.\" Copyright, the authors of the Linux man-pages project
.\"
.\" %%%LICENSE_START(FREELY_REDISTRIBUTABLE)
.\" may be freely modified and distributed
.\" %%%LICENSE_END
.\"
.\" FIXME Still to integrate are some points from Torvald Riegel's mail of
.\" 2015-01-23:
.\"       http://thread.gmane.org/gmane.linux.kernel/1703405/focus=7977
.\"
.\" FIXME Do we need to add some text regarding Torvald Riegel's 2015-01-24 mail
.\"       http://thread.gmane.org/gmane.linux.kernel/1703405/focus=1873242
.\"
.TH futex 2 2025-05-30 "Linux man-pages (unreleased)"
.SH NAME
futex \- fast user-space locking
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.BR "#include <linux/futex.h>" "  /* Definition of " FUTEX_* " constants */"
.BR "#include <sys/syscall.h>" "  /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.P
.BI "long syscall(SYS_futex, uint32_t *" uaddr ", int " op ", ...);"
.fi
.SH DESCRIPTION
The
.BR 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
.BR futex ()
system call only when it is likely that the program has to block for
a longer time until the condition becomes true.
Other
.BR futex ()
operations can be used to wake any processes or threads waiting
for a particular condition.
.P
A futex is a 32-bit value\[em]referred to below as a
.IR "futex word" \[em]whose
address is supplied to the
.BR 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)
.BR mmap (2)
or
.BR 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.
.P
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
.BR 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.
.\" Notes from Darren Hart (Dec 2015):
.\"     Totally ordered with respect futex operations refers to semantics
.\"     of the ACQUIRE/RELEASE operations and how they impact ordering of
.\"     memory reads and writes. The kernel futex operations are protected
.\"     by spinlocks, which ensure that all operations are serialized
.\"     with respect to one another.
.\"
.\"     This is a lot to attempt to define in this document. Perhaps a
.\"     reference to linux/Documentation/memory-barriers.txt as a footnote
.\"     would be sufficient? Or perhaps for this manual, "serialized" would
.\"     be sufficient, with a footnote regarding "totally ordered" and a
.\"     pointer to the memory-barrier documentation?
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.
.\" FIXME(Torvald Riegel):
.\" Eventually we want to have some text in NOTES to satisfy
.\" the reference in the following sentence
.\"     See NOTES for a detailed specification of
.\"     the synchronization semantics.
.P
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
.BR futex ()
wait operation.
This
.BR 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
.BR futex (7)
for more detail on how to use futexes.
.P
Besides the basic wait and wake-up futex functionality, there are further
futex operations aimed at supporting more complex use cases.
.P
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
.BR FUTEX_WAIT (2const)
are being performed on a particular futex word.
.\"
.SS Arguments
The
.I 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
.I op
argument.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.SS Futex operations
The
.I 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
.I op
are as follows:
.TP
.BR FUTEX_PRIVATE_FLAG " (since Linux 2.6.22)"
.\" commit 34f01cc1f512fa783302982776895c73714ebbc2
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.
.\" I.e., It allows the kernel choose the fast path for validating
.\" the user-space address and avoids expensive VMA lookups,
.\" taking reference counts on file backing store, and so on.
.IP
As a convenience,
.I <linux/futex.h>
defines a set of constants with the suffix
.B _PRIVATE
that are equivalents of all of the operations listed below,
.\" except the obsolete FUTEX_FD(2const), for which the "private" flag was
.\" meaningless
but with the
.B FUTEX_PRIVATE_FLAG
ORed into the constant value.
Thus, there are
.BR FUTEX_WAIT_PRIVATE ,
.BR FUTEX_WAKE_PRIVATE ,
and so on.
.TP
.BR FUTEX_CLOCK_REALTIME " (since Linux 2.6.28)"
.\" commit 1acdac104668a0834cfa267de9946fac7764d486
This option bit can be employed only with the
.BR FUTEX_WAIT_BITSET (2const),
.BR FUTEX_WAIT_REQUEUE_PI (2const),
(since Linux 4.5)
.\" commit 337f13046ff03717a9e99675284a817527440a49
.BR FUTEX_WAIT (2const),
and
(since Linux 5.14)
.\" commit bf22a6976897977b0a3f1aeba6823c959fc4fdae
.BR FUTEX_LOCK_PI2 (2const)
operations.
.IP
If this option is set, the kernel measures the
.I timeout
against the
.B CLOCK_REALTIME
clock.
.IP
If this option is not set, the kernel measures the
.I timeout
against the
.B CLOCK_MONOTONIC
clock.
.P
The operation specified in
.I op
is one of the following:
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_WAIT (2const)
.TQ
.BR FUTEX_WAKE (2const)
.TQ
.BR FUTEX_FD (2const)
.TQ
.BR FUTEX_REQUEUE (2const)
.TQ
.BR FUTEX_CMP_REQUEUE (2const)
.TQ
.BR FUTEX_WAKE_OP (2const)
.TQ
.BR FUTEX_WAIT_BITSET (2const)
.TQ
.BR FUTEX_WAKE_BITSET (2const)
.\"
.SS 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.
.P
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.
.P
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.)
.\"
.\" Quoting Darren Hart:
.\"     These opcodes paired with the PI futex value policy (described below)
.\"     defines a "futex" as PI aware. These were created very specifically
.\"     in support of PI pthread_mutexes, so it makes a lot more sense to
.\"     talk about a PI aware pthread_mutex, than a PI aware futex, since
.\"     there is a lot of policy and scaffolding that has to be built up
.\"     around it to use it properly (this is what a PI pthread_mutex is).
.P
.\"       mtk: The following text is drawn from the Hart/Guniguntala paper
.\"       (listed in SEE ALSO), but I have reworded some pieces
.\"       significantly.
.\"
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:
.IP \[bu] 3
If the lock is not acquired, the futex word's value shall be 0.
.IP \[bu]
If the lock is acquired, the futex word's value shall
be the thread ID (TID;
see
.BR gettid (2))
of the owning thread.
.IP \[bu]
If the lock is owned and there are threads contending for the lock,
then the
.B FUTEX_WAITERS
bit shall be set in the futex word's value; in other words, this value is:
.IP
.in +4n
.EX
FUTEX_WAITERS | TID
.EE
.in
.IP
(Note that is invalid for a PI futex word to have no owner and
.B FUTEX_WAITERS
set.)
.P
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
.I 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.
.P
If a futex is already acquired (i.e., has a nonzero value),
waiters must employ the
.BR FUTEX_LOCK_PI (2const)
or
.BR FUTEX_LOCK_PI2 (2const)
operations to acquire the lock.
If other threads are waiting for the lock, then the
.B FUTEX_WAITERS
bit is set in the futex value;
in this case, the lock owner must employ the
.BR FUTEX_UNLOCK_PI (2const)
operation to release the lock.
.P
In the cases where callers are forced into the kernel
(i.e., required to perform a
.BR 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.
.P
It is important to note
.\" tglx (July 2015):
.\"     If there are multiple waiters on a pi futex then a wake pi operation
.\"     will wake the first waiter and hand over the lock to this waiter. This
.\"     includes handing over the rtmutex which represents the futex in the
.\"     kernel. The strict requirement is that the futex owner and the rtmutex
.\"     owner must be the same, except for the update period which is
.\"     serialized by the futex internal locking. That means the kernel must
.\"     update the user-space value prior to returning to user space
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
.B FUTEX_WAITERS
bit set.)
.P
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
.B 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
.B FUTEX_OWNER_DIED
bit and is then responsible for cleaning up the stale state left over by
the dead owner.
.\" tglx (July 2015):
.\"     The FUTEX_OWNER_DIED bit can also be set on uncontended futexes, where
.\"     the kernel has no state associated. This happens via the robust futex
.\"     mechanism. In that case the futex value will be set to
.\"     FUTEX_OWNER_DIED. The robust futex mechanism is also available for non
.\"     PI futexes.
.P
PI futexes are operated on by specifying one of the values listed below in
.IR op .
Note that the PI futex operations must be used as paired operations
and are subject to some additional requirements:
.IP \[bu] 3
.BR FUTEX_LOCK_PI (2const),
.BR FUTEX_LOCK_PI2 (2const),
and
.BR FUTEX_TRYLOCK_PI (2const)
pair with
.BR FUTEX_UNLOCK_PI (2const).
.BR 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
.B EPERM
results.
.IP \[bu]
.BR FUTEX_WAIT_REQUEUE_PI (2const)
pairs with
.BR FUTEX_CMP_REQUEUE_PI (2const).
This must be performed from a non-PI futex to a distinct PI futex
(or the error
.B EINVAL
results).
Additionally,
the number of waiters to be woken must be 1
(or the error
.B EINVAL
results).
.P
The PI futex operations are as follows:
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_LOCK_PI (2const)
.TQ
.BR FUTEX_LOCK_PI2 (2const)
.TQ
.BR FUTEX_TRYLOCK_PI (2const)
.TQ
.BR FUTEX_UNLOCK_PI (2const)
.TQ
.BR FUTEX_CMP_REQUEUE_PI (2const)
.TQ
.BR FUTEX_WAIT_REQUEUE_PI (2const)
.P
The
.BR FUTEX_WAIT_REQUEUE_PI (2const)
and
.BR 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
.BR FUTEX_WAIT_REQUEUE_PI (2const)
operation, the user-space application pre-specifies the target
of the requeue that takes place in the
.BR FUTEX_CMP_REQUEUE_PI (2const)
operation.
.\"
.\" Darren Hart notes that a patch to allow glibc to fully support
.\" PI-aware pthreads condition variables has not yet been accepted into
.\" glibc. The story is complex, and can be found at
.\" https://sourceware.org/bugzilla/show_bug.cgi?id=11588
.\" Darren notes that in the meantime, the patch is shipped with various
.\" PREEMPT_RT-enabled Linux systems.
.\"
.\" Related to the preceding, Darren proposed that somewhere, man-pages
.\" should document the following point:
.\"
.\"     While the Linux kernel, since Linux 2.6.31, supports requeueing of
.\"     priority-inheritance (PI) aware mutexes via the
.\"     FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI futex operations,
.\"     the glibc implementation does not yet take full advantage of this.
.\"     Specifically, the condvar internal data lock remains a non-PI aware
.\"     mutex, regardless of the type of the pthread_mutex associated with
.\"     the condvar. This can lead to an unbounded priority inversion on
.\"     the internal data lock even when associating a PI aware
.\"     pthread_mutex with a condvar during a pthread_cond*_wait
.\"     operation. For this reason, it is not recommended to rely on
.\"     priority inheritance when using pthread condition variables.
.\"
.\" The problem is that the obvious location for this text is
.\" the pthread_cond*wait(3) man page. However, such a man page
.\" does not currently exist.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.SH RETURN VALUE
On error,
\-1 is returned,
and
.I errno
is set to indicate the error.
.P
The return value on success depends on the operation.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.SH ERRORS
.TP
.B EACCES
No read access to the memory of a futex word.
.TP
.B EFAULT
.I uaddr
did not point to a valid user-space address.
.TP
.B EINVAL
.I uaddr
does not point to a valid object\[em]that is,
the address is not four-byte-aligned.
.TP
.B EINVAL
Invalid argument.
.TP
.B ENOSYS
Invalid operation specified in
.IR op .
.TP
.B ENOSYS
The
.B FUTEX_CLOCK_REALTIME
option was specified in
.IR op ,
but the accompanying operation was neither
.BR FUTEX_WAIT_BITSET (2const),
.BR FUTEX_WAIT_REQUEUE_PI (2const),
nor
.BR FUTEX_LOCK_PI2 (2const).
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.SH STANDARDS
Linux.
.SH HISTORY
Linux 2.6.0.
.P
Initial futex support was merged in Linux 2.5.7 but with different
semantics from what was described above.
A four-argument system call with the semantics
described in this page was introduced in Linux 2.5.40.
A fifth argument was added in Linux 2.5.70,
and a sixth argument was added in Linux 2.6.7.
.SH 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
.I 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:
.P
.in +4n
.EX
.RB $ " ./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
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (futex.c)
.EX
/* 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 \[aq]num\-loops\[aq] 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 \[aq]futexp\[aq]: 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 \[aq]futexp\[aq]: 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\[rs]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\[rs]n", (intmax_t) getpid(), j);
        fpost(futex1);
    }
\&
    wait(NULL);
\&
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.ad l
.BR get_robust_list (2),
.BR restart_syscall (2),
.BR pthread_mutexattr_getprotocol (3),
.BR futex (7),
.BR sched (7)
.P
The following kernel source files:
.IP \[bu] 3
.I Documentation/pi\-futex.txt
.IP \[bu]
.I Documentation/futex\-requeue\-pi.txt
.IP \[bu]
.I Documentation/locking/rt\-mutex.txt
.IP \[bu]
.I Documentation/locking/rt\-mutex\-design.txt
.IP \[bu]
.I Documentation/robust\-futex\-ABI.txt
.P
Franke, H., Russell, R., and Kirwood, M., 2002.
.br
.UR http://kernel.org/\:doc/\:ols/\:2002/\:ols2002\-pages\-479\-495.pdf
.I Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux
.UE
(from proceedings of the Ottawa Linux Symposium 2002).
.P
Hart, D., 2009.
.UR http://lwn.net/\:Articles/\:360699/
.I A futex overview and update
.UE .
.P
Hart, D.\& and Guniguntala, D., 2009.
.UR http://lwn.net/\:images/\:conf/\:rtlws11/\:papers/\:proc/\:p10.pdf
.I Requeue-PI: Making glibc Condvars PI-Aware
.UE
(from proceedings of the 2009 Real-Time Linux Workshop).
.P
Drepper, U., 2011.
.UR http://www.akkadia.org/\:drepper/\:futex.pdf
.I Futexes Are Tricky
.UE .
.P
Futex example library,
.UR https://mirrors.kernel.org/\:pub/\:linux/\:kernel/\:people/\:rusty/
futex\-*.tar.bz2
.UE .
.\"
.\" FIXME(Torvald) We should probably refer to the glibc code here, in
.\" particular the glibc-internal futex wrapper functions that are
.\" WIP, and the generic pthread_mutex_t and perhaps condvar
.\" implementations.