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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 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.

       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 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.  According to
       POSIX.1-2001, pipes only need to be unidirectional.  Portable  applica‐
       tions 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)

       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2005-12-08			       PIPE(7)

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