Contain ordinary data. Messages allocated by allocb(9F) are M_DATA type by default. M_DATA messages are generally sent bidirectionally on a Stream and their contents can be passed between a process and the Stream head. In the getmsg(2) and putmsg(2) system calls, the contents of M_DATA message blocks are referred to as the data part. Messages composed of multiple message blocks typically have M_DATA as the message type for all message blocks following the first.