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PIPE(7)                    Linux Programmer's Manual                   PIPE(7)

       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 example.

       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, assum-
       ing 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.  Non-blocking 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 non-atomic: 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 non-blocking (O_NONBLOCK), whether there are multi-
       ple 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 imme-
              diately, writing all n bytes; otherwise write(2) fails, with  errno  set  to

       O_NONBLOCK disabled, n > PIPE_BUF
              The  write is non-atomic: 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

       O_NONBLOCK enabled, n > PIPE_BUF
              If  the pipe is full, then write(2) fails, with errno set to EAGAIN.  Other-
              wise, 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

   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 bidirec-
       tional pipe semantics.

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

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-

Linux                             2005-12-08                           PIPE(7)

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