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

NAME         top

       pipe - overview of pipes and FIFOs

DESCRIPTION         top

       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

       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

   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.

       The following ioctl(2) operation, which can be applied to a file
       descriptor that refers to either end of a pipe, places a count of the
       number of unread bytes in the pipe in the int buffer pointed to by
       the final argument of the call:

           ioctl(fd, FIONREAD, &nbytes);

       The FIONREAD operation is not specified in any standard, but is
       provided on many implementations.

       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.  The target for delivery of signals must be set using
       the fcntl(2) F_SETOWN command.  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.

SEE ALSO         top

       mkfifo(1), dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2),
       socketpair(2), splice(2), stat(2), mkfifo(3), epoll(7), fifo(7)

COLOPHON         top

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Linux                            2016-10-08                          PIPE(7)