mirror of https://github.com/mkerrisk/man-pages
86 lines
4.2 KiB
Groff
86 lines
4.2 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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.TH FUTEX 7 2002-12-31 "" "Linux Programmer's Manual"
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.SH NAME
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futex \- Fast Userspace Locking
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.SH SYNOPSIS
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.nf
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#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 Userspace muTexes') as a building block for fast userspace
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locking and semaphores. Futexes are very basic and lend themselves well for building higher level
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locking abstractions such as POSIX mutexes.
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.PP
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This page does not set out to document all design decisions but restricts itself to issues relevant for
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application and library development. Most programmers will in fact not be using futexes directly but
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instead rely on system libraries built on them, such as the NPTL pthreads implementation.
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.PP
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A futex is identified by a piece of memory which can be shared between different processes. In these
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different processes, it need not have identical addresses. In its bare form, a futex has semaphore
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semantics; it is a counter that can be incremented and decremented atomically; processes can wait for the
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value to become positive.
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.PP
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Futex operation is entirely userspace for the non-contended case. The kernel is only involved to arbitrate
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the contended case. As any sane design will strive for non-contention,
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futexes are also optimised
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for this situation.
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.PP
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In its bare form, a futex is an aligned integer which is only touched by atomic assembler
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instructions. Processes can share this integer using
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.BR mmap (),
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via shared memory segments or because they
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share memory space, in which case the application is commonly called multithreaded.
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.SH "SEMANTICS"
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.PP
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Any futex operation starts in userspace, but it may necessary to communicate with the kernel using the
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\fBfutex\fR(2) system call.
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.PP
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To 'up' a futex, execute the proper assembler instructions that will cause the host CPU to atomically
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increment the integer. Afterwards, 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. This is the non-contended case which is fast and
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should be common.
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.PP
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In the contended case, the atomic increment changed the counter from \-1 (or some other negative number). If this is detected,
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there are waiters. Userspace should now set the counter to 1 and instruct the kernel to wake up any
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waiters using the FUTEX_WAKE operation.
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.PP
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Waiting on a futex, to 'down' it, is the reverse operation. Atomically decrement the counter and
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check if it changed to 0, in which case the operation is done and the futex was uncontended. In all
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other circumstances, the process should set the counter to \-1 and request that the kernel wait for
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another process to up the futex. This is done using the FUTEX_WAIT operation.
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.PP
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The
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.BR futex()
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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. In this case, semantics are more complex and the programmer is referred
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to \fBfutex\fR(2) for
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more details. The same holds for asynchronous futex waiting.
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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 abstraction for end-users. Implementors
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are expected to be assembly literate and to have read the sources of the futex userspace 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 \fBfutex\fR(2) 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 (IBM Thomas J. Watson Research Center),
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Matthew Kirkwood, Ingo Molnar (Red Hat) and Rusty Russell (IBM Linux Technology Center). This page written
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by bert hubert.
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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 but with different semantics from those described above.
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Current semantics are available from Linux 2.5.40 onwards.
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.SH "SEE ALSO"
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.PP
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\fBfutex\fR(2),
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`Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux' (proceedings of the Ottawa Linux
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Symposium 2002),
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futex example library, futex-*.tar.bz2 <URL:ftp://ftp.kernel.org:/pub/linux/kernel/people/rusty/>.
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