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FUTEX(2) | Linux Programmer's Manual | FUTEX(2) |
NAME¶
futex - fast user-space lockingSYNOPSIS¶
#include <linux/futex.h> #include <sys/time.h>int futex(int *uaddr, int op, int val, const struct timespec *timeout,int *uaddr2, int val3);
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). This system call 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 user space, 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_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.
- FUTEX_WAKE
- This operation wakes at most val processes waiting
on this futex address (i.e., inside FUTEX_WAIT). The arguments
timeout, uaddr2 and val3 are ignored.
- 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.
- 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.
RETURN VALUE¶
In the event of an error, all operations return -1, and set errno to indicate the error. The return value on success depends on the operation, as described in the following list:- FUTEX_WAIT
- Returns 0 if the process was woken by a FUTEX_WAKE call. See ERRORS for the various possible error returns.
- 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 detected that the value pointed to by uaddr is not equal to the expected value val3. (This probably indicates a race; use the safe FUTEX_WAKE now.)
- EFAULT
- Error retrieving timeout information from user space.
- EINTR
- A FUTEX_WAIT operation was interrupted by a signal (see signal(7)) or a spurious wakeup.
- EINVAL
- Invalid argument.
- ENFILE
- The system limit on the total number of open files has been reached.
- ENOSYS
- Invalid operation specified in op.
- ETIMEDOUT
- Timeout during the FUTEX_WAIT operation.
- EWOULDBLOCK
- op was FUTEX_WAIT and the value pointed to by uaddr was not equal to the expected value val at the time of the call.
VERSIONS¶
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 described in this page 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.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. (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 user-space library referenced below.SEE ALSO¶
futex(7) Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceedings of the Ottawa Linux Symposium 2002), online atCOLOPHON¶
This page is part of release 3.44 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/.2012-08-13 | Linux |