pipemod(7M)

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 configura-
     tions 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 put-
     next() 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(). Simi-
     larly, 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