SetCurrentDirectory path limit

Julian Hansen
Julian Hansen used Ask the Experts™
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According to MSDN docs for SetCurrentDirectory (https://msdn.microsoft.com/en-us/library/windows/desktop/aa365530%28v=vs.85%29.aspx) path must be MAX_PATH -2 or less bytes (without trailing '\') MAX_PATH-1 if '\' is included.

I have a situation where I have a path (Windows 10) that is MAX_PATH -1 without trailing '\'.

Question if the OS allows this path - how do I SetCurrentDirectory to it ?
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Hello,
If the OS (Windows 10) allows you can do set it. For example Windows 10 explorer use same logic that your C++ program running on Windows use.

-FA
Top Expert 2016

Commented:
If the OS (Windows 10) allows you can do set it.
that is not true. windows allows path lengths up to 1024 what is nearly double of MAX_PATH, which currently is 260. MAX_PATH-1 however is limitation of most api functions and if you pass zero terminated path strings longer than this limitation, the function either would fail or - worse - write beyond array boundaries or might skip the termination and cut the path at the last character.

see https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx#maxpath for more information.

Sara
Most Valuable Expert 2017
Distinguished Expert 2018

Author

Commented:
So, is the solution then that you cannot use SetCurrentDirectory if you want to change path to one that is longer than 258 chars? Seems like a bit of an oversight.

_chdir has the same limitation.

Anyone have any ideas about how to programmatically change directory into a path > 258?
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Top Expert 2016

Commented:
it is possible by using the \\?\ prefix as described in the link i posted:

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".
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 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.

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 normalization that 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 is not able to interpret properly.

Sara
Top Expert 2016
Commented:
another way out is to use 8.3 short name for each folder in the path. winapi functions like FindFile additionally provide the short names.

i personally don't think that file paths with huge lengths should be supported by user applications. it doesn't help if a few programs can handle that, and finally the user can't print it or can't add it to their mail or link. i think it is better to refuse the file in the first case.

i remember a case where a user took the whole contents of a text message to build the filename and the file itself was empty. one of these files was shown (partly) in the explorer but all actions of the right-click menu failed with "file not found".

Sara
Most Valuable Expert 2017
Distinguished Expert 2018

Author

Commented:
Tried it with the \\?\ prefix - same problem - choking on the path 259 bytes in length.

Going to try the UniCode version to see if that is better - the docs seem to indicate that it should work. I will need to convert the routine to wide char - will check back when that is done.
Most Valuable Expert 2017
Distinguished Expert 2018

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Commented:
Thanks for the input pushed me in the right direction.

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