I had to help someone delete a folder that had weird characters in it that caused the path to be re-interepeted as a different path:
c:\test. -> c:\test
It took me a while to recall the \\?\ construct, since I have no idea what it's called or how to search for it. Once I remembered it, though, it was easy:
\\?\c:\test. -> c:\test.
What is the name of this construct, that I (and others) may search for it?
I don't think it has an official name in widespread use, so I doubt that you'll get very far in any searches. It is described here: https://msdn.microsoft.com/en-gb/library/windows/desktop/aa365247.aspx#maxpath
The Windows API has many functions that also have Unicode versions to permit an extended-length path for a maximum total path length of 32,767 characters. This type of path is composed of components separated by backslashes, each up to the value returned in the lpMaximumComponentLength parameter of the GetVolumeInformation function (this value is commonly 255 characters). To specify an extended-length path, use the "\\?\" prefix. For example, "\\?\D:\very long path".
For this usage it might be called the extended-length path prefix. However the prefix serves other purposes, most specifically suppressing user mode path canonicalisation, the purpose that you were availing yourself of.
As you can see from comments to this answer, there are lots of varied opinions on the most suitable name. I think that we can all agree that there is no single officially used name for this thing!
Related
I found a reference to a file in a log that had the following format:
\\?\C:\Path\path\file.log
I cannot find a reference to what the sequence of \?\ means. I believe the part between the backslashes refers to a hostname.
For instance, on my Windows computer, the following works just fine:
dir \\?\C:\
and also, just fine with same result:
dir \\.\C:\
Questions:
Is there a reference to what the question mark means in this particular path format?
What might generate a file path in such a format?
A long read, but worth reading if you are in this domain: http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247%28v=vs.85%29.aspx
Extract:
The Windows API has many functions that also have Unicode versions to
permit an extended-length path for a maximum total path length of
32,767 characters. This type of path is composed of components
separated by backslashes, each up to the value returned in the
lpMaximumComponentLength parameter of the GetVolumeInformation
function (this value is commonly 255 characters). To specify an
extended-length path, use the "\\?\" prefix. For example,
"\\?\D:\very long path".
and:
The "\\?\" prefix can also be used with paths constructed according to
the universal naming convention (UNC). To specify such a path using
UNC, use the "\\?\UNC\" prefix. For example, "\\?\UNC\server\share",
where "server" is the name of the computer and "share" is the name of
the shared folder. These 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.
Because you cannot use the "\\?\" prefix with a relative path,
relative paths are always limited to a total of MAX_PATH characters.
The Windows API parses input strings for file I/O. Among other things, it translates / to \ as part of converting the name to an NT-style name, or interpreting the . and .. pseudo directories. With few exceptions, the Windows API also limits path names to 260 characters.
The documented purpose of the \\?\ prefix is:
For file I/O, the "\\?\" prefix to a path string tells the Windows APIs to disable all string parsing and to send the string that follows it straight to the file system.
Among other things, this allows using otherwise reserved symbols in path names (such as . or ..). Opting out of any translations, the system no longer has to maintain an internal buffer, and the arbitrary limit of 260 characters can also be lifted (as long as the underlying filesystem supports it). Note, that this is not the purpose of the \\?\ prefix, rather than a corollary, even if the prefix is primarily used for its corollary.
I'm reading this. Here I've found some code lines, for example: wsprintf(szDrive, "\\\\.\\%c:", *lpszSrc); I want to ask, what does this string give?
I tried to look for information but all that I've found is:
In the ANSI version of this function, the name is limited to MAX_PATH
characters. To extend this limit to 32,767 wide characters, call the
Unicode version of the function and prepend "\\?\" to the path. For
more information, see Naming Files, Paths, and Namespaces.
and this do not answer into my question, so asking here. As I think it should be connected with windows specific or NTFS but not sure about that.
The %c is the single character format specifier for wsprintf.
The code is used to generate path names of this form:
\\.\C:
This is the path to a physical volume. You use such a path when performing file operations directly on a volume, bypassing the file system. So you'd use such a path when implementing raw disk copy, for example. The documentation for CreateFile has more detail.
This all ties in with the fact that the code you found this in performs a raw disk copy.
http://msdn.microsoft.com/en-us/library/windows/desktop/aa365740%28v=vs.85%29.aspx
cFileName
The name of the file.
The value of MAX_PATH is the same as almost the same as _MAX_FNAME, but using the first is misleading in this case. Anyway, if they meant the full path with name, should it be cFileName[MAX_PATH+_MAX_FNAME];? What was the purpose of using MAX_PATH?
The MAX_PATH constant represents the maximum length of an entire path, including the file name and the extension. It is irrelevant whether it is a relative path, a fully-qualified path, or even just a file name; they all have the same maximum length.
Thus, you would never see MAX_PATH + _MAX_FNAME because that would exceed the maximum length allowed for a path.
You'll find pretty much everything you ever wanted to know about paths in Win32 in this article. Note that some APIs (these will generally be called out explicitly in the docs ) accept long path names, indicated with a special prefix, which are not subject to the limitations of MAX_PATH.
The function signature is correct in this case. Sometimes, though, there are snippets of sample code on MSDN that accompany the docs, and this sample code ranges anywhere from stylistically curious to an utter abomination. It's always worth using a good healthy dose of common sense when trying to adapt sample code you find.
There's no mistake here. In Windows APIs that are subject to limits on length, the maximum length of a name is (usually) MAX_PATH. It doesn't matter whether the name is a fully specified absolute path, a relative path, or just a file name, the length limit is still MAX_PATH.
It may well be that the underlying file system has different limits. It's perfectly plausible that the native file system limit could be less than 260. But if the API declares a limit of 260 characters, then that's the limit when using that particular API.
If ever you think that the MSDN library is incorrect, it's easy enough to check. Take a look at the definition in the Windows header file and compare it with that given in the MSDN library. Invariably you will find that the MSDN library is accurate.
I'm having a look at the code of FastCopy, where I want to add a few features.
Internally, it seems that FastCopy stores its paths with a \\?\ before the path. eg. \\?\c:\Program Files\Adobe. These paths are passed on directly to Windows API functions like DeleteFile, RemoveDirectory, etc. so it seems Windows understands the format.
But what do these extra characters mean and why do FastCopy stores them that way?
The thing that's probably most relevant for FastCopy is that it allows you to work with file names more than ~256 characters long.
If memory serves, it also prevents Windows from parsing a file name looking for things like \\server\file to access a shared file (though you can still use \\?\UNC\whatever), but that's probably not what's really intended/relevant here.
You are referring to Long UNC paths : https://en.wikipedia.org/wiki/Path_%28computing%29 Hope that helps.
Generally that means it's supporting long file names - names up to about 32K in length.
It can also be used to specify UNC paths, e.g. \\?\UNC\server\share.
Without that support Fastcopy wouldn't be able to access all files properly.
More detail at http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
I have several level of directories in the folder path. when the path exceeds 256, then I could not create a sub-folder or file from it. Is there any chance to build paths more than this length.
Can anyone help me out.
In fact the limit on path strings is 260 characters. The underlying OS, these days, can support much longer path names, up to 32,767 characters. In order to name a path with a long name you need to use the magic \\?\ prefix, and use the Unicode version of the API.
However, many tools don't support such long names. A classic example of such a tool is Explorer which won't let you create objects with names longer than 260 characters. Because of this I strongly advise you to avoid creating such long names—doing so will save you much heartache in the long run.
This should get you started: http://msdn.microsoft.com/en-us/library/aa365247%28v=vs.85%29.aspx#maxpath
Sadly it's an issue that I don't think will be going away any time soon, so you'd do well to familiarize yourself with that stuff.
As an aside, if you have access to robocopy (comes packaged with Windows Vista and 7, but is also available for XP), which supports long paths, you could create your files/subfolders in a higher-up folder and then use robocopy to move the subfolder to its desired location deeper in the folder tree.
According to the documentation here http://msdn.microsoft.com/en-us/library/Aa365247, the maximum length is actually about 32,000, but most windows APIs still limit you to MAX_PATH which is 260. There are some unicode APIs that let you go beyond the 260 limit.
See here, http://msdn.microsoft.com/en-us/library/aa363856.
In the ANSI version of this function, the name is limited to MAX_PATH characters. To extend this limit to 32,767 wide characters, call the Unicode version of the function and prepend \\?\ to the path. For more information, see Naming a File.
This is an addendum to the answers above. I extracted only a summary to what I think is relevant, from Microsoft's official documentation:
Maximum Path Length Limitation
In the Windows API (with some exceptions), the maximum length for a path is MAX_PATH,
which is defined as 260 characters. A local path is structured in the following order:
drive letter, colon, backslash, name components separated by backslashes, and a terminating null character.
Example: "D:\some 256-character-path-string" -> 256
Using long paths
The Windows API has many functions that also have Unicode versions to permit an extended-length path for a maximum total path length of 32,767 characters.
To specify an extended-length path, use the "\?" prefix. For example, "\?\D:\very long path".
Relative paths
Relative paths are always limited to a total of MAX_PATH characters.
Enable Long Paths in Win10
Starting in Windows 10.1607, MAX_PATH limitations have been removed from common Win32 file and directory functions.
However, you must opt-in to the new behavior.
From Microsoft documentation:
https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file?redirectedfrom=MSDN#maximum-path-length-limitation
Warning for Delphi users:
There is a problem in IOUtils. It cannot be used in conjunction with Max_Path. It uses InternalCheckDirPathParam all over the place!
Details: TDirectory.GetDirectoryRoot does not handle correctly paths of Max_Path characters