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#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). 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 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:
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.
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).
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 onward.
Depending on which operation was executed, the returned value for a successful call can have differing meanings.
In the event of an error, all operations return -1, and set errno to indicate the error.
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.
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/.