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 memory in separate
processes may not be equal, the kernel maps them internally so the same memory
mapped in different locations will correspond for futex calls). It is
typically used to implement the contended case of a lock in shared memory, as
described in futex(4).
When a futex(4) operation did not finish uncontended in userspace, 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(4). As these semantics involve writing non-portable assembly
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 parameter, along
with a value val.
Five operations are currently defined:
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 describe the maximum duration of the wait, which is
infinite otherwise. The arguments uaddr2 and val3 are
For futex(4), 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.
This operation wakes at most val processes waiting
on this futex address (ie. inside FUTEX_WAIT). The arguments
timeout, uaddr2 and val3 are ignored.
For futex(4), 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).
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.
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 processes, and requeues all other waiters on the futex at
address uaddr2. The arguments timeout and val3 are
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, an error EAGAIN is returned. The argument
timeout is ignored.
Depending on which operation was executed, the returned value can have differing
Returns 0 if the process was woken by a FUTEX_WAKE call. In
case of timeout, ETIMEDOUT is returned. If the futex was not equal to the
expected value, the operation returns EWOULDBLOCK. Signals (or other
spurious wakeups) cause FUTEX_WAIT to return EINTR.
Returns the number of processes woken up.
Returns the new file descriptor associated with the
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 userspace library referenced below.
Initial futex support was merged in Linux 2.5.7 but with different semantics
from what was described above. A 4-parameter system call with the semantics
given here was introduced in Linux 2.5.40. In Linux 2.5.70 one parameter was
added. In Linux 2.6.7 a sixth parameter was added - messy, especially on the
futex(4), `Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux'
(proceedings of the Ottawa Linux Symposium 2002), futex example library,