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       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 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.

       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

       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.


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


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