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PIPE(7)                    Linux Programmer's Manual                   PIPE(7)



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  pro-
       cess  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 oper-
       ation 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 pro-
       cess 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 pro-
              cesses.

   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
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2005-12-08                           PIPE(7)