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simple windows COM port wrapper library

• 6:00 pm on Dec 19, 2011 | #more | tags: hobby

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  searching for a way to read the serial port in windows using C (usually i do my programming under linux), i found a cute little wrapper for the serial port here. i don't like .NET that's why i've wanted a pure C implementation. here's the code (most of it from the source). read the comments as it saved me a lot of problems accessing higher COM ports. remember to escape the characters in the string!

  HANDLE openSerialConsole (LPCSTR p) {
  // LPCSTR p is a string containing the name of the device
  // in Windows namespace, the safest way to access the COM ports
  // is to use the \\.\COMxx syntax. remember, p is a string
  // and \ escapes characters so p should really be "\\\\.\\COMxx"
      HANDLE h;
      DCB params = {0};
      COMMTIMEOUTS timeouts = {0};
  
      h = CreateFile ( p,
          GENERIC_READ | GENERIC_WRITE, // the way we want to interact with the COM port
          0, // not the wisest way to open the device as it takes exclusive control over
          NULL, // no security attributes, meaning no child process access
          OPEN_EXISTING, // only if we have a COM port
          0, // some things i didn't understood completely
          NULL);
      if (h == INVALID_HANDLE_VALUE) {
          printf("Error: Could not create handle!\n");
          return (void *) NULL;
          }
  
      params.DCBlength = sizeof(params);
      if (!GetCommState(h, &params)) {
          printf("Error: Setting COM State!\n");
          CloseHandle (h);
          return (void *) NULL;
          }
  
      // 9600 bauds, with a 8N1 format
      params.BaudRate=CBR_9600;
      params.ByteSize=8;
      params.Parity=NOPARITY;
      params.StopBits=ONESTOPBIT;
      if (!SetCommState(h, &params)) {
          printf("Error: Setting BAUD!\n");
          CloseHandle (h);
          return (void *) NULL;
          }
  
      // timeouts, avoid hanging
      timeouts.ReadIntervalTimeout=50;
      timeouts.ReadTotalTimeoutConstant=50;
      timeouts.ReadTotalTimeoutMultiplier=10;
      timeouts.WriteTotalTimeoutConstant=50;
      timeouts.WriteTotalTimeoutMultiplier=10;
      if (!SetCommTimeouts(h, &timeouts)) {
          printf("Error: Setting timeouts!\n");
          CloseHandle (h);
          return (void *) NULL;
          }
  
      return h;
      }
  
  unsigned long int readFromSerialConsole ( HANDLE h, char * buffer, int size ) {
      unsigned long int bytes = 0;
      COMSTAT status;
      unsigned long int errors;
  
      ClearCommError (h, &errors, &status);
  
      if (status.cbInQue > 0) {
          size = status.cbInQue > size ? size : status.cbInQue;
          if (ReadFile(h, buffer, size, &bytes, NULL) && bytes != 0)
              return bytes;
          }
      return bytes;
      }
  
  unsigned long int writeToSerialConsole(HANDLE h, char * buffer, int size) {
      unsigned long int bytes = 0;
      if (!WriteFile(h, buffer, size, &bytes, NULL));
      return bytes;
      }
  
  void closeSerialConsole (HANDLE h) {
      CloseHandle (h);
      }