# futex

## NAME



           futex - fast user-space locking
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <linux/futex.h>  **/* Definition of **FUTEX_*** constants */
           **#include <sys/syscall.h>  **/* Definition of **SYS_*** constants */
           **#include <unistd.h>**
    
           **long syscall(SYS_futex, uint32_t ***_uaddr_**, int** _op_**, ...);**
    


## DESCRIPTION



           The **futex**() system call provides a method for waiting until a
           certain condition becomes true.  It is typically used as a
           blocking construct in the context of shared-memory
           synchronization.  When using futexes, the majority of the
           synchronization operations are performed in user space.  A user-
           space program employs the **futex**() system call only when it is
           likely that the program has to block for a longer time until the
           condition becomes true.  Other **futex**() operations can be used to
           wake any processes or threads waiting for a particular condition.
    
           A futex is a 32-bit value—referred to below as a _futex word_ —whose
           address is supplied to the **futex**() system call.  (Futexes are 32
           bits in size on all platforms, including 64-bit systems.)  All
           futex operations are governed by this value.  In order to share a
           futex between processes, the futex is placed in a region of shared
           memory, created using (for example) mmap(2) or shmat(2).  (Thus,
           the futex word may have different virtual addresses in different
           processes, but these addresses all refer to the same location in
           physical memory.)  In a multithreaded program, it is sufficient to
           place the futex word in a global variable shared by all threads.
    
           When executing a futex operation that requests to block a thread,
           the kernel will block only if the futex word has the value that
           the calling thread supplied (as one of the arguments of the
           **futex**() call) as the expected value of the futex word.  The
           loading of the futex word's value, the comparison of that value
           with the expected value, and the actual blocking will happen
           atomically and will be totally ordered with respect to concurrent
           operations performed by other threads on the same futex word.
           Thus, the futex word is used to connect the synchronization in
           user space with the implementation of blocking by the kernel.
           Analogously to an atomic compare-and-exchange operation that
           potentially changes shared memory, blocking via a futex is an
           atomic compare-and-block operation.
    
           One use of futexes is for implementing locks.  The state of the
           lock (i.e., acquired or not acquired) can be represented as an
           atomically accessed flag in shared memory.  In the uncontended
           case, a thread can access or modify the lock state with atomic
           instructions, for example atomically changing it from not acquired
           to acquired using an atomic compare-and-exchange instruction.
           (Such instructions are performed entirely in user mode, and the
           kernel maintains no information about the lock state.)  On the
           other hand, a thread may be unable to acquire a lock because it is
           already acquired by another thread.  It then may pass the lock's
           flag as a futex word and the value representing the acquired state
           as the expected value to a **futex**() wait operation.  This **futex**()
           operation will block if and only if the lock is still acquired
           (i.e., the value in the futex word still matches the "acquired
           state").  When releasing the lock, a thread has to first reset the
           lock state to not acquired and then execute a futex operation that
           wakes threads blocked on the lock flag used as a futex word (this
           can be further optimized to avoid unnecessary wake-ups).  See
           futex(7) for more detail on how to use futexes.
    
           Besides the basic wait and wake-up futex functionality, there are
           further futex operations aimed at supporting more complex use
           cases.
    
           Note that no explicit initialization or destruction is necessary
           to use futexes; the kernel maintains a futex (i.e., the kernel-
           internal implementation artifact) only while operations such as
           FUTEX_WAIT(2const) are being performed on a particular futex word.
    
       **Arguments**
           The _uaddr_ argument points to the futex word.  On all platforms,
           futexes are four-byte integers that must be aligned on a four-byte
           boundary.  The operation to perform on the futex is specified in
           the _op_ argument.
    
       **Futex operations**
           The _op_ argument consists of two parts: a command that specifies
           the operation to be performed, bitwise ORed with zero or more
           options that modify the behaviour of the operation.  The options
           that may be included in _op_ are as follows:
    
           **FUTEX_PRIVATE_FLAG**(since Linux 2.6.22)
                  This option bit can be employed with all futex operations.
                  It tells the kernel that the futex is process-private and
                  not shared with another process (i.e., it is being used for
                  synchronization only between threads of the same process).
                  This allows the kernel to make some additional performance
                  optimizations.
    
                  As a convenience, _< linux/futex.h>_ defines a set of
                  constants with the suffix **_PRIVATE** that are equivalents of
                  all of the operations listed below, but with the
                  **FUTEX_PRIVATE_FLAG** ORed into the constant value.  Thus,
                  there are **FUTEX_WAIT_PRIVATE** , **FUTEX_WAKE_PRIVATE** , and so
                  on.
    
           **FUTEX_CLOCK_REALTIME**(since Linux 2.6.28)
                  This option bit can be employed only with the
                  FUTEX_WAIT_BITSET(2const), FUTEX_WAIT_REQUEUE_PI(2const),
                  (since Linux 4.5) FUTEX_WAIT(2const), and (since Linux
                  5.14) FUTEX_LOCK_PI2(2const) operations.
    
                  If this option is set, the kernel measures the _timeout_
                  against the **CLOCK_REALTIME** clock.
    
                  If this option is not set, the kernel measures the _timeout_
                  against the **CLOCK_MONOTONIC** clock.
    
           The operation specified in _op_ is one of the following:
    
           FUTEX_WAIT(2const)
           FUTEX_WAKE(2const)
           FUTEX_FD(2const)
           FUTEX_REQUEUE(2const)
           FUTEX_CMP_REQUEUE(2const)
           FUTEX_WAKE_OP(2const)
           FUTEX_WAIT_BITSET(2const)
           FUTEX_WAKE_BITSET(2const)
    
       **Priority-inheritance futexes**
           Linux supports priority-inheritance (PI) futexes in order to
           handle priority-inversion problems that can be encountered with
           normal futex locks.  Priority inversion is the problem that occurs
           when a high-priority task is blocked waiting to acquire a lock
           held by a low-priority task, while tasks at an intermediate
           priority continuously preempt the low-priority task from the CPU.
           Consequently, the low-priority task makes no progress toward
           releasing the lock, and the high-priority task remains blocked.
    
           Priority inheritance is a mechanism for dealing with the priority-
           inversion problem.  With this mechanism, when a high-priority task
           becomes blocked by a lock held by a low-priority task, the
           priority of the low-priority task is temporarily raised to that of
           the high-priority task, so that it is not preempted by any
           intermediate level tasks, and can thus make progress toward
           releasing the lock.  To be effective, priority inheritance must be
           transitive, meaning that if a high-priority task blocks on a lock
           held by a lower-priority task that is itself blocked by a lock
           held by another intermediate-priority task (and so on, for chains
           of arbitrary length), then both of those tasks (or more generally,
           all of the tasks in a lock chain) have their priorities raised to
           be the same as the high-priority task.
    
           From a user-space perspective, what makes a futex PI-aware is a
           policy agreement (described below) between user space and the
           kernel about the value of the futex word, coupled with the use of
           the PI-futex operations described below.  (Unlike the other futex
           operations described above, the PI-futex operations are designed
           for the implementation of very specific IPC mechanisms.)
    
           The PI-futex operations described below differ from the other
           futex operations in that they impose policy on the use of the
           value of the futex word:
    
           •  If the lock is not acquired, the futex word's value shall be 0.
    
           •  If the lock is acquired, the futex word's value shall be the
              thread ID (TID; see gettid(2)) of the owning thread.
    
           •  If the lock is owned and there are threads contending for the
              lock, then the **FUTEX_WAITERS** bit shall be set in the futex
              word's value; in other words, this value is:
    
                  FUTEX_WAITERS | TID
    
              (Note that is invalid for a PI futex word to have no owner and
              **FUTEX_WAITERS** set.)
    
           With this policy in place, a user-space application can acquire an
           unacquired lock or release a lock using atomic instructions
           executed in user mode (e.g., a compare-and-swap operation such as
           _cmpxchg_ on the x86 architecture).  Acquiring a lock simply
           consists of using compare-and-swap to atomically set the futex
           word's value to the caller's TID if its previous value was 0.
           Releasing a lock requires using compare-and-swap to set the futex
           word's value to 0 if the previous value was the expected TID.
    
           If a futex is already acquired (i.e., has a nonzero value),
           waiters must employ the FUTEX_LOCK_PI(2const) or
           FUTEX_LOCK_PI2(2const) operations to acquire the lock.  If other
           threads are waiting for the lock, then the **FUTEX_WAITERS** bit is
           set in the futex value; in this case, the lock owner must employ
           the FUTEX_UNLOCK_PI(2const) operation to release the lock.
    
           In the cases where callers are forced into the kernel (i.e.,
           required to perform a **futex**() call), they then deal directly with
           a so-called RT-mutex, a kernel locking mechanism which implements
           the required priority-inheritance semantics.  After the RT-mutex
           is acquired, the futex value is updated accordingly, before the
           calling thread returns to user space.
    
           It is important to note that the kernel will update the futex
           word's value prior to returning to user space.  (This prevents the
           possibility of the futex word's value ending up in an invalid
           state, such as having an owner but the value being 0, or having
           waiters but not having the **FUTEX_WAITERS** bit set.)
    
           If a futex has an associated RT-mutex in the kernel (i.e., there
           are blocked waiters) and the owner of the futex/RT-mutex dies
           unexpectedly, then the kernel cleans up the RT-mutex and hands it
           over to the next waiter.  This in turn requires that the user-
           space value is updated accordingly.  To indicate that this is
           required, the kernel sets the **FUTEX_OWNER_DIED** bit in the futex
           word along with the thread ID of the new owner.  User space can
           detect this situation via the presence of the **FUTEX_OWNER_DIED** bit
           and is then responsible for cleaning up the stale state left over
           by the dead owner.
    
           PI futexes are operated on by specifying one of the values listed
           below in _op_.  Note that the PI futex operations must be used as
           paired operations and are subject to some additional requirements:
    
           •  FUTEX_LOCK_PI(2const), FUTEX_LOCK_PI2(2const), and
              FUTEX_TRYLOCK_PI(2const) pair with FUTEX_UNLOCK_PI(2const).
              FUTEX_UNLOCK_PI(2const) must be called only on a futex owned by
              the calling thread, as defined by the value policy, otherwise
              the error **EPERM** results.
    
           •  FUTEX_WAIT_REQUEUE_PI(2const) pairs with
              FUTEX_CMP_REQUEUE_PI(2const).  This must be performed from a
              non-PI futex to a distinct PI futex (or the error **EINVAL**
              results).  Additionally, the number of waiters to be woken must
              be 1 (or the error **EINVAL** results).
    
           The PI futex operations are as follows:
    
           FUTEX_LOCK_PI(2const)
           FUTEX_LOCK_PI2(2const)
           FUTEX_TRYLOCK_PI(2const)
           FUTEX_UNLOCK_PI(2const)
           FUTEX_CMP_REQUEUE_PI(2const)
           FUTEX_WAIT_REQUEUE_PI(2const)
    
           The FUTEX_WAIT_REQUEUE_PI(2const) and FUTEX_CMP_REQUEUE_PI(2const)
           were added to support a fairly specific use case: support for
           priority-inheritance-aware POSIX threads condition variables.  The
           idea is that these operations should always be paired, in order to
           ensure that user space and the kernel remain in sync.  Thus, in
           the FUTEX_WAIT_REQUEUE_PI(2const) operation, the user-space
           application pre-specifies the target of the requeue that takes
           place in the FUTEX_CMP_REQUEUE_PI(2const) operation.
    


## RETURN VALUE



           On error, -1 is returned, and _errno_ is set to indicate the error.
    
           The return value on success depends on the operation.
    


## ERRORS



           **EACCES** No read access to the memory of a futex word.
    
           **EFAULT** _uaddr_ did not point to a valid user-space address.
    
           **EINVAL** _uaddr_ does not point to a valid object—that is, the address
                  is not four-byte-aligned.
    
           **EINVAL** Invalid argument.
    
           **ENOSYS** Invalid operation specified in _op_.
    
           **ENOSYS** The **FUTEX_CLOCK_REALTIME** option was specified in _op_ , but
                  the accompanying operation was neither
                  FUTEX_WAIT_BITSET(2const), FUTEX_WAIT_REQUEUE_PI(2const),
                  nor FUTEX_LOCK_PI2(2const).
    


## STANDARDS



           Linux.
    


## EXAMPLES



           The program below demonstrates use of futexes in a program where a
           parent process and a child process use a pair of futexes located
           inside a shared anonymous mapping to synchronize access to a
           shared resource: the terminal.  The two processes each write
           _nloops_ (a command-line argument that defaults to 5 if omitted)
           messages to the terminal and employ a synchronization protocol
           that ensures that they alternate in writing messages.  Upon
           running this program we see output such as the following:
    
               $ **./futex_demo** ;
               Parent (18534) 0
               Child  (18535) 0
               Parent (18534) 1
               Child  (18535) 1
               Parent (18534) 2
               Child  (18535) 2
               Parent (18534) 3
               Child  (18535) 3
               Parent (18534) 4
               Child  (18535) 4
    
       **Program source**
    
           /* futex_demo.c
    
              Usage: futex_demo [nloops]
                               (Default: 5)
    
              Demonstrate the use of futexes in a program where parent and child
              use a pair of futexes located inside a shared anonymous mapping to
              synchronize access to a shared resource: the terminal. The two
              processes each write 'num-loops' messages to the terminal and employ
              a synchronization protocol that ensures that they alternate in
              writing messages.
           */
           #define _GNU_SOURCE
           #include <err.h>
           #include <errno.h>
           #include <linux/futex.h>
           #include <stdatomic.h>
           #include <stdint.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/mman.h>
           #include <sys/syscall.h>
           #include <sys/time.h>
           #include <sys/wait.h>
           #include <unistd.h>
    
           static uint32_t *futex1, *futex2, *iaddr;
    
           static int
           futex(uint32_t *uaddr, int op, uint32_t val,
                 const struct timespec *timeout, uint32_t *uaddr2, uint32_t val3)
           {
               return syscall(SYS_futex, uaddr, op, val,
                              timeout, uaddr2, val3);
           }
    
           /* Acquire the futex pointed to by 'futexp': wait for its value to
              become 1, and then set the value to 0. */
    
           static void
           fwait(uint32_t *futexp)
           {
               long            s;
               const uint32_t  one = 1;
    
               /* atomic_compare_exchange_strong(ptr, oldval, newval)
                  atomically performs the equivalent of:
    
                      if (*ptr == *oldval)
                          *ptr = newval;
    
                  It returns true if the test yielded true and *ptr was updated. */
    
               while (1) {
    
                   /* Is the futex available? */
                   if (atomic_compare_exchange_strong(futexp, &one, 0))
                       break;      /* Yes */
    
                   /* Futex is not available; wait. */
    
                   s = futex(futexp, FUTEX_WAIT, 0, NULL, NULL, 0);
                   if (s == -1 && errno != EAGAIN)
                       err(EXIT_FAILURE, "futex-FUTEX_WAIT");
               }
           }
    
           /* Release the futex pointed to by 'futexp': if the futex currently
              has the value 0, set its value to 1 and then wake any futex waiters,
              so that if the peer is blocked in fwait(), it can proceed. */
    
           static void
           fpost(uint32_t *futexp)
           {
               long            s;
               const uint32_t  zero = 0;
    
               /* atomic_compare_exchange_strong() was described
                  in comments above. */
    
               if (atomic_compare_exchange_strong(futexp, &zero, 1)) {
                   s = futex(futexp, FUTEX_WAKE, 1, NULL, NULL, 0);
                   if (s  == -1)
                       err(EXIT_FAILURE, "futex-FUTEX_WAKE");
               }
           }
    
           int
           main(int argc, char *argv[])
           {
               pid_t         childPid;
               unsigned int  nloops;
    
               setbuf(stdout, NULL);
    
               nloops = (argc > 1) ? atoi(argv[1]) : 5;
    
               /* Create a shared anonymous mapping that will hold the futexes.
                  Since the futexes are being shared between processes, we
                  subsequently use the "shared" futex operations (i.e., not the
                  ones suffixed "_PRIVATE"). */
    
               iaddr = mmap(NULL, sizeof(*iaddr) * 2, PROT_READ | PROT_WRITE,
                            MAP_ANONYMOUS | MAP_SHARED, -1, 0);
               if (iaddr == MAP_FAILED)
                   err(EXIT_FAILURE, "mmap");
    
               futex1 = &iaddr[0];
               futex2 = &iaddr[1];
    
               *futex1 = 0;        /* State: unavailable */
               *futex2 = 1;        /* State: available */
    
               /* Create a child process that inherits the shared anonymous
                  mapping. */
    
               childPid = fork();
               if (childPid == -1)
                   err(EXIT_FAILURE, "fork");
    
               if (childPid == 0) {        /* Child */
                   for (unsigned int j = 0; j < nloops; j++) {
                       fwait(futex1);
                       printf("Child  (%jd) %u\n", (intmax_t) getpid(), j);
                       fpost(futex2);
                   }
    
                   exit(EXIT_SUCCESS);
               }
    
               /* Parent falls through to here. */
    
               for (unsigned int j = 0; j < nloops; j++) {
                   fwait(futex2);
                   printf("Parent (%jd) %u\n", (intmax_t) getpid(), j);
                   fpost(futex1);
               }
    
               wait(NULL);
    
               exit(EXIT_SUCCESS);
           }
    


* * *