hci_acldata_hdr_t
#define HCI_ACL_PKT_SIZE (sizeof(hci_acldata_hdr_t) + 0xffff)
hci_acldata_hdr_t hdr;
hci_acldata_hdr_t *hdr;
hdr = mtod(m, hci_acldata_hdr_t *);