directory-file-name

directory-file-name is a built-in function in `C source code'.

(directory-file-name DIRECTORY)

Returns the file name of the directory named DIRECTORY.
This is the name of the file that holds the data for the directory DIRECTORY.
This operation exists because a directory is also a file, but its name as
a directory is different from its name as a file.
In Unix-syntax, this function just removes the final slash.