Thread: Using CreateFile() to get handle to COM port

Hi,
Using the C code outlined below (extract from a larger program) in Visual Studio 2110 express, I am having difficulty using CreateFile to return a valid pointer. It continues to return 0xfffff (the Invalid_Handle_Value).

I have followed numerous guides on the internet to open COM ports in C in a Windows environment and have followed the syntax exactly, but with no success. Any suggestions why I can only get an invalid pointer would be appreciated.

Thanks in advance,
Mr. Whippy.
(sorry about the formatting - first post - see attached file)
temp1.c

Code:

#include<stdlib.h>
#include<stdio.h>
#include<string.h>
#include<windows.h>
#include<conio.h>
HANDLE hSerialIn = INVALID_HANDLE_VALUE;
DCB dcbSerialParams = { 0 };

DWORD WINAPI GetLastError(void);
LPCWSTR lpFileName;
long lLastError = ERROR_SUCCESS;
char portIn[16];
int main(void)
{
  sprintf(portIn, "COM1");
  lpFileName = (LPCWSTR) portIn;
  printf("lpFileName: %s\n", lpFileName);
  hSerialIn =
      CreateFile(lpFileName, GENERIC_READ | GENERIC_WRITE, 0, NULL,
                 OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

  if (hSerialIn == INVALID_HANDLE_VALUE) {
    if (GetLastError() == ERROR_FILE_NOT_FOUND) {
      printf("\nError: \nThe system cannot find the file specified (%s)\n", portIn);  //error code 0x02
    }
    else if(GetLastError() == ERROR_INVALID_NAME) {
      printf("\nError: \n%s 'filename, directory name, or volume label syntax is incorrect'\n", portIn);  //error code 0x7B
    }
    else
    {
      printf("\nHandle creation error code: %x\n", GetLastError());
    }
    puts("\t...CreateFile returned an invalid handle value");
  }
  if (!GetCommState(hSerialIn, &dcbSerialParams)) {
    printf("\nError: \ncould not get %s state!\n", portIn);
  }
  CloseHandle(hSerialIn);
  puts("\npress any key to exit...");
  getchar();
  return 1;

}

> Using the C code outlined below (extract from a larger program) in Visual Studio 2110 express
Are you telling us that MS is still going 100 years from now, and that you're from the future?

When pasting code, make sure you "paste as text".


> lpFileName = (LPCWSTR) portIn;
Do you know why you're doing this?
casting does not magically transform your string into the correct format. In particular, newer versions of visual studio default to using UNICODE for strings, whereas you're using plain old ASCII.

To turn UNICODE off, read this -> visual studio - How do I turn off Unicode in a VC++ project? - Stack Overflow
That should at least make your program work (assuming nothing else is wrong).

Longer term, read this -> Unicode and Multibyte Character Set (MBCS) Support
And start to write things like
TCHAR portIn = _T("COM1");

What's your latest code?

Personally, I would only call getLastError() once

Code:

if (hSerialIn == INVALID_HANDLE_VALUE) {
    DWORD err = GetLastError();
    if (err == ERROR_FILE_NOT_FOUND) {
      printf("\nError: \nThe system cannot find the file specified (%s)\n", portIn);  //error code 0x02
    }
    else if(err == ERROR_INVALID_NAME) {
      printf("\nError: \n%s 'filename, directory name, or volume label syntax is incorrect'\n", portIn);  //error code 0x7B
    }
    else
    {
      printf("\nHandle creation error code: %x\n", err);
    }
    puts("\t...CreateFile returned an invalid handle value");
  }

Which error message are you seeing?


> (Could this be a C vs. C++ issue? This is a *.c file and I am compiling it as such...)
Unlikely, it's all down to usage of the Win32 API.