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
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
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 optimized 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
In the contended case, the atomic increment changed the counter from -1 (or some
other negative number). If this is detected, there are waiters. User space
should now set the counter to 1 and instruct the kernel to wake up any waiters
using the FUTEX_WAKE
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 circumstances,
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
(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
for more details. The same holds for asynchronous futex waiting.
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
To reiterate, bare futexes are not intended as an easy-to-use abstraction for
end users. Implementors are expected to be assembly literate and to have read
the sources of the futex user-space library referenced below.
This man page illustrates the most common use of the futex
it is by no means the only one.
Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux
of the Ottawa Linux Symposium 2002), futex example library, futex-*.tar.bz2