1pipemod(7M) STREAMS Modules pipemod(7M)
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6 pipemod - STREAMS pipe flushing module
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9 The typical stream is composed of a stream head connected to modules
10 and terminated by a driver. Some stream configurations such as pipes
11 and FIFOs do not have a driver and hence certain features commonly sup‐
12 ported by the driver need to be provided by other means. Flushing is
13 one such feature, and it is provided by the pipemod module.
14 Pipes and FIFOs in their simplest configurations only have stream
17 heads. A write side is connected to a read side. This remains true when
18 modules are pushed. The twist occurs at a point known as the mid-point.
19 When an M_FLUSH message is passed from a write queue to a read queue
20 the FLUSHR and/or FLUSHW bits have to be switched. The mid-point of a
21 pipe is not always easily detectable, especially if there are numerous
22 modules pushed on either end of the pipe. In that case there needs to
23 be a mechanism to intercept all message passing through the stream. If
24 the message is an M_FLUSH message and it is at the mid-point, the flush
25 bits need to be switched. This bit switching is handled by the pipemod
26 module.
27 pipemod should be pushed onto a pipe or FIFO where flushing of any kind
30 will take place. The pipemod module can be pushed on either end of the
31 pipe. The only requirement is that it is pushed onto an end that previ‐
32 ously did not have modules on it. That is, pipemod must be the first
33 module pushed onto a pipe so that it is at the mid-point of the pipe
34 itself.
35 The pipemod module handles only M_FLUSH messages. All other messages
38 are passed on to the next module using the putnext() utility routine.
39 If an M_FLUSH message is passed to pipemod and the FLUSHR and FLUSHW
40 bits are set, the message is not processed but is passed to the next
41 module using the putnext() routine. If only the FLUSHR bit is set, the
42 FLUSHR bit is turned off and the FLUSHW bit is set. The message is then
43 passed on to the next module using putnext(). Similarly, if the FLUSHW
44 bit is the only bit set in the M_FLUSH message, the FLUSHW bit is
45 turned off and the FLUSHR bit is turned on. The message is then passed
46 to the next module on the stream.
47 The pipemod module can be pushed on any stream that desires the bit
50 switching. It must be pushed onto a pipe or FIFO if any form of flush‐
51 ing must take place.
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54 STREAMS Programming Guide
55SunOS 5.11 21 Aug 1992 pipemod(7M)