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.
STREAMS Programming Guide