futex
NAME
futex - Fast Userspace Locking system call
SYNOPSIS
#include <linux/futex.h> #include <sys/time.h> I int futex(int * uaddr , int op , int val \ ", const struct timespec *" timeout ,
I int * uaddr2 , int val3 );
DESCRIPTION
The
R 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
R 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
R val .
Five operations are currently defined:
FUTEX_WAIT
This operation atomically verifies that the futex address
uaddr
still contains the value
R 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
R FUTEX_WAIT ).
The arguments
R 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
R FUTEX_WAKE ,
the process will receive the signal number that was passed in
R val .
The calling process must close the returned file descriptor after use.
The arguments
R 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.
R 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
R uaddr2 .
The arguments
timeout
and
val3
are ignored.
R FUTEX_CMP_REQUEUE (since Linux 2.6.7)
There was a race in the intended use of
R FUTEX_REQUEUE ,
so
FUTEX_CMP_REQUEUE
was introduced.
This is similar to
R FUTEX_REQUEUE ,
but first checks whether the location
uaddr
still contains the value
R 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
R EWOULDBLOCK .
Signals (or other spurious wakeups)
cause
FUTEX_WAIT
to return
R 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.
SEE ALSO
futex(7),
Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux
(proceedings of the Ottawa Linux Symposium 2002),
futex example library, futex-*.tar.bz2
<URL:ftp://ftp.nl.kernel.org:/pub/linux/kernel/people/rusty/>.
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