1FUTEX(2) Linux Programmer's Manual FUTEX(2)
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6 futex - Fast Userspace Locking system call
7
9 #include <linux/futex.h>
10 #include <sys/time.h>
11 int futex(int *uaddr, int op, int val, const struct timespec *timeout,
13 int *uaddr2, int val3);
14
16 The futex() system call provides a method for a program to wait for a
17 value at a given address to change, and a method to wake up anyone
18 waiting on a particular address (while the addresses for the same mem‐
19 ory in separate processes may not be equal, the kernel maps them inter‐
20 nally so the same memory mapped in different locations will correspond
21 for futex() calls). It is typically used to implement the contended
22 case of a lock in shared memory, as described in futex(7).
23 When a futex(7) operation did not finish uncontended in userspace, a
25 call needs to be made to the kernel to arbitrate. Arbitration can
26 either mean putting the calling process to sleep or, conversely, waking
27 a waiting process.
28 Callers of this function are expected to adhere to the semantics as set
30 out in futex(7). As these semantics involve writing non-portable
31 assembly instructions, this in turn probably means that most users will
32 in fact be library authors and not general application developers.
33 The uaddr argument needs to point to an aligned integer which stores
35 the counter. The operation to execute is passed via the op argument,
36 along with a value val.
37 Five operations are currently defined:
39 FUTEX_WAIT
41 This operation atomically verifies that the futex address uaddr
42 still contains the value val, and sleeps awaiting FUTEX_WAKE on
43 this futex address. If the timeout argument is non-NULL, its
44 contents describe the maximum duration of the wait, which is
45 infinite otherwise. The arguments uaddr2 and val3 are ignored.
46 For futex(7), this call is executed if decrementing the count
48 gave a negative value (indicating contention), and will sleep
49 until another process releases the futex and executes the
50 FUTEX_WAKE operation.
51 FUTEX_WAKE
53 This operation wakes at most val processes waiting on this futex
54 address (i.e., inside FUTEX_WAIT). The arguments timeout,
55 uaddr2 and val3 are ignored.
56 For futex(7), this is executed if incrementing the count showed
58 that there were waiters, once the futex value has been set to 1
59 (indicating that it is available).
60 FUTEX_FD (present up to and including Linux 2.6.25)
62 To support asynchronous wakeups, this operation associates a
63 file descriptor with a futex. If another process executes a
64 FUTEX_WAKE, the process will receive the signal number that was
65 passed in val. The calling process must close the returned file
66 descriptor after use. The arguments timeout, uaddr2 and val3
67 are ignored.
68 To prevent race conditions, the caller should test if the futex
70 has been upped after FUTEX_FD returns.
71 Because it was inherently racy, FUTEX_FD has been removed from
73 Linux 2.6.26 onwards.
74 FUTEX_REQUEUE (since Linux 2.5.70)
76 This operation was introduced in order to avoid a "thundering
77 herd" effect when FUTEX_WAKE is used and all processes woken up
78 need to acquire another futex. This call wakes up val pro‐
79 cesses, and requeues all other waiters on the futex at address
80 uaddr2. The arguments timeout and val3 are ignored.
81 FUTEX_CMP_REQUEUE (since Linux 2.6.7)
83 There was a race in the intended use of FUTEX_REQUEUE, so
84 FUTEX_CMP_REQUEUE was introduced. This is similar to
85 FUTEX_REQUEUE, but first checks whether the location uaddr still
86 contains the value val3. If not, the operation fails with the
87 error EAGAIN. The argument timeout is ignored.
88
90 Depending on which operation was executed, the returned value for a
91 successful call can have differing meanings.
92 FUTEX_WAIT
94 Returns 0 if the process was woken by a FUTEX_WAKE call. In
95 case of timeout, the operation fails with the error ETIMEDOUT.
96 If the futex was not equal to the expected value, the operation
97 fails with the error EWOULDBLOCK. Signals (see signal(7)) or
98 other spurious wakeups cause FUTEX_WAIT to fail with the error
99 EINTR.
100 FUTEX_WAKE
102 Returns the number of processes woken up.
103 FUTEX_FD
105 Returns the new file descriptor associated with the futex.
106 FUTEX_REQUEUE
108 Returns the number of processes woken up.
109 FUTEX_CMP_REQUEUE
111 Returns the number of processes woken up.
112 In the event of an error, all operations return -1, and set errno to
114 indicate the error.
115
117 EACCES No read access to futex memory.
118 EAGAIN FUTEX_CMP_REQUEUE found an unexpected futex value. (This proba‐
120 bly indicates a race; use the safe FUTEX_WAKE now.)
121 EFAULT Error in getting timeout information from userspace.
123 EINVAL An operation was not defined or error in page alignment.
125 ENFILE The system limit on the total number of open files has been
127 reached.
128 ENOSYS Invalid operation specified in op.
130
132 Initial futex support was merged in Linux 2.5.7 but with different
133 semantics from what was described above. A 4-argument system call with
134 the semantics given here was introduced in Linux 2.5.40. In Linux
135 2.5.70 one argument was added. In Linux 2.6.7 a sixth argument was
136 added — messy, especially on the s390 architecture.
137
139 This system call is Linux-specific.
140
142 To reiterate, bare futexes are not intended as an easy-to-use abstrac‐
143 tion for end-users. (There is no wrapper function for this system call
144 in glibc.) Implementors are expected to be assembly literate and to
145 have read the sources of the futex userspace library referenced below.
146
148 futex(7)
149 Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceed‐
151 ings of the Ottawa Linux Symposium 2002), futex example library,
152 futex-*.tar.bz2 <URL:ftp://ftp.nl.kernel.org/pub/linux/kernel/peo‐
153 ple/rusty/>.
154
156 This page is part of release 3.22 of the Linux man-pages project. A
157 description of the project, and information about reporting bugs, can
158 be found at http://www.kernel.org/doc/man-pages/.
159Linux 2008-11-27 FUTEX(2)