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files and directories - What kind of check does DirectoryQ exactly perform?


I'm using Mathematica 9.0.1 on Windows XP and my drive letters are assigned as follows: C: and D: are hard local disks, while E: is assigned to some removable media. Now, the situation is as follows


DirectoryQ[# <> ":\\"] & /@ {"c", "d", "e"}


returns {True,True,True}, even when an optical disk isn't in the drive.


To outflank the problem I use a directoryQ function, defined in the code below.


However, it obviously fails against a full hard disk or when I don't have write access. Therefore, I' m wondering about the kind of check DirectoryQ basically performs. Is there a better solution to fix the problem?


   directoryQ[unit_]:=Module[
       {root,file,ret},
       ret=False;
       root=unit<>":\\";
       file = FileNameJoin[{root, "testFile.txt"}];
       If[
         DirectoryQ[root],

         Quiet[Export[file,"test","Text"]];
         If[
           FileExistsQ[file],
           DeleteFile[file];
           ret=True
         ];
       ];
       Return[ret];
];


directoryQ["c"]
directoryQ["e"]

Answer



Since Mathematica is written in the C programming language, it is likely that any file-system status operation is doing nothing more than calling C's built-in functions. In this case, that would be stat.


So let's test what stat does on Windows. According to the C language documentation, stat is given the name of something in the file system, and returns a C structure whose fields describe the properties of that thing. In particular, the st_mode field seems relevant:



st_mode: Bit mask for file-mode information. The _S_IFDIR bit is set if path specifies a directory; the _S_IFREG bit is set if path specifies an ordinary file or a device. User read/write bits are set according to the file's permission mode; user execute bits are set according to the filename extension.



I wrote a simple test program in Microsoft Visual C that tests the stat function. Drive C: is my Windows system drive; drive E: is my DVD drive, which currently doesn't have anything in it. Here's what I found:


stat("C:\\WINDOWS") returns success and sets _S_IFDIR

stat("C:\\WINDOWS\\SYSTEM32\\NOTEPAD.EXE") returns success and does not set _S_IFDIR
stat("C:\\") returns success and sets _S_IFDIR
stat("E:\\") returns success and sets _S_IFDIR
stat("W:\\") returns failure

So we can conclude from this that Windows, or at least the stat function on Windows, considers empty DVD drives to be "there." I also have a flash memory stick that I can plug into a USB slot, which I have mapped to drive W, but that "isn't there" when it's unplugged.


This is consistent with what Windows Explorer shows me when I click on "My Computer". I see drive C: with a name, and I see drive E:, even though I get an error message when I double-click on it. I don't see drive W: (my flash memory stick) after I pulled it out of the socket. So Windows does consider an empty DVD drive to be present, which is probably why stat says it's a valid directory.


I have no idea how to find out whether a DVD drive is full or empty in C, much less in Mathematica. But at least we can see the core of the problem.


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