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       pipe - overview of pipes and FIFOs


       Pipes  and  FIFOs  (also known as named pipes) provide a unidirectional
       interprocess communication channel.  A pipe has a read end and a  write
       end.  Data written to the write end of a pipe can be read from the read
       end of the pipe.

       A pipe is created using pipe(2), which creates a new pipe  and  returns
       two  file  descriptors,  one referring to the read end of the pipe, the
       other referring to the write end.   Pipes  can  be  used  to  create  a
       communication  channel  between  related  processes; see pipe(2) for an

       A FIFO (short for First In First Out) has a name within the file system
       (created  using  mkfifo(3)),  and is opened using open(2).  Any process
       may open a FIFO, assuming the file permissions allow it.  The read  end
       is  opened  using  the O_RDONLY flag; the write end is opened using the
       O_WRONLY flag.  See fifo(7) for further details.  Note: although  FIFOs
       have  a  pathname  in  the  file  system, I/O on FIFOs does not involve
       operations on the underlying device (if there is one).

   I/O on Pipes and FIFOs
       The only difference between pipes and FIFOs is the manner in which they
       are  created  and opened.  Once these tasks have been accomplished, I/O
       on pipes and FIFOs has exactly the same semantics.

       If a process attempts to read from an empty  pipe,  then  read(2)  will
       block  until  data  is  available.  If a process attempts to write to a
       full pipe (see below), then write(2) blocks until sufficient  data  has
       been  read  from  the pipe to allow the write to complete.  Nonblocking
       I/O is possible by using the fcntl(2) F_SETFL operation to  enable  the
       O_NONBLOCK open file status flag.

       The communication channel provided by a pipe is a byte stream: there is
       no concept of message boundaries.

       If all file descriptors referring to the write end of a pipe have  been
       closed,  then  an attempt to read(2) from the pipe will see end-of-file
       (read(2) will return 0).  If all file descriptors referring to the read
       end  of  a  pipe have been closed, then a write(2) will cause a SIGPIPE
       signal to be generated for the calling process.  If the calling process
       is  ignoring this signal, then write(2) fails with the error EPIPE.  An
       application that uses pipe(2) and fork(2) should use suitable  close(2)
       calls  to  close  unnecessary  duplicate file descriptors; this ensures
       that end-of-file and SIGPIPE/EPIPE are delivered when appropriate.

       It is not possible to apply lseek(2) to a pipe.

   Pipe Capacity
       A pipe has a limited capacity.  If the pipe is full,  then  a  write(2)
       will  block  or  fail,  depending on whether the O_NONBLOCK flag is set
       (see below).  Different implementations have different limits  for  the
       pipe  capacity.  Applications should not rely on a particular capacity:
       an application should be designed so that a  reading  process  consumes
       data  as  soon  as  it is available, so that a writing process does not
       remain blocked.

       In Linux versions before 2.6.11, the capacity of a pipe was the same as
       the  system  page size (e.g., 4096 bytes on i386).  Since Linux 2.6.11,
       the pipe capacity is 65536 bytes.

       POSIX.1-2001 says that write(2)s of less than PIPE_BUF  bytes  must  be
       atomic:  the  output  data  is  written  to  the  pipe  as a contiguous
       sequence.  Writes of more than PIPE_BUF bytes  may  be  nonatomic:  the
       kernel  may  interleave  the data with data written by other processes.
       POSIX.1-2001 requires PIPE_BUF to be at least 512  bytes.   (On  Linux,
       PIPE_BUF  is  4096 bytes.)  The precise semantics depend on whether the
       file descriptor is nonblocking (O_NONBLOCK), whether there are multiple
       writers to the pipe, and on n, the number of bytes to be written:

       O_NONBLOCK disabled, n <= PIPE_BUF
              All  n bytes are written atomically; write(2) may block if there
              is not room for n bytes to be written immediately

       O_NONBLOCK enabled, n <= PIPE_BUF
              If there is room to write n bytes to  the  pipe,  then  write(2)
              succeeds  immediately,  writing  all n bytes; otherwise write(2)
              fails, with errno set to EAGAIN.

       O_NONBLOCK disabled, n > PIPE_BUF
              The write is nonatomic:  the  data  given  to  write(2)  may  be
              interleaved with write(2)s by other process; the write(2) blocks
              until n bytes have been written.

       O_NONBLOCK enabled, n > PIPE_BUF
              If the pipe is full, then write(2)  fails,  with  errno  set  to
              EAGAIN.   Otherwise,  from  1 to n bytes may be written (i.e., a
              "partial write" may occur; the caller should  check  the  return
              value  from  write(2)  to  see  how  many  bytes  were  actually
              written), and these bytes may  be  interleaved  with  writes  by
              other processes.

   Open File Status Flags
       The  only  open file status flags that can be meaningfully applied to a
       pipe or FIFO are O_NONBLOCK and O_ASYNC.

       Setting the O_ASYNC flag for the read end of a  pipe  causes  a  signal
       (SIGIO  by default) to be generated when new input becomes available on
       the pipe (see fcntl(2) for details).  On Linux,  O_ASYNC  is  supported
       for pipes and FIFOs only since kernel 2.6.

   Portability notes
       On  some  systems (but not Linux), pipes are bidirectional: data can be
       transmitted in both directions between the  pipe  ends.   According  to
       POSIX.1-2001,   pipes   only   need  to  be  unidirectional.   Portable
       applications should avoid reliance on bidirectional pipe semantics.


       dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2),  socketpair(2),
       stat(2), mkfifo(3), epoll(7), fifo(7)


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