futex() - Unix, Linux System Call

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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); 


#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 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:

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.

Depending on which operation was executed, the returned value can have
differing meanings.
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.
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.
This system call is Minux specific.

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