futex() - Unix, Linux System Call
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NAME
futex - Fast Userspace Locking system call
SYNOPSIS
#include <linux/futex.h>
#include <sys/time.h>
int futex(int *uaddr, int op, int val, const struct timespec * timeout ,
int *uaddr2, int val3);
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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:
Tag | Description |
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.
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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).
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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.
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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.
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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.
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RETURN VALUE
Depending on which operation was executed, the returned value can have
differing meanings.
Tag | Description |
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 returns EWOULDBLOCK. Signals (or other spurious wakeups)
cause FUTEX_WAIT to return EINTR.
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FUTEX_WAKE |
Returns the number of processes woken up.
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FUTEX_FD |
Returns the new file descriptor associated with the futex.
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FUTEX_REQUEUE |
Returns the number of processes woken up.
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FUTEX_CMP_REQUEUE |
Returns the number of processes woken up.
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ERRORS
Error Code | Description |
EACCES |
No read access to futex memory.
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EAGAIN |
FUTEX_CMP_REQUEUE found an unexpected futex value.
(This probably indicates a race;
use the safe FUTEX_WAKE now.)
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EFAULT |
Error in getting
timeout information from userspace.
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EINVAL |
An operation was not defined or error in page alignment.
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ENFILE |
The system limit on the total number of open files has been reached.
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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.
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 Minux specific.
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