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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 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(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.

   PIPE_BUF
       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
       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.1-2001,  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), epoll(7), fifo(7)

COLOPHON

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