Ubuntu Manpage: futex - fast user-space locking

NAME

       futex - fast user-space locking

SYNOPSIS

       #include <linux/futex.h>

DESCRIPTION

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 POSIX mutexes.

This page does not set out to document all design decisions but restricts itself to issues relevant for
application and library development. Most programmers will in fact not be using futexes directly but
instead rely on system libraries built on them, such as the NPTL pthreads implementation.

A futex is identified by a piece of memory which can be shared between different processes. In these
different processes, it 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 is entirely 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.
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.

VERSIONS

       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.

NOTES

       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.

COLOPHON