You have a file descriptor (probably from a syscall), and you want the
corresponding pathname.

If you are on newer versions of DTrace, there is the fds[] array,

# dtrace -n 'syscall::read:entry { @[fds[arg0].fi_pathname] = count(); }'
dtrace: description 'syscall::read:entry ' matched 1 probe
^C

  /etc/minor_perm                                                   2
  /etc/mnttab                                                       2
  /etc/motd                                                         2
  /etc/magic                                                        4
  /usr/sbin/clri                                                    5
  /devices/pseudo/clone@0:ptm                                       6
  /sbin/mount                                                       6
  /dev/pts/28                                                       7
  /devices/pseudo/consms@0:mouse                                   31
  /devices/pseudo/conskbd@0:kbd                                    47
  <unknown>                                                       351

easy.

but if you are on an older version of DTrace, try this to convert from
this->fd to self->vpath,

        this->filep =
	    curthread->t_procp->p_user.u_finfo.fi_list[this->fd].uf_file;
        this->vnodep = this->filep != 0 ? this->filep->f_vnode : 0;
        self->vpath = this->vnodep ? (this->vnodep->v_path != 0 ?
            cleanpath(this->vnodep->v_path) : "<unknown>") : "<unknown>";