xenial (7) pipe.7.gz

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NAME

       pipe - overview of pipes and FIFOs

DESCRIPTION

       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 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
       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.  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.

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

SEE ALSO

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

COLOPHON

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