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WindowsCE.[Essential Windows CE Application Programming].Jon Wiley & Son.zip

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<B>Essential Windows CE Application Programming</B>
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<I>(Publisher: John Wiley & Sons, Inc.)</I>
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Author(s): Robert Burdick
<BR>
ISBN: 0471327476
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Publication Date: 03/01/99
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<H3><A NAME="Heading9"></A><FONT COLOR="#000077">Creating and Opening Files and Directories</FONT></H3>
<P>As under Windows NT, creating files and directories under Windows CE is done using the <I>CreateFile</I> and <I>CreateDirectory</I> functions. Under Windows CE, files are also opened using the <I>CreateFile</I> function, as we will soon see.</P>
<P><FONT SIZE="+1"><B>Creating and Opening Files</B></FONT></P>
<P>To create a file, your application calls the <I>CreateFile</I> function:</P>
<!-- CODE SNIP //-->
<PRE>
  CreateFile(lpFileName,dwDesiredAccess,dwShareMode,
    lpSecurityAttributes, dwCreationDistribution,
     dwFlagsAndAttributes, hTemplateFile);
</PRE>
<!-- END CODE SNIP //-->
<P><I>lpFileName</I> is the null-terminated Unicode string file name of the file to be created. Long file names are supported.</P>
<P><I>dwDesiredAccess</I> is used to indicate the access, or read-write mode, of the file. It can be any combination of the following values:</P>
<DL>
<DD><B>0.</B> Specifies device query access. This allows an application to query device attributes.
<DD><B>GENERIC_READ.</B> Specifies that the file is created/opened with read access.
<DD><B>GENERIC_WRITE.</B> Specifies that the file is created/opened with write access.
</DL>
<P>For example, to open a file called &#147;myfile.txt&#148; with read-write access, an application would do the following:
</P>
<!-- CODE SNIP //-->
<PRE>
  CreateFile(TEXT("myfile.txt"),
  (GENERIC_READ|GENERIC_WRITE),...);
</PRE>
<!-- END CODE SNIP //-->
<P>The third parameter to <I>CreateFile</I>, <I>dwShareMode</I>, is used to specify if and how the file can be shared. It can be a combination of one or more of the following values:</P>
<DL>
<DD><B>0.</B> Indicates that the file cannot be shared
<DD><B>FILE_SHARED_READ.</B> Subsequent open operations on the file will only succeed if read access is requested (via the <I>dwDesiredAccess</I> parameter).
<DD><B>FILE_SHARED_WRITE.</B> Subsequent open operations on the file will only succeed if write access is requested.
</DL>
<P>Under Windows CE, file security attributes are ignored. The <I>lpSecurityAttributes</I> parameter should therefore be set to NULL.</P>
<P>The <I>dwCreationDistribution</I> parameter controls how the <I>CreateFile</I> function behaves when attempting to create existing files, as well as what to do when the function tries to open a nonexistent file. This parameter can be one of the following:</P>
<DL>
<DD><B>CREATE_NEW.</B> The function creates a new file. If the specified file already exists, the <I>CreateFile</I> function fails.
<DD><B>CREATE_ALWAYS.</B> The function creates a new file. If the specified file already exists, it is overwritten by the <I>CreateFile</I> operation.
<DD><B>OPEN_EXISTING.</B> The function opens an existing file. If the specified file does not exist, <I>CreateFile</I> fails.
<DD><B>OPEN_ALWAYS.</B> The function opens an existing file. If the specified file does not already exist, it is created.
<DD><B>TRUNCATE_EXISTING.</B> The function opens the file, but truncates it to zero length. The file must be opened with GENERIC_WRITE access. <I>CreateFile</I> fails if the specified file does not exist.
</DL>
<P>By taking a look at the allowed <I>dwCreationDistribution</I> values, we can see how <I>CreateFile</I> can be used to both create new files and open existing files. For example, it is common to want to open a file or have the operating system create a file of that name if it does not exist, as follows:</P>
<!-- CODE SNIP //-->
<PRE>
  CreateFile(TEXT("myfile.txt"),
  GENERIC_READ|GENERIC_WRITE, 0,
    NULL, OPEN_ALWAYS,...);
</PRE>
<!-- END CODE SNIP //-->
<P><I>dwFlagsAndAttributes</I> determines the file attributes and several operating modes. We have already discussed file attributes. The flags portion can be any combination of the following values:</P>
<DL>
<DD><B>FILE_FLAG_WRITE_THROUGH.</B> Instructs Windows CE to write directly to the object store when writing to the specified file, as opposed to writing through any intermediate cache.
<DD><B>FILE_FLAG_RANDOM_ACCESS.</B> The file supports random access.
</DL>
<P>Most of the flags that are supported under Windows NT are not supported under Windows CE. Also note that the SECURITY_SQOS_ PRESENT flag, or any of the other values that can be used with it under Windows NT, are not supported under Windows CE.
</P>
<P>Finally, the <I>hTemplateFile</I> parameter is ignored under Windows CE and should be set to NULL.</P>
<P>If <I>CreateFile</I> is successful, a handle to the open file is returned. If it fails, the return value is INVALID_HANDLE_VALUE.</P>
<P>Open files are closed using the <I>CloseHandle</I> function:</P>
<!-- CODE SNIP //-->
<PRE>
  CloseHandle(hObject);
</PRE>
<!-- END CODE SNIP //-->
<P>where <I>hObject</I> is the handle of the file to close.</P>
<P><FONT SIZE="+1"><B>Creating Directories</B></FONT></P>
<P>Creating a directory is accomplished with the <I>CreateDirectory</I> function. The <I>CreateDirectoryEx</I> function available under Windows NT is not supported in Windows CE. The <I>CreateDirectory</I> function syntax is:</P>
<!-- CODE SNIP //-->
<PRE>
  CreateDirectory(lpPathName, lpSecurityAttributes);
</PRE>
<!-- END CODE SNIP //-->
<P><I>lpPathName</I> is a null-terminated Unicode string specifying the path of the directory to be created. The maximum allowed length of this name is the operating system&#150;defined value MAX_PATH. The second parameter to this function is ignored, as Windows CE does not support file security attributes. <I>lpSecurityAttributes</I> therefore should be set to NULL.</P>
<P>If <I>CreateDirectory</I> is successful, it returns TRUE. If unsuccessful, it returns FALSE. An application can get more detailed information about why the function failed by calling <I>GetLastError</I>.</P><P><BR></P>
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