readlink() will fail if:
[ENOTDIR]
A component of the path prefix is not a directory.
[ENAMETOOLONG]
A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters.
[ENOENT]
The named file does not exist.
[EACCES]
Search permission is denied for a component of the path prefix.
[ELOOP]
Too many symbolic links were encountered in translating the pathname.
[EINVAL]
The named file is not a symbolic link.
[EIO]
An I/O error occurred while reading from the file system.
[EFAULT]
buf extends outside the process's allocated address space.