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New page providing overview of pipes and FIFOs.
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Michael Kerrisk
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.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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.\" Copyright (C) 2005 Michael Kerrisk <mtk-manpages@gmx.net>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.TH PIPE 7 2005-12-08 "Linux 2.6.15" "Linux Programmer's Manual"
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.SH NAME
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pipe - overview of pipes and FIFOs
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.SH DESCRIPTION
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Pipes and FIFOs (also known as named pipes)
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provide a unidirectional interprocess communication channel.
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A pipe has a
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.I read end
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and a
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.IR "write end" .
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Data written to the write end of a pipe can be read
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by a process that has the read end of the pipe open.
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A pipe is created using
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.BR pipe (2),
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which creates a new pipe and returns two file descriptors,
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one referring to the read end of the pipe,
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the other referring to the write end.
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Pipes only allow communication between related processes:
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one process creates the pipe, and then allows another process to inherit
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duplicate file descriptors referring to the pipe as a result of calling
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.BR fork (2).
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A FIFO (short for First In First Out) has a name within the file
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system (created using
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.BR mkfifo (3)),
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and is opened using
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.BR open (2).
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Any process may open a FIFO, assuming the file permissions allow it.
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The read end is opened using the
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.B O_RDONLY
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flag; the write end is opened using the
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.B O_WRONLY
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flag.
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See
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.BR fifo (4)
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for further details.
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.IR Note :
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although FIFOs have a pathname in the file system,
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I/O on FIFOs does not involve disk operations.
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.SS "I/O on Pipes and FIFOs"
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The only difference between pipes and FIFOs is the manner in which
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they are created and opened.
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Once these tasks have been accomplished,
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I/O on pipes and FIFOs has exactly the same semantics.
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I/O is performed using
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.BR read (2)
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and
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.BR write (2).
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The communication channel provided by a pipe is a
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.IR "byte stream" :
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there is no concept of message boundaries;
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data can be read and written in arbitrary amounts.
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By default, I/O on pipes and FIFOs is blocking.
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If a process attempts to read from an empty pipe, then
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.BR read (2)
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will block until data is available.
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If a process attempts to write to a full pipe (see below), then
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.BR write (2)
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blocks until sufficient data has been read from the pipe
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to allow the write to complete.
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Non-blocking I/O is possible by using the
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.BR fcntl (2)
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.B F_SETFL
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operation to enable the
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.B O_NONBLOCK
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open file status flag.
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If all file descriptors referring to the write end of a pipe
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have been closed, then an attempt to
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.BR read (2)
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from the pipe will see end-of-file
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.RB ( read (2)
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will return 0).
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If all file descriptors referring to the read end of a pipe
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have been closed, then a
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.BR write (2)
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will cause a
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.B SIGPIPE
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signal to be generated for the calling process.
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If the calling process is ignoring this signal, then
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.BR write (2)
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fails with the error
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.BR EPIPE .
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An application that uses
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.BR pipe (2)
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and
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.BR fork (2)
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should use suitable
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.BR close (2)
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calls to close unnecessary duplicate file descriptors;
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this ensures that end-of-file and
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.BR SIGPIPE / EPIPE
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are delivered when appropriate.
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It is not possible to use
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.BR lseek (2)
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to randomly access the bytes in a pipe.
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.SS "Pipe Capacity"
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A pipe has a limited capacity.
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If the pipe is full, then a
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.BR write (2)
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will block or fail, depending on whether the
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.B O_NONBLOCK
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flag is set (see below).
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Different implementations have different limits for the pipe capacity.
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Applications should not rely on a particular capacity:
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an application should be designed so that a reading process consumes data
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as soon as it is available,
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so that a writing process does not remain blocked.
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In Linux versions before 2.6.11, the capacity of a pipe was the same as
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the system page size (e.g., 4096 bytes on x86).
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Since Linux 2.6.11, the pipe capacity is 65536 bytes.
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.SS PIPE_BUF
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POSIX.1 says that
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.BR write (2)s
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of less than
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.B PIPE_BUF
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bytes must be atomic: the output data is written to the pipe as a
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contiguous sequence.
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Writes of more than
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.B PIPE_BUF
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bytes may be non-atomic: the kernel may interleave the data
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with data written by other processes.
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POSIX.1 requires
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.B PIPE_BUF
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to be at least 512 bytes. (On Linux,
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.B PIPE_BUF
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is 4096 bytes.)
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The precise semantics depend on whether the file descriptor is non-blocking
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.RB ( O_NONBLOCK ),
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whether there are multiple writers to the pipe, and on
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.IR n ,
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the number of bytes to be written:
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.TP
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\fBO_NONBLOCK\fP disabled, \fIn\fP <= \fBPIPE_BUF\fP
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All
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.I n
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bytes are written atomically;
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.BR write (2)
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may block if there is not room for
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.I n
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bytes to be written immediately
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.TP
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\fBO_NONBLOCK\fP enabled, \fIn\fP <= \fBPIPE_BUF\fP
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If there is room to write
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.I n
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bytes to the pipe, then
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.BR write (2)
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succeeds immediately, writing all
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.I n
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bytes; otherwise
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.BR write (2)
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fails, with
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.I errno
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set to
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.BR EAGAIN .
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.TP
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\fBO_NONBLOCK\fP disabled, \fIn\fP > \fBPIPE_BUF\fP
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The write is non-atomic: the data given to
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.BR write (2)
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may be interleaved with
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.BR write (2)s
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by other process;
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the
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.BR write (2)
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blocks until
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.I n
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bytes have been written.
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.TP
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\fBO_NONBLOCK\fP enabled, \fIn\fP > \fBPIPE_BUF\fP
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If the pipe is full, then
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.BR write (2)
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fails, with
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.I errno
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set to
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.BR EAGAIN .
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Otherwise, from 1 to
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.I n
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bytes may be written (i.e., a "partial write" may occur;
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the caller should check the return value from
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.BR write (2)
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to see how many bytes were actually written),
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and these bytes may be interleaved with writes by other processes.
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.SS "Portability notes"
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On some systems (but not Linux), pipes are bidirectional:
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data can be transmitted in both directions between the pipe ends.
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According to POSIX.1, pipes only need to be unidirectional.
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Portable applications should avoid reliance on
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bidirectional pipe semantics.
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.SH "SEE ALSO"
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.BR dup (2),
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.BR fcntl (2),
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.BR open (2),
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.BR pipe (2),
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.BR poll (2),
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.BR select (2),
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.BR socketpair (2),
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.BR stat (2),
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.BR mkfifo (3),
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.BR fifo (4),
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.BR epoll (4)
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