mirror of https://github.com/mkerrisk/man-pages
128 lines
4.4 KiB
Groff
128 lines
4.4 KiB
Groff
.\" This manpage has been automatically generated by docbook2man
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.\" from a DocBook document. This tool can be found at:
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.\" <http://shell.ipoline.com/~elmert/comp/docbook2X/>
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.\" Please send any bug reports, improvements, comments, patches,
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.\" etc. to Steve Cheng <steve@ggi-project.org>.
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.\"
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.\" %%%LICENSE_START(MIT)
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.\" This page is made available under the MIT license.
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.\" %%%LICENSE_END
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.\"
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.TH FUTEX 7 2015-12-28 "Linux" "Linux Programmer's Manual"
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.SH NAME
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futex \- fast user-space locking
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.SH SYNOPSIS
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.nf
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.B #include <linux/futex.h>
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.fi
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.SH DESCRIPTION
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.PP
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The Linux kernel provides futexes ("Fast user-space mutexes")
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as a building block for fast user-space
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locking and semaphores.
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Futexes are very basic and lend themselves well for building higher-level
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locking abstractions such as
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mutexes, condition variables, read-write locks, barriers, and semaphores.
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.PP
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Most programmers will in fact not be using futexes directly but will
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instead rely on system libraries built on them,
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such as the Native POSIX Thread Library (NPTL) (see
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.BR pthreads (7)).
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.PP
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A futex is identified by a piece of memory which can be
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shared between processes or threads.
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In these different processes, the futex need not have identical addresses.
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In its bare form, a futex has semaphore semantics;
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it is a counter that can be incremented and decremented atomically;
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processes can wait for the value to become positive.
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.PP
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Futex operation occurs entirely in user space for the noncontended case.
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The kernel is involved only to arbitrate the contended case.
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As any sane design will strive for noncontention,
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futexes are also optimized for this situation.
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.PP
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In its bare form, a futex is an aligned integer which is
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touched only by atomic assembler instructions.
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This integer is four bytes long on all platforms.
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Processes can share this integer using
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.BR mmap (2),
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via shared memory segments, or because they share memory space,
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in which case the application is commonly called multithreaded.
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.SS Semantics
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.PP
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Any futex operation starts in user space,
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but it may be necessary to communicate with the kernel using the
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.BR futex (2)
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system call.
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.PP
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To "up" a futex, execute the proper assembler instructions that
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will cause the host CPU to atomically increment the integer.
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Afterward, check if it has in fact changed from 0 to 1, in which case
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there were no waiters and the operation is done.
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This is the noncontended case which is fast and should be common.
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.PP
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In the contended case, the atomic increment changed the counter
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from \-1 (or some other negative number).
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If this is detected, there are waiters.
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User space should now set the counter to 1 and instruct the
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kernel to wake up any waiters using the
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.B FUTEX_WAKE
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operation.
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.PP
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Waiting on a futex, to "down" it, is the reverse operation.
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Atomically decrement the counter and check if it changed to 0,
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in which case the operation is done and the futex was uncontended.
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In all other circumstances, the process should set the counter to \-1
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and request that the kernel wait for another process to up the futex.
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This is done using the
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.B FUTEX_WAIT
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operation.
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.PP
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The
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.BR futex (2)
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system call can optionally be passed a timeout specifying how long
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the kernel should
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wait for the futex to be upped.
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In this case, semantics are more complex and the programmer is referred
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to
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.BR futex (2)
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for
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more details.
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The same holds for asynchronous futex waiting.
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.SH VERSIONS
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.PP
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Initial futex support was merged in Linux 2.5.7
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but with different semantics from those described above.
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Current semantics are available from Linux 2.5.40 onward.
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.SH NOTES
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.PP
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To reiterate, bare futexes are not intended as an easy-to-use
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abstraction for end users.
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Implementors are expected to be assembly literate and to have read
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the sources of the futex user-space library referenced
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below.
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.PP
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This man page illustrates the most common use of the
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.BR futex (2)
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primitives; it is by no means the only one.
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.\" .SH AUTHORS
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.\" .PP
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.\" Futexes were designed and worked on by Hubertus Franke
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.\" (IBM Thomas J. Watson Research Center),
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.\" Matthew Kirkwood, Ingo Molnar (Red Hat) and
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.\" Rusty Russell (IBM Linux Technology Center).
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.\" This page written by bert hubert.
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.SH SEE ALSO
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.BR clone (2),
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.BR futex (2),
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.BR get_robust_list (2),
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.BR set_robust_list (2),
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.BR set_tid_address (2),
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.BR pthreads (7)
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.IR "Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux"
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(proceedings of the Ottawa Linux Symposium 2002),
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futex example library, futex-*.tar.bz2
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.UR ftp://ftp.kernel.org\:/pub\:/linux\:/kernel\:/people\:/rusty/
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.UE .
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