# comments the chunk may start at an arbitrary distance from the
    # end of the file. So if we do not find the header immediately
    # we have to extend the range of our search, possibly until we
    # have a large part of the archive in memory. We can fail only
    # after the whole file has been searched.

    set sz  [tell $fd]
    if {[info exists ::zip::max_header_seek]} {
        if {$::zip::max_header_seek < $sz} {
            set sz $::zip::max_header_seek
        }
    } 
    set len 512
    set at  512
    while {1} {
	if {$sz < $at} {set n -$sz} else {set n -$at}

	seek $fd $n end
	set hdr [read $fd $len]