Linux – How to Get Absolute Path from Relative Path Programmatically

Use realpath().

The realpath() function shall derive, from the pathname pointed to by file_name, an absolute pathname that names the same file, whose resolution does not involve '.', '..', or symbolic links. The generated pathname shall be stored as a null-terminated string, up to a maximum of {PATH_MAX} bytes, in the buffer pointed to by resolved_name.

If resolved_name is a null pointer, the behavior of realpath() is implementation-defined.

The following example generates an absolute pathname for the file identified by the symlinkpath argument. The generated pathname is stored in the actualpath array.

#include <stdlib.h>
...
char *symlinkpath = "/tmp/symlink/file";
char actualpath [PATH_MAX+1];
char *ptr;


ptr = realpath(symlinkpath, actualpath);

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).