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FUTEX(2) Linux Programmer's Manual FUTEX(2)
NAME
futex - fast user-space locking
SYNOPSIS
#include <linux/futex.h>
#include <sys/time.h>
int futex(int *uaddr, int op, int val, const struct timespec *timeout,
int *uaddr2, int val3);
Note: There is no glibc wrapper for this system call; see NOTES.
DESCRIPTION
The futex() system call provides a method for a program to wait for a
value at a given address to change, and a method to wake up anyone
waiting on a particular address (while the addresses for the same mem-
ory in separate processes may not be equal, the kernel maps them inter-
nally so the same memory mapped in different locations will correspond
for futex() calls). This system call is typically used to implement
the contended case of a lock in shared memory, as described in
futex(7).
When a futex(7) operation did not finish uncontended in user space, a
call needs to be made to the kernel to arbitrate. Arbitration can
either mean putting the calling process to sleep or, conversely, waking
a waiting process.
Callers of this function are expected to adhere to the semantics as set
out in futex(7). As these semantics involve writing nonportable assem-
bly instructions, this in turn probably means that most users will in
fact be library authors and not general application developers.
The uaddr argument needs to point to an aligned integer which stores
the counter. The operation to execute is passed via the op argument,
along with a value val.
Five operations are currently defined:
FUTEX_WAIT
This operation atomically verifies that the futex address uaddr
still contains the value val, and sleeps awaiting FUTEX_WAKE on
this futex address. If the timeout argument is non-NULL, its
contents specify the duration of the wait. (This interval will
be rounded up to the system clock granularity, and kernel sched-
uling delays mean that the blocking interval may overrun by a
small amount.) If timeout is NULL, the call blocks indefi-
nitely. The arguments uaddr2 and val3 are ignored.
For futex(7), this call is executed if decrementing the count
gave a negative value (indicating contention), and will sleep
until another process releases the futex and executes the
FUTEX_WAKE operation.
FUTEX_WAKE
This operation wakes at most val processes waiting on this futex
address (i.e., inside FUTEX_WAIT). The arguments timeout,
uaddr2 and val3 are ignored.
For futex(7), this is executed if incrementing the count showed
that there were waiters, once the futex value has been set to 1
(indicating that it is available).
FUTEX_FD (present up to and including Linux 2.6.25)
To support asynchronous wakeups, this operation associates a
file descriptor with a futex. If another process executes a
FUTEX_WAKE, the process will receive the signal number that was
passed in val. The calling process must close the returned file
descriptor after use. The arguments timeout, uaddr2 and val3
are ignored.
To prevent race conditions, the caller should test if the futex
has been upped after FUTEX_FD returns.
Because it was inherently racy, FUTEX_FD has been removed from
Linux 2.6.26 onward.
FUTEX_REQUEUE (since Linux 2.5.70)
This operation was introduced in order to avoid a "thundering
herd" effect when FUTEX_WAKE is used and all processes woken up
need to acquire another futex. This call wakes up val pro-
cesses, and requeues all other waiters on the futex at address
uaddr2. The arguments timeout and val3 are ignored.
FUTEX_CMP_REQUEUE (since Linux 2.6.7)
There was a race in the intended use of FUTEX_REQUEUE, so
FUTEX_CMP_REQUEUE was introduced. This is similar to
FUTEX_REQUEUE, but first checks whether the location uaddr still
contains the value val3. If not, the operation fails with the
error EAGAIN. The argument timeout is ignored.
RETURN VALUE
In the event of an error, all operations return -1, and set errno to
indicate the error. The return value on success depends on the opera-
tion, as described in the following list:
FUTEX_WAIT
Returns 0 if the process was woken by a FUTEX_WAKE call. See
ERRORS for the various possible error returns.
FUTEX_WAKE
Returns the number of processes woken up.
FUTEX_FD
Returns the new file descriptor associated with the futex.
FUTEX_REQUEUE
Returns the number of processes woken up.
FUTEX_CMP_REQUEUE
Returns the number of processes woken up.
ERRORS
EACCES No read access to futex memory.
EAGAIN FUTEX_CMP_REQUEUE detected that the value pointed to by uaddr is
not equal to the expected value val3. (This probably indicates
a race; use the safe FUTEX_WAKE now.)
EFAULT Error retrieving timeout information from user space.
EINTR A FUTEX_WAIT operation was interrupted by a signal (see sig-
nal(7)) or a spurious wakeup.
EINVAL Invalid argument.
ENFILE The system limit on the total number of open files has been
reached.
ENOSYS Invalid operation specified in op.
ETIMEDOUT
Timeout during the FUTEX_WAIT operation.
EWOULDBLOCK
op was FUTEX_WAIT and the value pointed to by uaddr was not
equal to the expected value val at the time of the call.
VERSIONS
Initial futex support was merged in Linux 2.5.7 but with different
semantics from what was described above. A 4-argument system call with
the semantics described in this page was introduced in Linux 2.5.40.
In Linux 2.5.70, one argument was added. In Linux 2.6.7, a sixth argu-
ment was added--messy, especially on the s390 architecture.
CONFORMING TO
This system call is Linux-specific.
NOTES
To reiterate, bare futexes are not intended as an easy-to-use abstrac-
tion for end-users. (There is no wrapper function for this system call
in glibc.) Implementors are expected to be assembly literate and to
have read the sources of the futex user-space library referenced below.
SEE ALSO
restart_syscall(2), futex(7)
Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceed-
ings of the Ottawa Linux Symposium 2002), online at
<http://kernel.org/doc/ols/2002/ols2002-pages-479-495.pdf>
Futex example library, futex-*.tar.bz2 at
<ftp://ftp.kernel.org/pub/linux/kernel/people/rusty/>
COLOPHON
This page is part of release 3.69 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
http://www.kernel.org/doc/man-pages/.
Linux 2014-05-21 FUTEX(2)
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