Author Topic: Filenames & Pathes >%MAX_PATH up to 32K?  (Read 5026 times)

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Offline Theo Gottwald

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Filenames & Pathes >%MAX_PATH up to 32K?
« on: November 07, 2009, 09:34:35 PM »
I am just looking on this Topic.
From what i see, we could theoretically work on files with a filename-lenght up to 32kb if we would use the
Unicode-API's (CreateFile, DeleteFile) instead of the ANSI API's.

Has anybody used these Unicode API's, which are they, and are there disadvantages?
If not, why don't we use generally these and have the advnatages of much longer Pathes?

MSDN-Infos here.

Maximum Path Length Limitation

In the Windows API (with some exceptions discussed in the following paragraphs), 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. For example, the maximum path on drive D is "D:\<some 256 character path string><NUL>" where "<NUL>" represents the invisible terminating null character for the current system codepage. (The characters < > are used here for visual clarity and cannot be part of a valid path string.)

Note  File I/O functions in the Windows API convert "/" to "\" as part of converting the name to an NT-style name, except when using the "\\?\" prefix as detailed in the following sections.

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>". (The characters < > are used here for visual clarity and cannot be part of a valid path string.)

Note  The maximum path of 32,767 characters is approximate, because the "\\?\" prefix may be expanded to a longer string by the system at run time, and this expansion applies to the total length.

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 machine 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.

There is no need to perform any Unicode normalization on path and file name strings for use by the Windows file I/O API functions because the file system treats path and file names as an opaque sequence of WCHARs. Any normalizations your application requires should be performed with this in mind, external of any calls to related Windows file I/O API functions.

When using an API to create a directory, the specified path cannot be so long that you cannot append an 8.3 file name (that is, the directory name cannot exceed MAX_PATH minus 12).

The shell and the file system have different requirements. It is possible to create a path with the Windows API that the shell user interface might not be able to interpret properly.

« Last Edit: November 07, 2009, 09:38:58 PM by Theo Gottwald »

Offline José Roca

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Re: Filenames & Pathes >%MAX_PATH up to 32K?
« Reply #1 on: November 07, 2009, 11:55:02 PM »
Disadvantages? Too much typing. Don't ask me to type a 32 Kb file name.

Which one are they? The same that the ansi ones but with an ending "W", e.g. CreateFileW, DeleteFileW.

Offline Peter Weis

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Re: Filenames & Pathes >%MAX_PATH up to 32K?
« Reply #2 on: March 30, 2017, 06:14:13 PM »
« Last Edit: March 30, 2017, 06:29:16 PM by Peter Weis »