How to find relative path given two absolute paths?

Question

Given two absolute paths, e.g.

• /a/path/to/a
• /a/path/to/somewhere/else

How can I get a relative path from one to the other, ../a?

In a sense, the opposite of what realpath does.

Answer 1

I answered a similar question here: Resolving a relative path without referencing the current directory on Windows.

There is no standard function for this. There is a function in vi-like-emacs for this purpose. A quick check of apropos relative shows me few other programs which likely implement this: revpath for example).

It could be done as a string-manipulation (no need to compute working directories):

• start by finding the longest common prefix which ends with a path-separator.
• if there is no common prefix, you are done
• strip the common prefix from (a copy of...) the current and target strings
• replace each directory-name in the current string with ".."
• add that (with a path-separator) in front of the target string
• return that combined string

The "done" in the second step presumes that you want to use a relative path to shorten the result. On the other hand, you might want to use a relative pathname regardless of the length. In that case, just skip the step (the result will be longer, but relative).

Answer 2

Find the longest common path (in this case, /a/path/to) and delete it from both absolute paths. That would give:

• /a
• /somewhere/else

Now, replace each path component in the starting path with ../ and prepend the result to the destination path. If you want to go from directory else to directory a, that would give you:

../../a

If you want to go the other way, you'd instead have:

../somewhere/else

Answer 3

Build a tree with the first absolute path, then add the second path to that tree, and then walk from one leaf to the other: a step from one node to its parent is translated to a "../" sequence, and a step from a node to one of its children is translated to the name of that children. Notice that there might be more than one solution. For example:

1) /a/path/to/a

And

2) /a/path/to/a/new/one

The obvious path from (1) to (2) is new/one but ../../../a/path/to/a/new/one is also valid. When you write the algorithm to do the walking in your tree you have to be aware of this

Answer 4

Using cwalk you can use cwk_path_get_relative, which even works cross-platform:

#include <cwalk.h>
#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
  char buffer[FILENAME_MAX];

  cwk_path_get_relative("/hello/there/", "/hello/world", buffer, sizeof(buffer));
  printf("The relative path is: %s", buffer);

  return EXIT_SUCCESS;
}

Output:

The relative path is: ../world