How to get real full path of a file or directory having one of its paths?

Question

The same file system entry can be accessible in several paths.

1. real full path - /home/user/dir1/file1
2. path which contains parent dirs - /home/user/dir1/../dir1/file1
3. path with direct symlinks - /home/user/dir1/symlink_to_file1
4. path with indirect symlinks - /home/user/symlink_to_dir1/file1
5. ...

I want two write a function which for given two paths will tell whether the file or directory specified by the second path is inside (including sub-directories) the directory specified by the first path.

I think the most obvious solution is to find real full paths of both file system entries then check whether the first real path is a prefix of the second. That is why the title of question is about finding real full paths .

NOTE: I want to write the function for both Windows and POSIX compatible systems.

NOTE: boost::filesystem cannot be used.

Answer 1

In Windows and Unix-land alike there is no single “real path”. In particular a file can have many different directory entries, called hardlinks, in Unix-land created via ln and in Windows 7 and later via mklink . But also, in Windows you can very simply define a local logical drive mapped to some directory, via the subst command, and drives mapped to file server directories via eg net use , and you can mount a drive as a directory, eg via the mountvol command.

However, the “real path” problem is just an imagined solution to the real problem, which is to establish whether a file or directory is inside a directory specified via a path.

For that, establish a system-specfic ID for the filesystem entity that you're searching for, and scan up the parent directory chain looking for that ID. Sorry, I misread the question. I can't think of any efficient way to do this, it sounds like brute force ID search through all possible directories, unless you can avail yourself of indexing information.

Answer 2

The question you need to know up front is this: How many ways are there to get to /path/to/filename? With symbolic links the answer is infinite (well, within the bound of the filesystem size). Any symbolic link anywhere on any portion of the filesystem could redirect to the file (or some portion of the path above the file). Even without considering hard links the search space must be the entire filesystem under /base/path/of/interest/ (which may be the entire filesystem).

Allowing symbolic links, and without further limitations, there is no non-brute-force method for establishing whether /path/to/filename is reachable within /base/path/of/interest/.