futex — Fast Userspace Locking system call
#include <linux/futex.h> #include <sys/time.h>
int futex( |
int *uaddr, |
| int op, | |
| int val, | |
| const struct timespec *timeout, | |
| int *uaddr2, | |
int val3); |
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 nonportable 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 argument, along
with a value val.
Five operations are currently defined:
FUTEX_WAITThis 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_WAKEThis operation wakes at most val processes waiting on
this futex address (i.e., 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 (present up to and including
Linux 2.6.25)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 was inherently racy, FUTEX_FD has been removed from Linux
2.6.26 onwards.
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, the operation fails with the error EAGAIN. The argument timeout is ignored.
Depending on which operation was executed, the returned value for a successful call can have differing meanings.
FUTEX_WAITReturns 0 if the process was woken by a FUTEX_WAKE call. In case of timeout,
the operation fails with the error ETIMEDOUT. If the futex was not
equal to the expected value, the operation fails with
the error EWOULDBLOCK.
Signals (see signal(7)) or other
spurious wakeups cause FUTEX_WAIT to fail with the error
EINTR.
FUTEX_WAKEReturns the number of processes woken up.
FUTEX_FDReturns the new file descriptor associated with the futex.
FUTEX_REQUEUEReturns the number of processes woken up.
FUTEX_CMP_REQUEUEReturns the number of processes woken up.
In the event of an error, all operations return −1,
and set errno to indicate the
error.
No read access to futex memory.
FUTEX_CMP_REQUEUE
found an unexpected futex value. (This probably
indicates a race; use the safe FUTEX_WAKE now.)
Error in getting timeout information from
userspace.
An operation was not defined or error in page alignment.
The system limit on the total number of open files has been reached.
Invalid operation specified in op.
Initial futex support was merged in Linux 2.5.7 but with different semantics from what was described above. A 4-argument system call with the semantics given here was introduced in Linux 2.5.40. In Linux 2.5.70 one argument was added. In Linux 2.6.7 a sixth argument was added — messy, especially on the s390 architecture.
To reiterate, bare futexes are not intended as an easy-to-use abstraction for end-users. (There is no wrapper function for this system call in glibc.) Implementors are expected to be assembly literate and to have read the sources of the futex userspace library referenced below.
Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceedings of the Ottawa Linux Symposium 2002), online at
http://kernel.org/doc/ols/2002/ols2002-pages-479-495.pdf
Futex example library, futex-*.tar.bz2 at
ftp://ftp.nl.kernel.org/pub/linux/kernel/people/rusty/.
This page is part of release 3.25 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
|
Page by b.hubert - may be freely modified and distributed Niki A. Rahimi (LTC Security Development, narahimius.ibm.com) added ERRORS section. Modified 2004-06-17 mtk Modified 2004-10-07 aeb, added FUTEX_REQUEUE, FUTEX_CMP_REQUEUE FIXME 2.6.14 adds FUTEX_WAKE_OP 2.6.18 adds (Ingo Molnar) priority inheritance support: FUTEX_LOCK_PI, FUTEX_UNLOCK_PI, and FUTEX_TRYLOCK_PI. These need to be documented in the manual page. Probably there is sufficient material in the kernel source file Documentation/pi-futex.txt. 2.6.25 adds FUTEX_WAKE_BITSET, FUTEX_WAIT_BITSET |