mirror of https://github.com/mkerrisk/man-pages
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Groff
420 lines
13 KiB
Groff
.\" Copyright (C) 2005 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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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.\" manual page may be incorrect or out-of-date.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. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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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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.\" %%%LICENSE_END
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.\"
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.TH PIPE 7 2017-09-15 "Linux" "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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from the read end of the pipe.
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.PP
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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 can be used to create a communication channel between related
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processes; see
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.BR pipe (2)
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for an example.
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.PP
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A FIFO (short for First In First Out) has a name within the filesystem
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(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 (7)
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for further details.
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.IR Note :
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although FIFOs have a pathname in the filesystem,
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I/O on FIFOs does not involve operations on the underlying device
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(if there is one).
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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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.PP
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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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Nonblocking 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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.PP
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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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.PP
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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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.PP
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It is not possible to apply
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.BR lseek (2)
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to 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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.PP
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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 i386).
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Since Linux 2.6.11, the pipe capacity is 16 pages
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(i.e., 65,536 bytes in a system with a page size of 4096 bytes).
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Since Linux 2.6.35, the default pipe capacity is 16 pages,
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but the capacity can be queried and set using the
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.BR fcntl (2)
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.BR F_GETPIPE_SZ
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and
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.BR F_SETPIPE_SZ
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operations.
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See
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.BR fcntl (2)
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for more information.
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.PP
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The following
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.BR ioctl (2)
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operation, which can be applied to a file descriptor
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that refers to either end of a pipe,
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places a count of the number of unread bytes in the pipe in the
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.I int
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buffer pointed to by the final argument of the call:
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.PP
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ioctl(fd, FIONREAD, &nbytes);
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.PP
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The
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.B FIONREAD
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operation is not specified in any standard,
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but is provided on many implementations.
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.\"
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.SS /proc files
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On Linux, the following files control how much memory can be used for pipes:
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.TP
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.IR /proc/sys/fs/pipe\-max\-pages " (only in Linux 2.6.34)"
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.\" commit b492e95be0ae672922f4734acf3f5d35c30be948
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An upper limit, in pages, on the capacity that an unprivileged user
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(one without the
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.BR CAP_SYS_RESOURCE
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capability)
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can set for a pipe.
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.IP
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The default value for this limit is 16 times the default pipe capacity
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(see above); the lower limit is two pages.
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.IP
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This interface was removed in Linux 2.6.35, in favor of
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.IR /proc/sys/fs/pipe\-max\-size .
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.TP
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.IR /proc/sys/fs/pipe\-max\-size " (since Linux 2.6.35)"
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.\" commit ff9da691c0498ff81fdd014e7a0731dab2337dac
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The maximum size (in bytes) of individual pipes that can be set
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.\" This limit is not checked on pipe creation, where the capacity is
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.\" always PIPE_DEF_BUFS, regardless of pipe-max-size
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by users without the
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.B CAP_SYS_RESOURCE
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capability.
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The value assigned to this file may be rounded upward,
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to reflect the value actually employed for a convenient implementation.
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To determine the rounded-up value,
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display the contents of this file after assigning a value to it.
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.IP
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The default value for this file is 1048576 (1\ MiB).
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The minimum value that can be assigned to this file is the system page size.
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Attempts to set a limit less than the page size cause
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.BR write (2)
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to fail with the error
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.BR EINVAL .
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.IP
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Since Linux 4.9,
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.\" commit 086e774a57fba4695f14383c0818994c0b31da7c
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the value on this file also acts as a ceiling on the default capacity
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of a new pipe or newly opened FIFO.
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.TP
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.IR /proc/sys/fs/pipe\-user\-pages\-hard " (since Linux 4.5)"
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.\" commit 759c01142a5d0f364a462346168a56de28a80f52
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The hard limit on the total size (in pages) of all pipes created or set by
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a single unprivileged user (i.e., one with neither the
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.B CAP_SYS_RESOURCE
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nor the
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.B CAP_SYS_ADMIN
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capability).
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So long as the total number of pages allocated to pipe buffers
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for this user is at this limit,
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attempts to create new pipes will be denied,
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and attempts to increase a pipe's capacity will be denied.
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.IP
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When the value of this limit is zero (which is the default),
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no hard limit is applied.
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.\" The default was chosen to avoid breaking existing applications that
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.\" make intensive use of pipes (e.g., for splicing).
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.TP
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.IR /proc/sys/fs/pipe\-user\-pages\-soft " (since Linux 4.5)"
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.\" commit 759c01142a5d0f364a462346168a56de28a80f52
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The soft limit on the total size (in pages) of all pipes created or set by
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a single unprivileged user (i.e., one with neither the
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.B CAP_SYS_RESOURCE
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nor the
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.B CAP_SYS_ADMIN
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capability).
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So long as the total number of pages allocated to pipe buffers
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for this user is at this limit,
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individual pipes created by a user will be limited to one page,
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and attempts to increase a pipe's capacity will be denied.
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.IP
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When the value of this limit is zero, no soft limit is applied.
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The default value for this file is 16384,
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which permits creating up to 1024 pipes with the default capacity.
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.PP
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Before Linux 4.9, some bugs affected the handling of the
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.IR pipe\-user\-pages\-soft
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and
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.IR pipe\-user\-pages\-hard
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limits; see BUGS.
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.\"
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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 nonatomic: 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.
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(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 nonblocking
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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 nonatomic: 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 Open file status flags
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The only open file status flags that can be meaningfully applied to
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a pipe or FIFO are
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.B O_NONBLOCK
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and
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.BR O_ASYNC .
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.PP
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Setting the
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.B O_ASYNC
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flag for the read end of a pipe causes a signal
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.RB ( SIGIO
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by default) to be generated when new input becomes available on the pipe.
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The target for delivery of signals must be set using the
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.BR fcntl (2)
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.B F_SETOWN
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command.
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On Linux,
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.B O_ASYNC
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is supported for pipes and FIFOs only since kernel 2.6.
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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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POSIX.1 requires only unidirectional pipes.
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Portable applications should avoid reliance on
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bidirectional pipe semantics.
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.SS BUGS
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Before Linux 4.9, some bugs affected the handling of the
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.IR pipe\-user\-pages\-soft
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and
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.IR pipe\-user\-pages\-hard
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limits when using the
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.BR fcntl (2)
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.BR F_SETPIPE_SZ
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operation to change a pipe's capacity:
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.\" These bugs where remedied by a series of patches, in particular,
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.\" commit b0b91d18e2e97b741b294af9333824ecc3fadfd8 and
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.\" commit a005ca0e6813e1d796a7422a7e31d8b8d6555df1
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.IP (1) 5
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When increasing the pipe capacity, the checks against the soft and
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hard limits were made against existing consumption,
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and excluded the memory required for the increased pipe capacity.
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The new increase in pipe capacity could then push the total
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memory used by the user for pipes (possibly far) over a limit.
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(This could also trigger the problem described next.)
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.IP
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Starting with Linux 4.9,
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the limit checking includes the memory required for the new pipe capacity.
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.IP (2)
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The limit checks were performed even when the new pipe capacity was
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less than the existing pipe capacity.
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This could lead to problems if a user set a large pipe capacity,
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and then the limits were lowered, with the result that the user could
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no longer decrease the pipe capacity.
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.IP
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Starting with Linux 4.9, checks against the limits
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are performed only when increasing a pipe's capacity;
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an unprivileged user can always decrease a pipe's capacity.
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.IP (3)
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The accounting and checking against the limits were done as follows:
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.IP
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.RS
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.PD 0
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.IP (a) 4
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Test whether the user has exceeded the limit.
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.IP (b)
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Make the new pipe buffer allocation.
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.IP (c)
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Account new allocation against the limits.
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.PD
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.RE
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.IP
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This was racey.
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Multiple processes could pass point (a) simultaneously,
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and then allocate pipe buffers that were accounted for only in step (c),
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with the result that the user's pipe buffer
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allocation could be pushed over the limit.
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.IP
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Starting with Linux 4.9,
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the accounting step is performed before doing the allocation,
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and the operation fails if the limit would be exceeded.
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.PP
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Before Linux 4.9, bugs similar to points (1) and (3) could also occur
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when the kernel allocated memory for a new pipe buffer;
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that is, when calling
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.BR pipe (2)
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and when opening a previously unopened FIFO.
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.SH SEE ALSO
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.BR mkfifo (1),
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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 splice (2),
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.BR stat (2),
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.BR tee (2),
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.BR vmsplice (2),
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.BR mkfifo (3),
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.BR epoll (7),
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.BR fifo (7)
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