Message Types

All STREAMS messages are assigned message types to indicate their intended use by modules and drivers and to determine their handling by the stream head.

A driver or module can assign most types to a message it generates, and a module can modify a message type during processing. The stream head will convert certain system calls to specified message types and send them downstream. It will also respond to other calls by copying the contents of certain message types that were sent upstream. Messages exist only in the kernel, so a user process can only send and receive buffers. The process is not explicitly aware of the message type, but it may be aware of message boundaries, depending on the system call used (see the distinction between the getmsg system call and the read subroutine in Sending and Receiving Messages ).

Most message types are internal to STREAMS and can only be passed from one STREAMS module to another. A few message types, including M_DATA, M_PROTO, and M_PCPROTO, can also be passed between a stream and user processes. M_DATA messages carry data both within a stream and between a stream and a user process. M_PROTO and M_PCPROTO messages carry both data and control information. However, the distinction between control information and data is generally determined by the developer when implementing a particular stream. Control information includes two types of information: service interface information and condition or status information. Service interface information is carried between two stream entities that present service interfaces. Condition or status information can be sent between any two stream entities regardless of their interface. An M_PCPROTO message has the same general use as an M_PROTO message, but the former moves faster through a stream.