1FUTEX(7) Linux Programmer's Manual FUTEX(7)
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6 futex - Fast Userspace Locking
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9 #include <linux/futex.h>
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12 The Linux kernel provides futexes ("Fast Userspace muTexes") as a
13 building block for fast userspace locking and semaphores. Futexes are
14 very basic and lend themselves well for building higher level locking
15 abstractions such as POSIX mutexes.
16 This page does not set out to document all design decisions but
18 restricts itself to issues relevant for application and library devel‐
19 opment. Most programmers will in fact not be using futexes directly
20 but instead rely on system libraries built on them, such as the NPTL
21 pthreads implementation.
22 A futex is identified by a piece of memory which can be shared between
24 different processes. In these different processes, it need not have
25 identical addresses. In its bare form, a futex has semaphore seman‐
26 tics; it is a counter that can be incremented and decremented atomi‐
27 cally; processes can wait for the value to become positive.
28 Futex operation is entirely userspace for the noncontended case. The
30 kernel is only involved to arbitrate the contended case. As any sane
31 design will strive for noncontention, futexes are also optimized for
32 this situation.
33 In its bare form, a futex is an aligned integer which is only touched
35 by atomic assembler instructions. Processes can share this integer
36 using mmap(2), via shared memory segments or because they share memory
37 space, in which case the application is commonly called multithreaded.
38 Semantics
40 Any futex operation starts in userspace, but it may necessary to commu‐
41 nicate with the kernel using the futex(2) system call.
42 To "up" a futex, execute the proper assembler instructions that will
44 cause the host CPU to atomically increment the integer. Afterwards,
45 check if it has in fact changed from 0 to 1, in which case there were
46 no waiters and the operation is done. This is the noncontended case
47 which is fast and should be common.
48 In the contended case, the atomic increment changed the counter from -1
50 (or some other negative number). If this is detected, there are wait‐
51 ers. Userspace should now set the counter to 1 and instruct the kernel
52 to wake up any waiters using the FUTEX_WAKE operation.
53 Waiting on a futex, to "down" it, is the reverse operation. Atomically
55 decrement the counter and check if it changed to 0, in which case the
56 operation is done and the futex was uncontended. In all other circum‐
57 stances, the process should set the counter to -1 and request that the
58 kernel wait for another process to up the futex. This is done using
59 the FUTEX_WAIT operation.
60 The futex(2) system call can optionally be passed a timeout specifying
62 how long the kernel should wait for the futex to be upped. In this
63 case, semantics are more complex and the programmer is referred to
64 futex(2) for more details. The same holds for asynchronous futex wait‐
65 ing.
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68 Initial futex support was merged in Linux 2.5.7 but with different
69 semantics from those described above. Current semantics are available
70 from Linux 2.5.40 onwards.
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73 To reiterate, bare futexes are not intended as an easy to use abstrac‐
74 tion for end-users. Implementors are expected to be assembly literate
75 and to have read the sources of the futex userspace library referenced
76 below.
77 This man page illustrates the most common use of the futex(2) primi‐
79 tives: it is by no means the only one.
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82 futex(2)
83 Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceed‐
85 ings of the Ottawa Linux Symposium 2002), futex example library,
86 futex-*.tar.bz2 <URL:ftp://ftp.kernel.org/pub/linux/kernel/peo‐
87 ple/rusty/>.
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90 This page is part of release 3.25 of the Linux man-pages project. A
91 description of the project, and information about reporting bugs, can
92 be found at http://www.kernel.org/doc/man-pages/.
93Linux 2002-12-31 FUTEX(7)