PIPEMOD(4M) STREAMS Modules PIPEMOD(4M)

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
and FIFOs 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 and FIFOs 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 the FLUSHR 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 an M_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 requirement 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 the FLUSHR and FLUSHW
bits are set, the message is not processed but is passed to the next
module using the putnext() routine. If only the FLUSHR bit is set,
the FLUSHR 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 the M_FLUSH message, the FLUSHW
bit is turned off and the FLUSHR bit is turned on. The message is
then passed 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.

SEE ALSO

STREAMS Programming Guide

August 21, 1992 PIPEMOD(4M)