FUTEX(7) Linux Programmer's Manual FUTEX(7)
futex - fast user-space locking
The Linux kernel provides futexes ("Fast user-space mutexes") as a
building block for fast user-space locking and semaphores. Futexes are
very basic and lend themselves well for building higher-level locking
abstractions such as mutexes, condition variables, read-write locks,
barriers, and semaphores.
Most programmers will in fact not be using futexes directly but will
instead rely on system libraries built on them, such as the Native
POSIX Thread Library (NPTL) (see pthreads(7)).
A futex is identified by a piece of memory which can be shared between
processes or threads. In these different processes, the futex need not
have identical addresses. In its bare form, a futex has semaphore
semantics; it is a counter that can be incremented and decremented
atomically; processes can wait for the value to become positive.
Futex operation occurs entirely in user space for the noncontended
case. The kernel is involved only to arbitrate the contended case. As
any sane design will strive for noncontention, futexes are also opti-
mized for this situation.
In its bare form, a futex is an aligned integer which is touched only
by atomic assembler instructions. This integer is four bytes long on
all platforms. Processes can share this integer using mmap(2), via
shared memory segments, or because they share memory space, in which
case the application is commonly called multithreaded.
Any futex operation starts in user space, but it may be necessary to
communicate with the kernel using the futex(2) system call.
To "up" a futex, execute the proper assembler instructions that will
cause the host CPU to atomically increment the integer. Afterward,
check if it has in fact changed from 0 to 1, in which case there were
no waiters and the operation is done. This is the noncontended case
which is fast and should be common.
In the contended case, the atomic increment changed the counter from -1
(or some other negative number). If this is detected, there are wait-
ers. User space should now set the counter to 1 and instruct the ker-
nel to wake up any waiters using the FUTEX_WAKE operation.
Waiting on a futex, to "down" it, is the reverse operation. Atomically
decrement the counter and check if it changed to 0, in which case the
operation is done and the futex was uncontended. In all other circum-
stances, the process should set the counter to -1 and request that the
kernel wait for another process to up the futex. This is done using
the FUTEX_WAIT operation.
The futex(2) system call can optionally be passed a timeout specifying
how long the kernel should wait for the futex to be upped. In this
case, semantics are more complex and the programmer is referred to
futex(2) for more details. The same holds for asynchronous futex wait-
Initial futex support was merged in Linux 2.5.7 but with different
semantics from those described above. Current semantics are available
from Linux 2.5.40 onward.
To reiterate, bare futexes are not intended as an easy-to-use abstrac-
tion for end users. Implementors are expected to be assembly literate
and to have read the sources of the futex user-space library referenced
This man page illustrates the most common use of the futex(2) primi-
tives; it is by no means the only one.
clone(2), futex(2), get_robust_list(2), set_robust_list(2),
Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceed-
ings of the Ottawa Linux Symposium 2002), futex example library,
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