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futex(7) Miscellaneous Information Manual futex(7)
futex - fast user-space locking
#include <linux/futex.h>
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 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.
Semantics
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 waiters. User space should now set the counter to 1 and
instruct the kernel 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 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 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 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 onward.
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(2)
primitives; it is by no means the only one.
clone(2), futex(2), get_robust_list(2), set_robust_list(2),
set_tid_address(2), pthreads(7)
Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux
(proceedings of the Ottawa Linux Symposium 2002), futex example
library, futex-*.tar.bz2
⟨https://mirrors.kernel.org/pub/linux/kernel/people/rusty/⟩.
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 futex(7)
Pages that refer to this page: futex(2), pthreads(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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