does anyone know how or if it's possible to change permissions on a numbers of registry keys in a subtree, but also also in the process, exclude several keys i don't want changing?
the trouble is they all have different names and if i create a batch file to change the ones i want to change, later on part of a key's name will change so i'll have to keep updating the batch, can i use wild cards to target the keys by the part of the name that stays the stay the same?
i.e. \somekey_0123456789 (the number bit changes but somekey doesn't)
hope this made some sense.
Related
Will UUID (or GUID) algorithms ever generate an "artificial" number?
Such as:
00000000-0000-0000-0000-000000000001
11111111-2222-3333-4444-555555555555
12345678-...
AAAAAAAA-...
I want to know if it is safe to set a GUID manually as a PK for a table that generates its own PK (NEWID or NEWSEQUENTIALID in TSQL).
The probability of generating any specific GUID is 2-128, so no matter what GUIDs you set as PKs, the chances of generating a duplicate are tiny. How tiny? Your chances of winning NY lottery are about one nonillion , or 1030, higher than generating a duplicate GUID, so you are pretty safe.
When you need to use fixed IDs, it is common practice to generate GUIDs on your computer, say, through the Visual Studio menu, or by writing a tiny program, and then hard-coding that GUID into your program as a "well-known value".
The format of many sorts of GUID is specified in http://en.wikipedia.org/wiki/Universally_unique_identifier and http://www.ietf.org/rfc/rfc4122.txt, and it is likely that an arbitrary choice will not conflict with any properly created GUID in any case.
It is however possible that an arbitrary GUID that you choose could be the same as an arbitrary GUID that somebody else chooses - as you are both breaking the rules. To avoid this, I suggest that you get hold of a properly created GUID of your own and either put it in as a config setting or hardwire it into your your code. A command line program to generate a GUID called uuidgen exists on linux and comes with Windows SDKs. A quick search suggests that you can also generate one via web pages such as http://www.famkruithof.net/uuid/uuidgen and https://www.guidgenerator.com/online-guid-generator.aspx.
Here are some freshly generated GUIDs anyway
{CB58921C-66A9-4C64-B2F4-413166736071}
{8FE41A9D-A0AC-4D32-8C10-48796DEFBD9C}
{E280FE84-0E60-4880-91DC-7A7BB886E1EE}
(From a script I found at http://www.somacon.com/p113.php).
The typical algorithm for generating a GUID on a Windows PC is to concatenate two hex encoded values, usually a portion of the MAC address and the current timestamp (so all GUIDs generated by the same PC will differ only by the timestamp portion). So, in your case it would be nearly impossible for the PC to generate your sample values randomly.
Is it possible to change the value Xcode 5 substitutes for ___FULLUSERNAME___ when expanding file templates? Looks like this was possible in the past via defaults but it now pulls from the system's full username.
My problem is that I have two user accounts, one work and one personal. I like this separation, however, OSX prevents setting the same full username value on both accounts. I was surprised by this restriction since I assumed it was just a UI thing since the underlying account names are obviously different. Alternatively, I would accept an answer that works around this limitation.
EDIT
I don't want to copy all the standard templates into ~/Library/Developer/Xcode/Templates/File\ Templates/ and do a find a replace with my name since that creates duplicates of every template.
I achieve this by renaming my user name of OS X, check this document by Apple.
I am working on an application that involves maintaining consistency between two local directories. Specifically, the directories should be identical, with the exception that all files in one of the directories are modified in some particular way (this part is not important to my question).
While running, my application runs two processes that listen for changes occurring under each of the paths, and performs relevant operations to bring them back in sync when necessary.
In terms of my specific question: I'm looking for advice on the tricker situation of when one starts the application. At this point, each process needs to check all files/folders under both the path that it is looking after, to see if anything has changed in anyway whilst the application was not running. (Let us assume that the application cannot be notified by the OS of anything that happened while it was shutdown, and thus will need to directly check every file/folder.)
Each process will have access to (and maintain) a persistent data-structure of all files/folder under its designated path. I was thinking that the following should be held within the data-structure for each of the files and folders:
File/folder name;
File hash (CRC32);
File/folder last mod data; and
File/folder size.
These pieces of information will obviously help to check for any changes to files/folder, but what is the best way to store them?
It seems to me that one sensible way to approach the situation of an application start is for each process to recursively scan through all files/folders under its designated path, and compare the metadata for each file scanned to the metadata stored in its data-structure. Then the processes should also iterate through the data-structures to look for things that have been removed from the paths. Some cases that may be encountered during this process are:
file modified (file name found in data-structure, but hash differs);
file added (no identical filename or hash found in data-structure);
file renamed (file with same hash exists in data-structure, but not with same filename);
folder added (no folder name in data-structure);
folder removed (folder name in data-structure, but not under path);
folder renamed (tricky one).
So, what's the best data-structure to use for this task? In my head I'm thinking some form of sorted associative array, e.g., a red-black tree, which store file and folder objects. Each file object contains name, hash and mod-date attributes , while each folder object contains name and children attributes, where children stores another associative array with everything underneath. Given the path to an arbitrary file, e.g., /foo/bar/file.txt, you begin at the root (foo), check for bar and so on until you get to file.txt's parent object.
Another alternative I can think of is to merely store everything flatly, such that there is one red-black tree where each key is the full path to each file/folder, and the value is the file / folder object. This would probably be quicker for retrieval, but it won't be possible to detect renamed files/folders without iterating through all values anyway, which sounds expensive. In the first approach, it may be the case that identifying a rename would only involves checking a portion of the data-structure rather than all of it.
Sorry the above ideas aren't terribly well thought out. What's the state of the art in this area, and are there any well-trodden approaches to these types of problems?
You're modelling a filesystem, so it's quite natural to use a hierarchical data structure. After all, you don't need to compare the file at dir1\dir2\foo.txt to dir3\bar.txt, right? You didn't mention file moves between directories as something you're tracking.
So, the data structure could be:
interface IFSEntry {
string name
datetime creationDate
pure virtual bool Compare(IFSEntry other)
pure virtual void UpdateFrom(IFSEntry other)
pure virtual bool WasRenamed(Dictionary<string,IFSEntry> possibleOriginals, out string oldName)
...
}
class File : IFSEntry {
...
}
class Directory : IFSEntry {
private Dictionary<string,IFSEntry> children;
...
}
The Directory implementations of UpdateFrom and Compare would recurse down their children.
File renames would be relatively easy by comparing CRC's. You'd miss files that changed in both places and were renamed. You could add a CRC dictionary to the Directory class if the time to run the comparisons proves a performance problem.
For directory moves, if the child files also changed, then you've got a fuzzy logic situation. It would be best to have a merge tool that the user would operate for that situation.
If a file changes in both places, you also need a user-facing merge strategy if conflicting changes occur. I'd argue that is always a good idea, just to let the user eyeball that the document didn't lose coherence.
GetDllDirectory produces an ambiguous value. When the string this call produces is empty, it means one of the following:
nobody has called SetDllDirectory
somebody passed NULL to SetDllDirectory
somebody passed an empty string to SetDllDirectory
The first two cases are equivalent for my purposes, but the third case is a problem. If I want to write save/restore code (call GetDllDirectory to save the "old" value, SetDllDirectory to set a "new" value temporarily, and later SetDllDirectory again to restore the "old" value), I run the risk of reversing some other programmer's intent.
If the other programmer intended for the current working directory to be in the DLL search order (in other words, one of the first two bullets is true), and I pass an empty string to SetDllDirectory, I will be taking the current working directory out of the DLL search order, reversing the other programmer's intent.
Can anyone suggest an approach to eliminate or work around this ambiguity?
P.S. I know having the current working directory in the DLL search order could be interpreted as a security hole. Nevertheless, it is the default behavior, and my code is not in a position to undo that; my code needs to be compatible with the expectations of all potential callers, many of which are large and old and beyond my control.
No fix for this. Between a rock and a hard place, you ought to assume that NULL was passed. There is already a way to enable safe searching with a registry setting.
After reading this, it sounds like a great idea to store files using the SHA-1 for the directory.
I have no idea what this means however, all I know is that SHA-1 and MD5 are hashing algorithms. If I calculate the SHA-1 hash using this ruby script, and I change the file's content (which changes the hash), how do I know where the file is stored then?
My question is then, what are the basics of implementing a SHA-1/file-storage system?
If all of the files are changing content all the time, is there a better solution for storing them, or do you just have to keep updating the hash?
I'm just thinking about how to create a generic file storing system like GoogleDocs, Flickr, Youtube, DropBox, etc., something that you could reuse in different environments (such as storing PubMed journal articles or Cramster homework assignments and tests, or just images like on Flickr). I'd probably store them on Amazon EC2. Just some system so I can say "this is how I'll 99% of the time do file storing from now on", so I can stop thinking about building a solid/consistent way to store files and get onto some real problems.
First of all, if the contents of the files are changing, filename from SHA-digest approach is not very suitable, because the name and location of the file in filesystem must change when the contents of the file changes.
Basically you first compute a SHA-1 or MD5 digest (= hash value) from the contents of the file.
When you have a digest, for example, 00e4f56c0de1c61fdb926e79e8a0a65bd12930c9, you generate a file location and filename from the digest. For example, you split the first few characters from the digest to directory structure and rest of the characters to file name. For example:
00e4f56c0de1c61fdb926e79e8a0a65bd12930c9 => some/path/00/e4/f5/6c0de1c61fdb926e79e8a0a65bd12930c9.txt
This way you only need to store the SHA-1 digest of the file to database. You can then always find out the right location and the name of the file.
Directories usually also have maximum number of files they can contain, for example maximum of 32000 subdirectories and files per directory. A directory structure based on this kind of hashing makes it unlikely that you store too many files to same directory. Also using hashing like this make sure that every directory has about the same number of files, you won't get into situation where all your files are in same directory.
The idea is not to change the file content, but rather its name (and path), by using a hash value.
Changing the content with a hash would be disastrous since a hash is normally not reversible.
I'm not sure of the motivivation for using a hash rather than the file name (or even rather than a long random number), but here are a few advantages of the hash appraoch:
the file names on the disk is uniform
the upper or lower parts of the hash value can be used to name the directories and hence distribute the files relatively uniformely
the name becomes a code, making it difficult for someone to
a) guess a file name
b) categorize pictures (would someone steal the hard drive content)
be able to retrieve the filename and location from the file contents itself (assuming the hash comes from such content. (not quite sure which use case would involve this... a bit contrieved...)
The general interest of using a hash is that unlike a file name, a hash is meaningless, and therefore one would require the database to relate images and "bibliographic" type data (name of uploader, date of upload, tags, ...)
In thinking about it, re-reading the referenced SO response, I don't really see much of an advantage of a hash, as compared to, say, a random number...
Furthermore... some hashes produce a numeric value, typically expressed in hexadecimal (as seen in the refernced SO question) and this could be seen as wasteful, by making the file names longer than they need to be, and hence putting more stress on the file system (bigger directories...)
One advantage I see with storing files using their hash is that the file data only needs to be stored once and then can be referenced multiple times within your database. This will save you space if you have a different users uploading the exact same file.
However the downside to this is when a user deletes what they think is there file from your app, you can't just physically delete the file from disk because other users that uploaded the same exact file may still be using it.
The idea is that you need to come up with a name for the photo, and you probably want to scatter the files among a number of directories. One easy way to come up with a unique name is to use the hash.
So the beginning of the hash was peeled off for a multi-level directory structure and the rest of the hash was used for a filename for the jpg.
This has the additional benefit of detecting duplicate uploads.