When the flush request is initiated from an ioctl(2) or from flushq(9F), the FLUSHR or the FLUSHW bits of an M_FLUSH message must be switched. Bits are switched at the point where the M_FLUSH message is passed from a write queue to a read queue. This point is also known as the midpoint of the pipe.
The midpoint of a pipe is not always easily detectable, especially if there are numerous modules pushed on either end of the pipe. In that case, some mechanism needs to intercept all messages passing through the stream. If the message is an M_FLUSH message and it is at the stream midpoint, 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(7M) 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(7M) must be the first module pushed onto a pipe so that it is at the midpoint of the pipe itself.
The pipemod(7M) module handles only M_FLUSH messages. All other messages are passed to the next module using the putnext(9F) utility routine. If an M_FLUSH message is passed to pipemod(7M) and the FLUSHR and FLUSHW bits are set, the message is not processed but is passed to the next module using putnext(9F). If only the FLUSHR bit is set, it is turned off and the FLUSHW bit is set. The message is then passed to the next module, using putnext(9F). Similarly, if the FLUSHW bit was the only bit set in the M_FLUSH message, it is turned off and the FLUSHR bit is turned on. The message is then passed to the next module on the stream.
The pipemod(7M) module can be pushed on any stream if it requires the bit switching.