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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  only  allow  communication
       between  related  processes:  one  process  creates  the pipe, and then
       allows another process to inherit duplicate file descriptors  referring
       to the pipe as a result of calling fork(2).

       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(4) 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 x86).   Since  Linux  2.6.11,
       the pipe capacity is 65536 bytes.

   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 non-atomic: 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  non-blocking  (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  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 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, pipes only need to be unidirectional.   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),
       stat(2), mkfifo(3), fifo(4), epoll(4)