Message Blocks

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A STREAMS message consists of one or more linked message blocks. That is, the first message block of a message may be attached to other message blocks that are part of the same message.

Multiple blocks in a message can occur, for example, as the result of processing that adds header or trailer data to the data contained in the message, or because of size limitations in the message buffer that cause the data to span multiple blocks. When a message is composed of multiple message blocks, the message type of the first block determines the type of the entire message, regardless of the types of the attached message blocks.

STREAMS allocates a message as a single block containing a buffer of a certain size. If the data for a message exceeds the size of the buffer containing the data, the procedure can allocate a new block containing a larger buffer, copy the current data to it, insert the new data, and deallocate the old block. Alternatively, the procedure can allocate an additional (smaller) block, place the new data in the new message block, and link it after or before the initial message block. Both alternatives yield one new message.

Messages can exist standalone when the message is being processed by a procedure. Alternatively, a message can await processing on a linked list of messages, called a message queue, in a QUEUE. In the Message Queue diagram (Figure 1), Message 1 is linked to Message 2.

Figure 1. Message Queue
This diagram shows the queue header on the left which is bordered by message 1. The message block (type) in message 1 has a two-way arrow connected to the queue header and also another two-way arrow to the message block in message 2. Below this message block (type) an arrow points to another message block and that block, in turn, points to another message block within the message 1 area. The lowest message block has an arrow that points downward. To the right of message 1, is message 2. A two-way arrow exits the message block (type) on the right and continues to the next message. Below the message block (type) of message 2, an arrow points to a message block. That message block has an arrow that points downward.

When a message is queued, the first block of the message contains links to preceding and succeeding messages on the same message queue, in addition to containing a link to the second block of the message (if present). The message queue head and tail are contained in the QUEUE.

STREAMS utility routines enable developers to manipulate messages and message queues.