So we all know that Windows programs by default are limited to dealing
with a maximum path length of 260 characters. However, this limit can easily be overcome by prefixing the path by the \\?\ character sequence.
For some reason, however, this isn't possible with relative paths, as MSDN says:
Because you cannot use the \\?\ prefix with a relative path,
relative paths are always limited to a total of MAX_PATH characters.
(source)
I don't really understand the reason why Microsoft decided to forbid relative paths to be prefixed with \\?\ so if there is some sort of rationale behind this decision, I'd be really glad to hear about it because it doesn't really make sense to me that \\?\ is only allowed for full paths.
My real question, though, is how to deal with this limitation: Should I simply call GetFullPathName() on relative paths to extend them to full paths, then add the \\?\ prefix, and then pass that path to fopen() etc., or what is the recommended way of dealing with this limitation?
You cannot use the \\?\ prefix with a relative path.
When relative path is passed to the system, it is parsed as absolute paths and then passed to the system. And as it is mentioned in the source:
The prefixes \\:\ are not used as part of the path itself. They
indicate that the path should be passed to the system with minimal
modification, which means that you cannot use forward slashes to
represent path separators, or a period to represent the current
directory, or double dots to represent the parent directory.
Related
I am working on a trie data structure which inserts and searches for normal paths.
A path can contain any character from unicode, so in order to represent it completely in utf-8, the array in trie needs to contain next nodes for all 256 ascii.
But I am also concerned about the space and insertion time taken by trie.
The conditions under which my trie is setup rarely would insert a character of unicode(I mean 128-255 ascii). So I just put an if condition to reject paths which contain above ascii 127. I don’t think the ascii 1-31 are relevant either, although I am unsure about this. As 1-31 chars are like carriage return, esc etc, can I simply continue the loop without inserting them? Like is it possible to encounter paths that are actually differentiable because of ascii 1-31 in a real scenario?
Answering this old question, on macOS ascii 13 is used to represent custom icons which may appear in many paths. Thanks to #EricPostpischil who told that in comments.
All other characters ranging between 1-31 appear pretty less in paths.
Also, macOS users mostly have a case-insensitive path, so generally considering both lowercase and uppercase is also useless.
PS:
Although this question seems to be opinion based, but it actually isn't because it can be answered quite concisely. It attempts to ask for frequency of appearance of characters in paths on macOS. (sorry for the confusing title, I was a noob that time, changing it now will make all comments on it absurd)
I have some unique codes that are generated from strings (ex: website host names) in various independent components of my application.
These codes are meant to be used by machines only so i would like to keep them as short as possible.
The below algorithm would be applied to every word in the string. The output words would be concatenated with a dash to generate the unique code.
The current algorithm I have used:
- Skip word if length is less than 6
- Leave first character as is
- Remove every wowel in the word from the second character onwards
architectural digest eu => archtctrl-dgst-eu
arizona foothills magazine => arzn-fthlls-mgzn
Is there a better way to shorten an English word leaving it as recognisable as possible to a human reader?
The output should be deterministic and produce the same shortened version whenever it is run on the same input.
A good algorithm should also minimise the number of clashes for similarly spelt words.
I have some unique codes that are generated from strings
I am afraid that is not true. There are many English words that will reduce to the same 'code word' when stripped of their vowels. For example, 'leaving' -> 'living' Given, this is fairly rare, it could still cause issues.
How important is it that these 'code words' remain human-readable if as you say, they are meant to be used by machines only? If its not that important, I'd suggest looking into some simpler compression algorithms like Huffman Coding or LZW Compression. Then if the user needs to see the translation of the code word, just uncompress it.
If you must keep it human-readable, I'm not sure that there is much more you can do to shorten it. You could take a look at specific latin + greek roots, and determine if you can shorten those any more by hand, and then just substitute those out automatically.
Alternatively, you could turn to a phonetic approach. Automatically search the pronunciation of the word, and then see if that is any shorter (or itself can be compressed, taking 'cee' to 'C', or 'kay' to 'K'). This would be much more time and CPU intensive, but its still an option if you really, really need short but yet readable codes.
What you're generating sounds like what's called a "slug". There are many libraries to handle this for blogs or site generators that should suit your purposes. Here's a usage example from a Python library called slugify:
txt = "___This is a test ---"
r = slugify(txt)
self.assertEqual(r, "this-is-a-test")
Slug libraries generally work like this:
replacing non-ascii linguistic characters via a mapping (ex: 影師嗎 -> ying-shi-ma)
replace accented latin letters with ascii equivalents via a mapping (ex: C'est déjà l'été. -> c-est-deja-l-ete)
remove beginning and trailing spaces/punctuation
convert remaining spaces and punctuation to dashes, collapsing multiple dashes in a row to a single dash
If you want to make slugs shorter you could remove vowels or, more simply, use a maximum length.
The POSIX shell command language is not easy to parse, largely because of tight coupling between lexing and parsing.
However, parsing expression grammars (PEGs) are often scannerless. By combining lexing and parsing, it seems that I could avoid these problems. The language that I am using (Rust) has a well-maintained PEG library. However, I know of three difficulties that could make it impractical to use this library:
Shells must be able to parse line by line, not reading characters past the end of the line.
Aliases are purely lexical, and can cause a token to be replaced by any sequence of other tokens in certain situations
Shell reserved words are only recognized in certain situations
Is a PEG suited to parsing the shell command language given these requirements, or is a hand-written recursive-descent parser more suitable?
Yes, a PEG can be used, and none of the issues you note should be a problem.
In particular:
1) parsing line by line: most PEG tools will not have any built-in white-space skipping. All white space including newlines must be explicitly handled by you, which means you can handle newline any way you like.
2) You should not use the parse tree from PEG as your AST. Instead you should descend the parse tree and build an AST. For aliases then, after the parse has completed and you're building your AST, you can detect the alias and insert the appropriate expansion for the alias instead.
3) Reserved words are not reserved unless you reserve them. That is, if you have a context where either a reserved word or another alphanumeric symbol can occur, you must first check for the reserved words explicitly, then the arbitrary alphanumeric symbol, because once the PEG decides it has a match, that will not back-track. Anywhere a reserved word is not permitted, simply don't check for it, and your generalised alphanumeric symbol rule will succeed instead.
We are working with a number of unix based filesystems, all of which share a similar set of restrictions on that certain characters can't be used in the username fields. One of those restrictions is no "#" , "_", or "." in the names. Being unix there are a number of other restrictions.
So the question is if there is a good known algorithm that can take an email address and turn that into a predictable unix filename. We would need to reverse this at some point to get the email.
I've considered doing thing like "."->"DOT", "#"->"AT", etc. But there are size limitations and other things that are generally problematic. I could also optimize by being able to map the #xyz.com part of the email to a special char or something. Each implementation would only have at most 3 domains it would need to support. I'm hoping someone has found a solution without a huge number of tradeoffs.
UPDATE:
-The two target filesystems are AFS and NFS.
-Base64 doesn't work as it has not compatible characters. "/"
-Readable is preferable.
Seems like the best answer would be to replace the #xyz.com domain to a single non-standard character, and then have a function that could shrink the first part of a name to something that fits in the username length restrictions of the various filesystems. But what is a good function for that?
You could try a modified version of the URL percent (%) encoding scheme used on for URIs.
If the percent symbol isn't allowed on your particular filesystem(s), simply replace it with a different, allowed character (and remember to encode any occurrences of that character properly).
Using this method:
mail.address#server.com
Would become:
mail%2Eaddress%40server%2Ecom
Or, if you had to substitute (for example), the letter a instead of the % symbol:
ma61ila2Ea61ddressa40servera2Ecom
Not exactly humanly-readable perhaps, but easily enough processed through an encoding algorithm. For the best space efficiency, your escape character should be a character allowed by the filesystem, yet one that is not likely to appear frequently in an address.
This encoding scheme has the advantage that there is no size increase for most normal characters. The string length will ONLY go up for characters not supported by the filesystem.
Check out base64. Encoding and decoding is well defined.
I'd prefer this over rolling my own format any day.
Hmm, from your question I'm not totally clear on this point, but since you wanted some conversion I'm assuming that you want something that is at least human readable?
Each OS may have different restrictions, but are you close enough to the platforms that you would be able to find out/test what is acceptable in a username? If you could find three 'special' characters that you could use just to do a replace on '#', '.', '_' you would be good to go. (Is that comprehensive? if not you would need to make sure you know all of them otherwise you could clash.) I searched a bit trying to find whether there was a POSIX standard, but wasn't able to find anything, so that's why I think if you can just test what's valid that would be the most direct route.
With even one special character, you could do URL encoding, either with '%' if it's available, or whatever you choose if not, say '!", then { '#'->'!40", '_'->'!5F', '.'-> '!2E' }. (The spec [RFC1738] http://www.rfc-editor.org/rfc/rfc1738.txt) defines the characters as US-ASCII so you can just find a table, e.g. in wikipedia's ASCII article and look up the correct hex digits there.) Or, you could just do your own simple mapping since you don't need the whole ASCII set, you could just do a map with two characters per escaped character and have, say, '!a','!u','!p' for at, underscore, period.
If you have two special characters, say, '%', and '!', you could delimit text that represents the character, say, %at!, &us!, and '&pd!'. (This is pretty much html-style encoding, but instead of '&' and ';' you are using the available ones, and you're making up your own mnemonics.) Another idea is that you could use runs of a symbol to determine the translated character, where each new character flops which symbol is being used. (This conveniently stops the run if we need to put two of the disallowed characters next to each other.) So assume '%' and '!', with period being 1, underscore 2, and at-sign being three, 'mickey._sample_#fake.out' would become 'mickey%!!sample%%!!!fake%out'. There are other variations but this one is easy to code.
If none of this is an option (e.g. no symbols at all, just [a-zA-Z0-9]), then really I think the Base64 answer sounds about right. Really once we're getting to anything other than a simple replacement (and even that) it's already getting hard to type if that's the goal. But if you really need to try to keep the email mostly readable, what you do is implement some sort of escaping. I'm thinking use '0' as your escape character, so now '0' becomes '00', '#' becomes '01', '.' becomes '02', and '_' becomes '03'. So now, 'mickey01._sample_#fake.out'would become 'mickey0010203sample0301fake02out'. Not beautiful but it should work; since we escaped any raw 0's, just always make sure you define a mapping for whatever you choose as your escape char and you should be fine..
That's all I can think of atm. :) Definitely if there's no need for these usernames to be readable in the raw it seems like apparently Base64 won't work, since it can produce slashes. Heck, ok, just the 2-digit US-ASCII hex value for each character and you're done...] is a good way to go; there's lots of nice debugged, heavily field-tested code out there for it and it solves your problem quite handily. :)
Given...
- the limited set of characters allowed in various file systems
- the desire to keep the encoded email address short (both for human readability and for possible concerns with file system limitations)
...a possible approach may be a two steps encoding logic whereby the email is
first compressed using a lossless compression algorithm such as Lempel-Ziv, effectively turning it into a "binary" form, stored in a shorter array of bytes
then this array of bytes is encoded using a Base64-like algorithm
The idea is to minimize the size of the binary representation, so that the expansion associated with the storage inefficiency of the encoding -which can only store roughly 6 bits (and probably a bit less) per character-, doesn't cause the encoded string to be too long.
Without getting overly sophisticated for the compression nor the encoding, such a system would likely produce encoded strings that are maybe 4/5 of the input string size (the email address): the compression should easily half the size, but the encoding, say Base32, would grow the binary form size by 8/5.
Efforts in improving the compression ratio may allow the selection of more "wasteful" encoding schemes (with smaller character sets) and this may help making the output more human-readable and also more broadly safe on various flavors of file systems. For example whereby a Base64 seems optimal. space-wise, using only uppercase letter (base 26) may ensure portability of the underlying scheme to file systems where the file names are not case sensitive.
Another benefit of the initial generic compression is that few, if any, assumptions need to be made about the syntax of valid input key (email addresses here).
Ideas for compression:
LZ seems like a good choice, 'though one may consider primin its initial buffer with common patterns found in email addresses (example ".com" or even "a.com", "b.com" etc.). This initial buffer would ensure several instances of "citations" per compressed email address, hence a better compression ratio overall). To further squeeze a few bytes, maybe LZH or other LZ-variations could be used.
Aside from the priming of the buffer mentioned above, another customization may be to use a shorter buffer than typical LZ algorithms, since the string we have to compress (email address instances) are themselves very short and would not benefit from say a 512 bytes buffer. (Shorter buffer sizes allow shorter codes for the citations)
Ideas for encoding:
Base64 is not suitable as-is because of the slash (/), plus (+) and equal (=) characters. Alternate characters could be used to replace these; dash (-) comes to mind, but finding three charcters, allowed by all "flavors" of the targeted file systems may be a stretch.
Never the less, Base64 and its 4 output characters per 3 payload bytes ratio provide what is probably the barely achievable upper limit of storage efficiency [for an acceptable character set].
At the lower end of this efficiency, is maybe an ASCII representation of the Hexadeciamal values of the bytes in the array. This format with a doubling of the payload bytes may be acceptable, length-wise, and is interesting because of its simplicity (there is a direct and simple relation between each nibble (4 bits) in the input and characters in the encoded string.
Base32 whereby A thru Z encode 0 thru 25 and 0 thru 5 encode 26 thru 31, respectively, essentially variation of Base64 with an 8 output characters per 5 payload bytes ratio may be a very viable compromise.
I was wondering, what is the longest possible name length allowed by the Windows kernel?
E.g.: I know the kernel uses UNICODE_STRING structures to hold all object paths, and since the byte length of a wide-character string is stored inside a USHORT, that allows for a maximum path length of 2^15 - 1 characters. Is there a similar, hard restriction on a file name (rather than path)? (I don't care if NTFS or FAT32 imposes a particular restriction; I'm looking for the longest possible theoretically allowed name in the kernel, assuming no additional file system or shell restrictions.)
(Edit: For those wondering why this even matters, consider that normally, traversing a directory is achieved by FindFirstFile/FindNextFile calls, one call per file. Given the function named NtQueryDirectoryFile, which is the underlying system call and which returns multiple file names per call, it's actually possible to take advantage of this maximum-length restriction on the path to make an extremely-fast directory traverser that uses solely the stack as a buffer. Now I'm trying to extend that concept, and I need to know the maximum size of a file name.)
The maximum length of a path is 32,767 characters whereby each path component (directory or file) can have a maximum length of 255 characters (to be more exact, the value returned in the lpMaximumComponentLength parameter of the GetVolumeInformation function).
This is documented on MSDN.
Ah, I found this page myself that guarantees that file names can't be longer than 255 characters:
A pathname MUST be no more than 32,760 characters in length.
...
Each pathname component MUST be no more than 255 characters in length.
Which makes me wonder:
Why does Windows use ULONGs for file name lengths, when it uses USHORTs for path lengths?!
If anyone knows why this is, please post/comment! I'm rather curious. :)