futex(2) — Linux manual pages

DESCRIPTION

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(7).

When a futex(7) 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(7). 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:

FUTEX_WAIT

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 ignored.

For futex(7), 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.

FUTEX_WAKE

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(7), 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).

FUTEX_FD

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.

Because it is inherently racy, FUTEX_FD is scheduled for removal in June 2007; any applications that use it should be fixed now.

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 ignored.

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.

RETURN VALUE

Depending on which operation was executed, the returned value can have differing meanings.

FUTEX_WAIT: 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 fails with the error EWOULDBLOCK. Signals (or other spurious wakeups) cause FUTEX_WAIT to return EINTR.
FUTEX_WAKE: Returns the number of processes woken up.
FUTEX_FD: Returns the new file descriptor associated with the futex.
FUTEX_REQUEUE: Returns the number of processes woken up.
FUTEX_CMP_REQUEUE: Returns the number of processes woken up.

ERRORS

EACCES: No read access to futex memory.
EAGAIN: FUTEX_CMP_REQUEUE found an unexpected futex value. (This probably indicates a race; use the safe FUTEX_WAKE now.)
EFAULT: Error in getting timeout information from userspace.
EINVAL: An operation was not defined or error in page alignment.
ENFILE: The system limit on the total number of open files has been reached.

VERSIONS

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 s390 architecture.

CONFORMING TO

This system call is Linux specific.

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 userspace library referenced below.

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