1futex(7) Miscellaneous Information Manual futex(7)
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6 futex - fast user-space locking
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9 #include <linux/futex.h>
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12 The Linux kernel provides futexes ("Fast user-space mutexes") as a
13 building block for fast user-space locking and semaphores. Futexes are
14 very basic and lend themselves well for building higher-level locking
15 abstractions such as mutexes, condition variables, read-write locks,
16 barriers, and semaphores.
17 Most programmers will in fact not be using futexes directly but will
19 instead rely on system libraries built on them, such as the Native
20 POSIX Thread Library (NPTL) (see pthreads(7)).
21 A futex is identified by a piece of memory which can be shared between
23 processes or threads. In these different processes, the futex need not
24 have identical addresses. In its bare form, a futex has semaphore se‐
25 mantics; it is a counter that can be incremented and decremented atomi‐
26 cally; processes can wait for the value to become positive.
27 Futex operation occurs entirely in user space for the noncontended
29 case. The kernel is involved only to arbitrate the contended case. As
30 any sane design will strive for noncontention, futexes are also opti‐
31 mized for this situation.
32 In its bare form, a futex is an aligned integer which is touched only
34 by atomic assembler instructions. This integer is four bytes long on
35 all platforms. Processes can share this integer using mmap(2), via
36 shared memory segments, or because they share memory space, in which
37 case the application is commonly called multithreaded.
38 Semantics
40 Any futex operation starts in user space, but it may be necessary to
41 communicate 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. Afterward,
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. User space should now set the counter to 1 and instruct the ker‐
52 nel 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 fu‐
64 tex(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 se‐
69 mantics from those described above. Current semantics are available
70 from Linux 2.5.40 onward.
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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 user-space 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 clone(2), futex(2), get_robust_list(2), set_robust_list(2), set_tid_ad‐
83 dress(2), pthreads(7)
84 Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceed‐
86 ings of the Ottawa Linux Symposium 2002), futex example library, fu‐
87 tex-*.tar.bz2 ⟨https://mirrors.kernel.org/pub/linux/kernel/people
88 /rusty/⟩.
89Linux man-pages 6.04 2022-10-30 futex(7)