In get_foo_message_len(...) - What value is passed into offset?

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/* determine PDU length of protocol foo */
static guint
get_foo_message_len(packet_info *pinfo _U_, tvbuff_t *tvb, int offset, void *data _U_)
{
    /* TODO: change this to your needs */
    return (guint)tvb_get_ntohl(tvb, offset+4); /* e.g. length is at offset 4 */
}

/* The main dissecting routine */
static int
dissect_foo(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data)
{
    tcp_dissect_pdus(tvb, pinfo, tree, TRUE, FRAME_HEADER_LEN,
                     get_foo_message_len, dissect_foo_message, data);
    return tvb_captured_length(tvb);
}