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       fifo - first-in first-out special file, named pipe


       A  FIFO  special  file  (a named pipe) is similar to a pipe, except that it is accessed as
       part of the filesystem.  It can be opened by multiple processes for  reading  or  writing.
       When  processes  are  exchanging  data via the FIFO, the kernel passes all data internally
       without writing it to the filesystem.  Thus, the FIFO special file has no contents on  the
       filesystem;  the filesystem entry merely serves as a reference point so that processes can
       access the pipe using a name in the filesystem.

       The kernel maintains exactly one pipe object for each FIFO special file that is opened  by
       at  least  one process.  The FIFO must be opened on both ends (reading and writing) before
       data can be passed.  Normally, opening the FIFO blocks until the other end is opened also.

       A process can open a FIFO in nonblocking mode.  In this case, opening for  read-only  will
       succeed  even if no-one has opened on the write side yet, opening for write-only will fail
       with ENXIO (no such device or address) unless the other end has already been opened.

       Under Linux, opening a FIFO  for  read  and  write  will  succeed  both  in  blocking  and
       nonblocking  mode.  POSIX leaves this behavior undefined.  This can be used to open a FIFO
       for writing while there are no readers available.  A process that uses both  ends  of  the
       connection in order to communicate with itself should be very careful to avoid deadlocks.


       When a process tries to write to a FIFO that is not opened for read on the other side, the
       process is sent a SIGPIPE signal.

       FIFO special files can be created by mkfifo(3), and are indicated by ls -l with  the  file
       type 'p'.


       mkfifo(1), open(2), pipe(2), sigaction(2), signal(2), socketpair(2), mkfifo(3), pipe(7)


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