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 commu-
       nication channel between related processes; see pipe(2) for an example.

       A  FIFO (short for First In First Out) has a name within the filesystem
       (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 filesystem, I/O on FIFOs does not involve opera-
       tions 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.  Since Linux 2.6.35, the default pipe
       capacity is 65536 bytes, but the capacity can be queried and set  using
       the  fcntl(2)  F_GETPIPE_SZ  and F_SETPIPE_SZ operations.  See fcntl(2)
       for more information.

       POSIX.1 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
       requires PIPE_BUF to be at least 512 bytes.   (On  Linux,  PIPE_BUF  is
       4096 bytes.)  The precise semantics depend on whether the file descrip-
       tor 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 inter-
              leaved  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 writ-
              ten), and these bytes may be interleaved with  writes  by  other

   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.  POSIX.1 requires
       only unidirectional pipes.  Portable applications should avoid reliance
       on bidirectional pipe semantics.

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

       This  page  is  part of release 4.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at

Linux                             2015-12-05                           PIPE(7)
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