Technical data

pipemod(7M) STREAMS Modules SunOS 5.5

NAME pipemod − STREAMS pipe ﬂushing module

DESCRIPTION The typical stream is composed of a stream head connected to modules and terminated

by a driver. Some stream conﬁgurations such as pipes andFIFOs do not have a driver

and hence certain features commonly supported by the driver need to be provided by

other means. Flushing is one such feature, and it is provided by the pipemod module.

Pipes andFIFOs in their simplest conﬁgurations only have stream heads. A write side is

connected to a read side. This remains true when modules are pushed. The twist occurs

at a point known as the mid-point. When an M_FLUSH message is passed from a write

queue to a read queue theFLUSHR and/or FLUSHW bits have to be switched. The mid-

point of a pipe is not always easily detectable, especially if there are numerous modules

pushed on either end of the pipe. In that case there needs to be a mechanism to intercept

all message passing through the stream. If the message is anM_FLUSH message and it is

at the mid-point, the ﬂush bits need to be switched. This bit switching is handled by the

pipemod module.

pipemod should be pushed onto a pipe or FIFO where ﬂushing of any kind will take

place. The pipemod module can be pushed on either end of the pipe. The only require-

ment is that it is pushed onto an end that previously did not have modules on it. That is,

pipemod must be the ﬁrst module pushed onto a pipe so that it is at the mid-point of the

pipe itself.

The pipemod module handles only M_FLUSH messages. All other messages are passed

on to the next module using the putnext() utility routine. If an M_FLUSH message is

passed to pipemod and theFLUSHR andFLUSHW bits are set, the message is not pro-

cessed but is passed to the next module using the putnext() routine. If only the FLUSHR

bit is set, theFLUSHR bit is turned off and the FLUSHW bit is set. The message is then

passed on to the next module using putnext(). Similarly, if the FLUSHW bit is the only bit

set in theM_FLUSH message, the FLUSHW bit is turned off and theFLUSHR bit is turned

on. The message is thenpassed to the next module on the stream.

The pipemod module can be pushed on any stream that desires the bit switching. It

must be pushed onto a pipe or FIFO if any form of ﬂushing must take place.