommState(serialinstance, &dcb) != true)
    {
        printf("ERROR setting new state of COM   %d \r\n", GetLastError());
        return false;
    }
    else{printf("debug   set new comstate\r\n");}
    /*
    if (!BuildCommDCB("115200,n,8,1", &dcb)) //fills in basic async details
    {
        printf("ERROR getting port comstate\r\n");
        return FALSE;
    }
    */
    if (!SetCommMask(serialinstance, EV_RXCHAR))
    {
        printf("ERROR setting new COM MASK   %d \r\n", GetLastError());
        return false;
    }
    else{printf("debug   commmask set\r\n");}
    timeouts.ReadIntervalTimeout = MAXDWORD;
    timeouts.ReadTotalTimeoutMultiplier = 20;
    timeouts.ReadTotalTimeoutConstant = 0;
    timeouts.WriteTotalTimeoutMultiplier = 0;
    timeouts.WriteTotalTimeoutConstant = 0;

    if (!SetCommTimeouts(serialinstance, &timeouts))
    {
        printf("ERROR setting timeout parameters\r\n");
        return false;
    }
    else{printf("debug   timeouts set\r\n");}
    osStatus.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
    if (osStatus.hEvent == NULL)
    {// error creating event; abort
        printf("ERROR creating Serial EVENT\r\n");
        return false;
    }
    else{printf("debug   event created set\r\n");}
    osStatus.Internal = 0;
    osStatus.InternalHigh = 0;
    osStatus.Offset = 0;
    osStatus.OffsetHigh = 0;
    assert(osStatus.hEvent);
    printf("debug   com port setting complete\r\n");






    return true;
}
//---------------------------------------------------------

bool read_serial_simple()
{
    char m[1000];
    LPDWORD bytesRead;



    if (WaitCommEvent(serialinstance, &dwCommEvent, &osStatus))
    {
        if(dwCommEvent & EV_RXCHAR)
        {
            ReadFile(serialinstance, &m, 1, bytesRead, &osStatus);
            printf("data read =   %d,       number bytes read =     %d  \r\n", m, bytesRead);
            return true;
        }
        else
        {
            int error = GetLastError();
            if(error == ERROR_IO_PENDING){printf(" waiting on incomplete IO\r\n");}
            else{printf("ERROR %d\r\n", error);}
            return false;
        }
    }
    return false;
}

};

So I stripped the read function down. I now get a char and it reports reading 1 byte but the value of the char is incorrect. I get a series of 48, 13, 10, and occasionally a 50 value for the byte. However the Arduino is sending a series a 0's then a 128 as verified with TerraTerm. What else do I need here?

bool read_serial_simple()
{
    unsigned char m = 0;
    DWORD bytesRead;
    if(ReadFile(serialinstance, &m, 1, &bytesRead, &osStatus) == true)
    {
        printf("data read =   %d,       number bytes read =     %d  \r\n", m, bytesRead);
        return true;
    }
    else{
        int error = GetLastError();
        if(error == ERROR_IO_PENDING){printf(" waiting on incomplete IO\r\n");}
        else{printf("ERROR %d\r\n", error);}
        return false;
    }

}

So now I can read a byte of data but I cannot write a byte or more to the port. I just get ERROR_IO_PENDING. Can someone help out with this as well. Write function of my class below.

bool write(DWORD noBytesToWrite)
{
    if(WriteFile(serialinstance, outBuffer, noBytesToWrite, NULL, &osStatus) == true)
    {
        printf("message sent\r\n");
        return true;
    }
    else
    {
        int error = GetLastError();
        if(error != ERROR_IO_PENDING){LastError();}
        return false;
    }
}

I'm calling both functions from main as follows

myserial.open_serial(COM12);
myserial.outBuffer[0] = 'H';
myserial.outBuffer[1] = 'e';
myserial.outBuffer[2] = 'L';
myserial.outBuffer[3] = 'l';
myserial.outBuffer[4] = 'O';
for(int n=0; n<5; n++){printf("%c", myserial.outBuffer[n]);}
printf("\r\n");

while(1)
{
    myserial.read();
    myserial.write(5);
    //system("PAUSE");
}

Currently the arduino is set up to read bytes in and repeat them back to the pc. It is doing this fine on the arduino IDE serial monitor so now I just need to get this pc program to write out.

Answer 1

Your bytesRead variable is an uninitialized pointer. You are passing an invalid address to ReadFile() to write to.

Replace LPDWORD bytesRead with DWORD bytesRead, then pass it to ReadFile() using &bytesRead.

Edit: Also eliminate the FILE_FLAG_OVERLAPPED. You are not handling it properly, and there is no point in using it if you WaitForSingleObject() before reading.

Answer 2

Sorry my answer is a bit late, but as I was checking up on another serial port detail I found this.

There is a bit flaw in the original code. You are calling CreateFile with the FILE_FLAG_OVERLAPPED flag. This means you want to use non-blocking calls. You either need to remove this flag, or change your ReadFile and WriteFile calls so that they include a pointer to an OVERLAPPED structure WriteFile.

Your code works with ReadFile as it will complete syncrhronously as there is a character already waiting. The WriteFile will return IO_PENDING result to indicate that the write has been queued.