futex.2: Terminology fixes

[thirdparty/man-pages.git] / man2 / futex.2

.\" Page by b.hubert
.\" and Copyright (C) 2015, Thomas Gleixner <tglx@linutronix.de>
.\" and Copyright (C) 2015, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(FREELY_REDISTRIBUTABLE)
.\" may be freely modified and distributed
.\" %%%LICENSE_END
.\"
.\" Niki A. Rahimi (LTC Security Development, narahimi@us.ibm.com)
.\" added ERRORS section.
.\"
.\" Modified 2004-06-17 mtk
.\" Modified 2004-10-07 aeb, added FUTEX_REQUEUE, FUTEX_CMP_REQUEUE
.\"
.\" 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 add some text regarding Torvald Riegel's 2015-01-24 mail
.\"       at http://thread.gmane.org/gmane.linux.kernel/1703405/focus=1873242
.\"
.TH FUTEX 2 2014-05-21 "Linux" "Linux Programmer's Manual"
.SH NAME
futex \- fast user-space locking
.SH SYNOPSIS
.nf
.sp
.B "#include <linux/futex.h>"
.B "#include <sys/time.h>"
.sp
.BI "int futex(int *" uaddr ", int " futex_op ", int " val ,
.BI "          const struct timespec *" timeout , \
" \fR  /* or: \fBu32 \fIval2\fP */ 
.BI "          int *" uaddr2 ", int " val3 );
.fi

.IR Note :
There is no glibc wrapper for this system call; see NOTES.
.SH DESCRIPTION
.PP
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: The program implements the majority of the
synchronization in user space, and uses one of operations of the system call
when it is likely that it has to block for a longer time until the condition
becomes true.  The program uses another operation of the system call to wake
anyone waiting for a particular condition.

The condition is represented by the futex word, which is an address in memory
supplied to the
.BR futex ()
system call, and the value at this memory location.
(While the virtual addresses for the same memory in separate
processes may not be equal,
the kernel maps them internally so that the same memory mapped
in different locations will correspond for
.BR futex ()
calls.)

When executing a futex operation that requests to block a thread, the kernel
will only block if the futex word has the value that the calling thread
supplied as expected value.  The load from the futex word, the comparison with
the expected value, and the actual blocking will happen atomically and totally
ordered with respect to concurrently executing futex operations on the same
futex word, such as operations that wake threads blocked on this futex word.
Thus, the futex word is used to connect the synchronization in user space with
the implementation of blocking by the kernel; similar to an atomic
compare-and-exchange operation that potentially changes shared memory,
blocking via a futex is an atomic compare-and-block operation. See NOTES for
a detailed specification of the synchronization semantics.

One example use of futexes is 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.  If
a thread cannot acquire a lock because it is already acquired by another
thread, it can request to block if and only the lock is still acquired by
using the lock's flag as futex word and expecting a value that represents the
acquired state.  When releasing the lock, a thread has to first reset the
lock state to not acquired and then execute the futex operation that wakes
one thread blocked on the futex word that is the lock's flag (this can be
be further optimized to avoid unnecessary wake-ups).  See
.BR 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.  Also note that
no explicit initialization or destruction are necessary to use futexes; the
kernel maintains a futex (i.e., the kernel-internal implementation artifact)
only while operations such as
.BR FUTEX_WAIT ,
described below, 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 futex_op
argument;
.IR val
is a value whose meaning and purpose depends on
.IR futex_op .

The remaining arguments
.RI ( timeout ,
.IR uaddr2 ,
and
.IR val3 )
are required only for certain of the futex operations described below.
Where one of these arguments is not required, it is ignored.

For several blocking operations, the
.I timeout
argument is a pointer to a
.IR timespec
structure that specifies a timeout for the operation.
However,  notwithstanding the prototype shown above, for some operations,
this argument is instead a four-byte integer whose meaning
is determined by the operation.
For these operations, the kernel casts the
.I timeout
value to
.IR u32 ,
and in the remainder of this page, this argument is referred to as
.I val2
when interpreted in this fashion.

Where it is required, the
.IR uaddr2
argument is a pointer to a second futex word that is employed by the operation.
The interpretation of the final integer argument,
.IR val3 ,
depends on the operation.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.SS Futex operations
The
.I futex_op
argument consists of two parts:
a command that specifies the operation to be performed,
bit-wise ORed with zero or or more options that
modify the behaviour of the operation.
The options that may be included in
.I futex_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 only being used for synchronization between
threads of the same process).
This allows the kernel to 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.

As a convenience,
.IR <linux/futex.h>
defines a set of constants with the suffix
.BR _PRIVATE
that are equivalents of all of the operations listed below,
.\" except the obsolete FUTEX_FD, for which the "private" flag was
.\" meaningless
but with the
.BR 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
and
.BR FUTEX_WAIT_REQUEUE_PI
operations.

If this option is set, the kernel treats
.I timeout
as an absolute time based on
.BR CLOCK_REALTIME .

If this option is not set, the kernel treats
.I timeout
as relative time,
.\" FIXME XXX I added CLOCK_MONOTONIC here. Okay?
measured against the
.BR CLOCK_MONOTONIC
clock.
.PP
The operation specified in
.I futex_op
is one of the following:
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_WAIT " (since Linux 2.6.0)"
.\" Strictly speaking, since some time in 2.5.x
This operation tests that the value at the
futex word pointed to by the address
.I uaddr
still contains the expected value
.IR val ,
and if so, then sleeps awaiting
.B FUTEX_WAKE
on the futex word.  The load of the value of the futex word is an atomic memory
access (i.e., using atomic machine instructions of the respective
architecture).  This load, the comparison with the expected value, and
starting to sleep are performed atomically and totally ordered with respect
to other futex operations on the same futex word.  If the thread starts to
sleep, it is considered a waiter on this futex word.
If the futex value does not match
.IR val ,
then the call fails immediately with the error
.BR EAGAIN .

The purpose of the comparison with the expected value is to prevent lost
wake-ups: If another thread changed the value of the futex word after the
calling thread decided to block based on the prior value, and if the other
thread executed a
.BR FUTEX_WAKE
operation (or similar wake-up) after the value change and before this
.BR FUTEX_WAIT
operation, then the latter will observe the value change and will not start
to sleep.

If the
.I timeout
argument is non-NULL, its contents specify a relative timeout for the wait,
.\" FIXME XXX I added CLOCK_MONOTONIC here. Okay?
measured according to the
.BR CLOCK_MONOTONIC
clock.
(This interval will be rounded up to the system clock granularity,
and kernel scheduling delays mean that the
blocking interval may overrun by a small amount.)
If
.I timeout
is NULL, the call blocks indefinitely.

The arguments
.I uaddr2
and
.I val3
are ignored.

.\" XXX I think we should remove this.  Or maybe adapt to a different example.
.\" For
.\" .BR futex (7),
.\" this call is executed if decrementing the count gave a negative value
.\" (indicating contention),
.\" and will sleep until another process or thread releases
.\" the futex and executes the
.\" .B FUTEX_WAKE
.\" operation.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_WAKE " (since Linux 2.6.0)"
.\" Strictly speaking, since Linux 2.5.x
This operation wakes at most
.I val
.\" XXX I believe FUTEX_WAIT_BITSET waiters, for example, could also be woken
.\" (therefore, make it e.g. instead of i.e.)?
of the waiters that are waiting (e.g., inside
.BR FUTEX_WAIT )
on the futex word at the address
.IR uaddr .
Most commonly,
.I val
is specified as either 1 (wake up a single waiter) or
.BR INT_MAX
(wake up all waiters).
.\" FIXME Please confirm that the following is correct:
No guarantee is provided about which waiters are awoken
(e.g., a waiter with a higher scheduling priority is not guaranteed
to be awoken in preference to a waiter with a lower priority).

The arguments
.IR timeout ,
.IR uaddr2 ,
and
.I val3
are ignored.

.\" XXX I think we should remove this.  Or maybe adapt to a different example.
.\" For
.\" .BR futex (7),
.\" this is executed if incrementing the count showed that there were waiters,
.\" FIXME How does "incrementing the count showed that there were waiters"?
.\" once the futex value has been set to 1 (indicating that it is available).
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_FD " (from Linux 2.6.0 up to and including Linux 2.6.25)"
.\" Strictly speaking, from Linux 2.5.x to 2.6.25
This operation creates a file descriptor that is associated with the futex at
.IR uaddr .
The caller must close the returned file descriptor after use.
When another process or thread performs a
.BR FUTEX_WAKE
on the futex word, the file descriptor indicates as being readable with
.BR select (2),
.BR poll (2),
and
.BR epoll (7)

The file descriptor can be used to obtain asynchronous notifications: if
.I val
is nonzero, then when another process or thread executes a
.BR FUTEX_WAKE ,
the caller will receive the signal number that was passed in
.IR val .

The arguments
.IR timeout ,
.I uaddr2
and
.I val3
are ignored.

.\" FIXME We never define "upped".  Maybe just remove that sentence?
To prevent race conditions, the caller should test if the futex has
been upped after
.B FUTEX_FD
returns.

Because it was inherently racy,
.B FUTEX_FD
has been removed
.\" commit 82af7aca56c67061420d618cc5a30f0fd4106b80
from Linux 2.6.26 onward.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_REQUEUE " (since Linux 2.6.0)"
.\" Strictly speaking: from Linux 2.5.70
.\" FIXME Is there some indication that it is broken in general, or is this
.\" comment implicitly speaking about the condvar (?) use case?  If the latter
.\" we might want to weaken the advice a little.
.IR "Avoid using this operation" .
It is broken for its intended purpose.
Use
.BR FUTEX_CMP_REQUEUE
instead.

This operation performs the same task as
.BR FUTEX_CMP_REQUEUE ,
except that no check is made using the value in
.IR  val3 .
(The argument
.I val3
is ignored.)
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_CMP_REQUEUE " (since Linux 2.6.7)"
This operation first checks whether the location
.I uaddr
still contains the value
.IR val3 .
If not, the operation fails with the error
.BR EAGAIN .
Otherwise, the operation wakes up a maximum of
.I val
waiters that are waiting on the futex at
.IR uaddr .
If there are more than
.I val
waiters, then the remaining waiters are removed
from the wait queue of the source futex at
.I uaddr
and added to the wait queue of the target futex at
.IR uaddr2 .
The
.I val2
argument specifies an upper limit on the number of waiters
that are requeued to the futex at
.IR uaddr2 .

.\" FIXME Is this correct?  Or is just the decision which threads to wake or
.\" requeue part of the atomic operation?
The load from
.I uaddr
is an atomic memory access (i.e., using atomic machine instructions of the
respective architecture).  This load, the comparison with
.IR val3 ,
and the requeueing of any waiters are performed atomically and totally ordered
with respect to other operations on the same futex word.

This operation was added as a replacement for the earlier
.BR FUTEX_REQUEUE .
The difference is that the check of the value at
.I uaddr
can be used to ensure that requeueing only happens under certain conditions.
Both operations can be used to avoid a "thundering herd" effect when
.B FUTEX_WAKE
is used and all of the waiters that are woken need to acquire another futex.

.\" FIXME Please review the following new paragraph to see if it is
.\"       accurate.
Typical values to specify for
.I val
are 0 or or 1.
(Specifying
.BR INT_MAX
is not useful, because it would make the
.BR FUTEX_CMP_REQUEUE
operation equivalent to
.BR FUTEX_WAKE .)
The limit value specified via
.I val2
is typically either 1 or
.BR INT_MAX .
(Specifying the argument as 0 is not useful, because it would make the
.BR FUTEX_CMP_REQUEUE
operation equivalent to
.BR FUTEX_WAIT .)
.\"
.\" FIXME Here, it would be helpful to have an example of how
.\"       FUTEX_CMP_REQUEUE might be used, at the same time illustrating
.\"       why FUTEX_WAKE is unsuitable for the same use case.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.\" FIXME I added a lengthy piece of text on FUTEX_WAKE_OP text,
.\"       and I'd be happy if someone checked it.
.TP
.BR FUTEX_WAKE_OP " (since Linux 2.6.14)"
.\" commit 4732efbeb997189d9f9b04708dc26bf8613ed721
.\"	Author: Jakub Jelinek <jakub@redhat.com>
.\"	Date:   Tue Sep 6 15:16:25 2005 -0700
.\" FIXME The glibc condvar implementation is currently being revised (e.g.,
.\" to not use an internal lock anymore).
.\" It is probably more future-proof to remove this paragraph.
This operation was added to support some user-space use cases
where more than one futex must be handled at the same time.
The most notable example is the implementation of
.BR pthread_cond_signal (3),
which requires operations on two futexes,
the one used to implement the mutex and the one used in the implementation
of the wait queue associated with the condition variable.
.BR FUTEX_WAKE_OP
allows such cases to be implemented without leading to
high rates of contention and context switching.

The
.BR FUTEX_WAIT_OP
operation is equivalent to execute the following code atomically and totally
ordered with respect to other futex operations on any of the two supplied
futex words:

.in +4n
.nf
int oldval = *(int *) uaddr2;
*(int *) uaddr2 = oldval \fIop\fP \fIoparg\fP;
futex(uaddr, FUTEX_WAKE, val, 0, 0, 0);
if (oldval \fIcmp\fP \fIcmparg\fP)
    futex(uaddr2, FUTEX_WAKE, val2, 0, 0, 0);
.fi
.in

In other words,
.BR FUTEX_WAIT_OP
does the following:
.RS
.IP * 3
saves the original value of the futex word at
.IR uaddr2
and performs an operation to modify the value of the futex at
.IR uaddr2 ;
this is an atomic read-modify-write memory access (i.e., using atomic machine
instructions of the respective architecture)
.IP *
wakes up a maximum of
.I val
waiters on the futex for the futex word at
.IR uaddr ;
and
.IP *
dependent on the results of a test of the original value of the futex word at
.IR uaddr2 ,
wakes up a maximum of
.I val2
waiters on the futex for the futex word at
.IR uaddr2 .
.RE
.IP
The operation and comparison that are to be performed are encoded
in the bits of the argument
.IR val3 .
Pictorially, the encoding is:

.in +8n
.nf
+---+---+-----------+-----------+
|op |cmp|   oparg   |  cmparg   |
+---+---+-----------+-----------+
  4   4       12          12    <== # of bits
.fi
.in

Expressed in code, the encoding is:

.in +4n
.nf
#define FUTEX_OP(op, oparg, cmp, cmparg) \\
                (((op & 0xf) << 28) | \\
                ((cmp & 0xf) << 24) | \\
                ((oparg & 0xfff) << 12) | \\
                (cmparg & 0xfff))
.fi
.in

In the above,
.I op
and
.I cmp
are each one of the codes listed below.
The
.I oparg
and
.I cmparg
components are literal numeric values, except as noted below.

The
.I op
component has one of the following values:

.in +4n
.nf
FUTEX_OP_SET        0  /* uaddr2 = oparg; */
FUTEX_OP_ADD        1  /* uaddr2 += oparg; */
FUTEX_OP_OR         2  /* uaddr2 |= oparg; */
FUTEX_OP_ANDN       3  /* uaddr2 &= ~oparg; */
FUTEX_OP_XOR        4  /* uaddr2 ^= oparg; */
.fi
.in

In addition, bit-wise ORing the following value into
.I op
causes
.IR "(1\ <<\ oparg)"
to be used as the operand:

.in +4n
.nf
FUTEX_OP_ARG_SHIFT  8  /* Use (1 << oparg) as operand */
.fi
.in

The
.I cmp
field is one of the following:

.in +4n
.nf
FUTEX_OP_CMP_EQ     0  /* if (oldval == cmparg) wake */
FUTEX_OP_CMP_NE     1  /* if (oldval != cmparg) wake */
FUTEX_OP_CMP_LT     2  /* if (oldval < cmparg) wake */
FUTEX_OP_CMP_LE     3  /* if (oldval <= cmparg) wake */
FUTEX_OP_CMP_GT     4  /* if (oldval > cmparg) wake */
FUTEX_OP_CMP_GE     5  /* if (oldval >= cmparg) wake */
.fi
.in

The return value of
.BR FUTEX_WAKE_OP
is the sum of the number of waiters woken on the futex
.IR uaddr
plus the number of waiters woken on the futex
.IR uaddr2 .
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_WAIT_BITSET " (since Linux 2.6.25)"
.\" commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d
This operation is like
.BR FUTEX_WAIT
except that
.I val3
is used to provide a 32-bit bitset to the kernel.
This bitset is stored in the kernel-internal state of the waiter.
See the description of
.BR FUTEX_WAKE_BITSET
for further details.

The
.BR FUTEX_WAIT_BITSET
operation also interprets the
.I timeout
argument differently from
.BR FUTEX_WAIT .
See the discussion of
.BR FUTEX_CLOCK_REALTIME ,
above.

The
.I uaddr2
argument is ignored.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_WAKE_BITSET " (since Linux 2.6.25)"
.\" commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d
This operation is the same as
.BR FUTEX_WAKE
except that the
.I val3 
argument is used to provide a 32-bit bitset to the kernel.
This bitset is used to select which waiters should be woken up.
The selection is done by a bit-wise AND of the "wake" bitset
(i.e., the value in
.IR val3 )
and the bitset which is stored in the kernel-internal
state of the waiter (the "wait" bitset that is set using
.BR FUTEX_WAIT_BITSET ).
All of the waiters for which the result of the AND is nonzero are woken up;
the remaining waiters are left sleeping.

.\" FIXME XXX Is this paragraph that I added okay?
The effect of
.BR FUTEX_WAIT_BITSET
and
.BR FUTEX_WAKE_BITSET
is to allow selective wake-ups among multiple waiters that are blocked
on the same futex.
Note, however, that using this bitset multiplexing feature on a
futex is less efficient than simply using multiple futexes,
because employing bitset multiplexing requires the kernel
to check all waiters on a futex,
including those that are not interested in being woken up
(i.e., they do not have the relevant bit set in their "wait" bitset).
.\" According to http://locklessinc.com/articles/futex_cheat_sheet/:
.\"
.\"    "The original reason for the addition of these extensions
.\"     was to improve the performance of pthread read-write locks
.\"     in glibc. However, the pthreads library no longer uses the
.\"     same locking algorithm, and these extensions are not used
.\"     without the bitset parameter being all ones.
.\" 
.\" The page goes on to note that the FUTEX_WAIT_BITSET operation
.\" is nevertheless used (with a bitset of all ones) in order to
.\" obtain the absolute timeout functionality that is useful
.\" for efficiently implementing Pthreads APIs (which use absolute
.\" timeouts); FUTEX_WAIT provides only relative timeouts.

The
.I uaddr2
and
.I timeout
arguments are ignored.

The
.BR FUTEX_WAIT
and
.BR FUTEX_WAKE
operations correspond to
.BR FUTEX_WAIT_BITSET
and
.BR FUTEX_WAKE_BITSET
operations where the bitsets are all ones.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.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.

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 latter's priority is temporarily raised to that of the former,
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 lock
held by another intermediate-priority task
(and so on, for chains of arbitrary length),
then both of those task
(or more generally, all of the tasks in a lock chain)
have their priorities raised to be the same as the high-priority task.

.\" FIXME XXX The following is my attempt at a definition of PI futexes,
.\"       based on mail discussions with Darren Hart. Does it seem okay?
From a user-space perspective,
what makes a futex PI-aware is a policy agreement between user space
and the kernel about the value of the futex word (described in a moment),
coupled with the use of the PI futex operations described below
(in particular,
.BR FUTEX_LOCK_PI ,
.BR FUTEX_TRYLOCK_PI ,
and
.BR FUTEX_CMP_REQUEUE_PI ).
.\" 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).

.\" FIXME XXX ===== Start of adapted Hart/Guniguntala text =====
.\"       The following text is drawn from the Hart/Guniguntala paper
.\"       (listed in SEE ALSO), but I have reworded some pieces
.\"       significantly. Please check it.
.\"
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 * 3
If the lock is not acquired, the futex word's value shall be 0.
.IP *
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 *
.\" FIXME XXX In the following line, I added "the lock is owned and". Okay?
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:

    FUTEX_WAITERS | TID

.PP
Note that a PI futex word never just has the value
.BR FUTEX_WAITERS ,
which is a permissible state for non-PI futexes.

With this policy in place,
a user-space application can acquire a not-acquired
lock or release a lock that no other threads try to acquire using atomic
instructions executed in user space (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.

If a futex is already acquired (i.e., has a nonzero value),
waiters must employ the
.B FUTEX_LOCK_PI
operation 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
.B FUTEX_UNLOCK_PI
operation to release the lock.

In the cases where callers are forced into the kernel
(i.e., required to perform a
.BR futex ()
operation),
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.
.\" FIXME ===== End of adapted Hart/Guniguntala text =====

It is important to note
.\" FIXME We need some explanation here of *why* it is important to
.\"       note this. Can someone explain?
that the kernel will update the futex word's value prior
to returning to user space.
Unlike the other futex operations described above,
the PI futex operations are designed
for the implementation of very specific IPC mechanisms.
.\"
.\" FIXME XXX In discussing errors for FUTEX_CMP_REQUEUE_PI, Darren Hart
.\"       made the observation that "EINVAL is returned if the non-pi 
.\"       to pi or op pairing semantics are violated."
.\"       Probably there needs to be a general statement about this
.\"       requirement, probably located at about this point in the page.
.\"       Darren, care to take a shot at this?
.\"
.\" FIXME Somewhere on this page (I guess under the discussion of PI
.\"       futexes) we need a discussion of the FUTEX_OWNER_DIED bit.
.\"       Can someone propose a text?

PI futexes are operated on by specifying one of the following values in
.IR futex_op :
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_LOCK_PI " (since Linux 2.6.18)"
.\" commit c87e2837be82df479a6bae9f155c43516d2feebc
.\"
.\" FIXME I did some significant rewording of tglx's text.
.\"       Please check, in case I injected errors.
.\"
This operation is used after after an attempt to acquire
the lock via an atomic user-space instruction failed
because the futex word has a nonzero value\(emspecifically,
because it contained the namespace-specific TID of the lock owner.
.\" FIXME In the preceding line, what does "namespace-specific" mean?
.\"       (I kept those words from tglx.)
.\"       That is, what kind of namespace are we talking about?
.\"       (I suppose we are talking PID namespaces here, but I want to
.\"       be sure.)

The operation checks the value of the futex word at the address
.IR uaddr .
If the value is 0, then the kernel tries to atomically set
the futex value to the caller's TID.
If that fails,
.\" FIXME What would be the cause of failure?
or the futex word's value is nonzero,
the kernel atomically sets the
.B FUTEX_WAITERS
bit, which signals the futex owner that it cannot unlock the futex in
user space atomically by setting the futex value to 0.
After that, the kernel tries to find the thread which is
associated with the owner TID,
.\" FIXME Could I get a bit more detail on the next two lines?
.\"       What is "creates or reuses kernel state" about?
creates or reuses kernel state on behalf of the owner
and attaches the waiter to it.
.\" FIXME In the next line, what type of "priority" are we talking about?
.\"       Realtime priorities for SCHED_FIFO and SCHED_RR?
.\"       Or something else?
The enqueueing of the waiter is in descending priority order if more
than one waiter exists.
.\" FIXME What does "bandwidth" refer to in the next line?
The owner inherits either the priority or the bandwidth of the waiter.
.\" FIXME In the preceding line, what determines whether the
.\"       owner inherits the priority versus the bandwidth?
.\"
.\" FIXME Could I get some help translating the next sentence into
.\"       something that user-space developers (and I) can understand?
.\"       In particular, what are "nested locks" in this context?
This inheritance follows the lock chain in the case of
nested locking and performs deadlock detection.

.\" FIXME tglx says "The timeout argument is handled as described in
.\"       FUTEX_WAIT." However, it appears to me that this is not right.
.\"       Is the following formulation correct?
The
.I timeout
argument provides a timeout for the lock attempt.
It is interpreted as an absolute time, measured against the
.BR CLOCK_REALTIME
clock.
If
.I timeout
is NULL, the operation will block indefinitely.

The
.IR uaddr2 ,
.IR val ,
and
.IR val3
arguments are ignored.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_TRYLOCK_PI " (since Linux 2.6.18)"
.\" commit c87e2837be82df479a6bae9f155c43516d2feebc
This operation tries to acquire the futex at
.IR uaddr .
.\" FIXME I think it would be helpful here to say a few more words about
.\"       the difference(s) between FUTEX_LOCK_PI and FUTEX_TRYLOCK_PI.
.\"       Can someone propose something?
.\"
.\" FIXME Additionally, we claim above that just FUTEX_WAITERS is never an
.\"       allowed state.
It deals with the situation where the TID value at
.I uaddr
is 0, but the
.B FUTEX_WAITERS
bit is set.
.\" FIXME How does the situation in the previous sentence come about?
.\"       Probably it would be helpful to say something about that in
.\"       the man page.
.\" FIXME And *how* does FUTEX_TRYLOCK_PI deal with this situation?
User space cannot handle this condition in a race-free manner

The
.IR uaddr2 ,
.IR val ,
.IR timeout ,
and
.IR val3
arguments are ignored.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_UNLOCK_PI " (since Linux 2.6.18)"
.\" commit c87e2837be82df479a6bae9f155c43516d2feebc
This operation wakes the top priority waiter that is waiting in
.B FUTEX_LOCK_PI
on the futex address provided by the
.I uaddr
argument.

This is called when the user space value at
.I uaddr
cannot be changed atomically from a TID (of the owner) to 0.

The
.IR uaddr2 ,
.IR val ,
.IR timeout ,
and
.IR val3
arguments are ignored.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_CMP_REQUEUE_PI " (since Linux 2.6.31)"
.\" commit 52400ba946759af28442dee6265c5c0180ac7122
This operation is a PI-aware variant of
.BR FUTEX_CMP_REQUEUE .
It requeues waiters that are blocked via
.B FUTEX_WAIT_REQUEUE_PI
on
.I uaddr
from a non-PI source futex
.RI ( uaddr )
to a PI target futex
.RI ( uaddr2 ).

As with
.BR FUTEX_CMP_REQUEUE ,
this operation wakes up a maximum of
.I val
waiters that are waiting on the futex at
.IR uaddr .
However, for
.BR FUTEX_CMP_REQUEUE_PI ,
.I val
is required to be 1
(since the main point is to avoid a thundering herd).
The remaining waiters are removed from the wait queue of the source futex at
.I uaddr
and added to the wait queue of the target futex at
.IR uaddr2 .

The
.I val2
.\" val2 is the cap on the number of requeued waiters.
.\" In the glibc pthread_cond_broadcast() implementation, this argument
.\" is specified as INT_MAX, and for pthread_cond_signal() it is 0.
and
.I val3
arguments serve the same purposes as for
.BR FUTEX_CMP_REQUEUE .
.\"
.\" FIXME The page at http://locklessinc.com/articles/futex_cheat_sheet/
.\"       notes that "priority-inheritance Futex to priority-inheritance
.\"       Futex requeues are currently unsupported". Do we need to say
.\"       something in the man page about that?
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.TP
.BR FUTEX_WAIT_REQUEUE_PI " (since Linux 2.6.31)"
.\" commit 52400ba946759af28442dee6265c5c0180ac7122
.\"
.\" FIXME I find the next sentence (from tglx) pretty hard to grok.
.\"       Could someone explain it a bit more?
Wait operation to wait on a non-PI futex at
.I uaddr
and potentially be requeued onto a PI futex at
.IR uaddr2 .
The wait operation on
.I uaddr
is the same as
.BR FUTEX_WAIT .
.\"
.\" FIXME I'm not quite clear on the meaning of the following sentence.
.\"       Is this trying to say that while blocked in a
.\"       FUTEX_WAIT_REQUEUE_PI, it could happen that another
.\"       task does a FUTEX_WAKE on uaddr that simply causes
.\"       a normal wake, with the result that the FUTEX_WAIT_REQUEUE_PI
.\"       does not complete? What happens then to the FUTEX_WAIT_REQUEUE_PI
.\"       opertion? Does it remain blocked, or does it unblock
.\"       In which case, what does user space see?
The waiter can be removed from the wait on
.I uaddr
via
.BR FUTEX_WAKE
without requeueing on
.IR uaddr2 .

.\" FIXME Please check the following. tglx said "The timeout argument
.\"       is handled as described in FUTEX_WAIT.", but the truth is
.\"       as below, AFAICS
If
.I timeout
is not NULL, it specifies a timeout for the wait operation;
this timeout is interpreted as outlined above in the description of the
.BR FUTEX_CLOCK_REALTIME
option.
If
.I timeout
is NULL, the operation can block indefinitely.

The
.I val3
argument is ignored.
.\" FIXME Re the preceding sentence... Actually 'val3' is internally set to
.\"       FUTEX_BITSET_MATCH_ANY before calling futex_wait_requeue_pi().
.\"       I'm not sure we need to say anything about this though.
.\"       Comments?

The
.BR FUTEX_WAIT_REQUEUE_PI
and
.BR FUTEX_CMP_REQUEUE_PI
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
operation, the user-space application pre-specifies the target
of the requeue that takes place in the
.BR FUTEX_CMP_REQUEUE_PI
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 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
.PP
In the event of an error, all operations return \-1 and set
.I errno
to indicate the cause of the error.
The return value on success depends on the operation,
as described in the following list:
.TP
.B FUTEX_WAIT
Returns 0 if the caller was woken up.  Note that a wake-up can also be
caused by common futex usage patterns in unrelated code that happened to have
previously used the futex word's memory location (e.g., typical futex-based
implementations of Pthreads mutexes can cause this under some conditions).
Therefore, callers should always conservatively assume that a return value of
0 can mean a spurious wake-up, and use the futex word's value (i.e., the user
space synchronization scheme) to decide whether to continue to block or not.
.TP
.B FUTEX_WAKE
Returns the number of waiters that were woken up.
.TP
.B FUTEX_FD
Returns the new file descriptor associated with the futex.
.TP
.B FUTEX_REQUEUE
Returns the number of waiters that were woken up.
.TP
.B FUTEX_CMP_REQUEUE
Returns the total number of waiters that were woken up or
requeued to the futex for the futex word at
.IR uaddr2 .
If this value is greater than
.IR val ,
then difference is the number of waiters requeued to the futex for the futex
word at
.IR uaddr2 .
.TP
.B FUTEX_WAKE_OP
Returns the total number of waiters that were woken up.
This is the sum of the woken waiters on the two futexes for the futex words at
.I uaddr
and
.IR uaddr2 .
.TP
.B FUTEX_WAIT_BITSET
Returns 0 if the caller was woken up. See
.B FUTEX_WAIT
for how to interpret this correctly in practice.
.TP
.B FUTEX_WAKE_BITSET
Returns the number of waiters that were woken up.
.TP
.B FUTEX_LOCK_PI
Returns 0 if the futex was successfully locked.
.TP
.B FUTEX_TRYLOCK_PI
Returns 0 if the futex was successfully locked.
.TP
.B FUTEX_UNLOCK_PI
Returns 0 if the futex was successfully unlocked.
.TP
.B FUTEX_CMP_REQUEUE_PI
Returns the total number of waiters that were woken up or
requeued to the futex for the futex word at
.IR uaddr2 .
If this value is greater than
.IR val ,
then difference is the number of waiters requeued to the futex for the futex
word at
.IR uaddr2 .
.TP
.B FUTEX_WAIT_REQUEUE_PI
Returns 0 if the caller was successfully requeued to the futex for the futex
word at
.IR uaddr2 .
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.SH ERRORS
.TP
.B EACCES
No read access to the memory of a futex word.
.TP
.B EAGAIN
.RB ( FUTEX_WAIT ,
.BR FUTEX_WAIT_BITSET ,
.BR FUTEX_WAIT_REQUEUE_PI )
The value pointed to by
.I uaddr
was not equal to the expected value
.I val
at the time of the call.

.BR Note :
on Linux, the symbolic names
.B EAGAIN
and
.B EWOULDBLOCK
(both of which appear in different parts of the kernel futex code)
have the same value.
.TP
.B EAGAIN
.RB ( FUTEX_CMP_REQUEUE ,
.BR FUTEX_CMP_REQUEUE_PI )
The value pointed to by
.I uaddr
is not equal to the expected value
.IR val3 .
.\" FIXME: Is the following sentence correct?
.\" I would prefer to remove this sentence. --triegel@redhat.com
(This probably indicates a race;
use the safe
.B FUTEX_WAKE
now.)
.\" 
.\" FIXME XXX Should there be an EAGAIN case for FUTEX_TRYLOCK_PI?
.\"       It seems so, looking at the handling of the rt_mutex_trylock()
.\"       call in futex_lock_pi()
.\"       (Davidlohr also thinks so.)
.\" 
.TP
.BR EAGAIN
.RB ( FUTEX_LOCK_PI ,
.BR FUTEX_TRYLOCK_PI ,
.BR FUTEX_CMP_REQUEUE_PI )
The futex owner thread ID of
.I uaddr
(for
.BR FUTEX_CMP_REQUEUE_PI :
.IR uaddr2 )
is about to exit,
but has not yet handled the internal state cleanup.
Try again.
.TP
.BR EDEADLK
.RB ( FUTEX_LOCK_PI ,
.BR FUTEX_TRYLOCK_PI ,
.BR FUTEX_CMP_REQUEUE_PI )
The futex word at
.I uaddr
is already locked by the caller.
.TP
.BR EDEADLK
.\" FIXME I reworded tglx's text somewhat; is the following okay?
.\" FIXME XXX I see that kernel/locking/rtmutex.c uses EDEADLK in some places,
.\"       and EDEADLOCK in others. On almost all architectures these
.\"       constants are synonymous. Is there a reason that both names
.\"       are used?
.RB ( FUTEX_CMP_REQUEUE_PI )
While requeueing a waiter to the PI futex for the futex word at
.IR uaddr2 ,
the kernel detected a deadlock.
.TP
.B EFAULT
A required pointer argument (i.e.,
.IR uaddr ,
.IR uaddr2 ,
or
.IR timeout )
did not point to a valid user-space address.
.TP
.B EINTR
A
.B FUTEX_WAIT
or
.B FUTEX_WAIT_BITSET
operation was interrupted by a signal (see
.BR signal (7)).
In kernels before Linux 2.6.22, this error could also be returned for
on a spurious wakeup; since Linux 2.6.22, this no longer happens.
.TP
.B EINVAL
The operation in
.IR futex_op
is one of those that employs a timeout, but the supplied
.I timeout
argument was invalid
.RI ( tv_sec
was less than zero, or
.IR tv_nsec
was not less than 1000,000,000).
.TP
.B EINVAL
The operation specified in
.IR futex_op
employs one or both of the pointers
.I uaddr
and
.IR uaddr2 ,
but one of these does not point to a valid object\(emthat is,
the address is not four-byte-aligned.
.TP
.B EINVAL
.RB ( FUTEX_WAIT_BITSET ,
.BR FUTEX_WAKE_BITSET )
The bitset supplied in
.IR val3
is zero.
.TP
.B EINVAL
.RB ( FUTEX_CMP_REQUEUE_PI )
.I uaddr
equals
.IR uaddr2
(i.e., an attempt was made to requeue to the same futex).
.TP
.BR EINVAL
.RB ( FUTEX_FD )
The signal number supplied in
.I val
is invalid.
.TP
.B EINVAL
.RB ( FUTEX_WAKE ,
.BR FUTEX_WAKE_OP ,
.BR FUTEX_WAKE_BITSET ,
.BR FUTEX_REQUEUE ,
.BR FUTEX_CMP_REQUEUE )
The kernel detected an inconsistency between the user-space state at
.I uaddr
and the kernel state\(emthat is, it detected a waiter which waits in
.BR FUTEX_LOCK_PI
on
.IR uaddr .
.TP
.B EINVAL
.RB ( FUTEX_LOCK_PI ,
.BR FUTEX_TRYLOCK_PI ,
.BR FUTEX_UNLOCK_PI )
The kernel detected an inconsistency between the user-space state at
.I uaddr
and the kernel state.
This indicates either state corruption
.\" FIXME tglx did not mention the "state corruption" for FUTEX_UNLOCK_PI.
.\"       Does that case also apply for FUTEX_UNLOCK_PI?
or that the kernel found a waiter on
.I uaddr
which is waiting via
.BR FUTEX_WAIT
or
.BR FUTEX_WAIT_BITSET .
.TP
.B EINVAL
.RB ( FUTEX_CMP_REQUEUE_PI )
The kernel detected an inconsistency between the user-space state at
.I uaddr2
and the kernel state;
that is, the kernel detected a waiter which waits via
.BR FUTEX_WAIT
.\" FIXME tglx did not mention FUTEX_WAIT_BITSET here,
.\"       but should that not also be included here?
on
.IR uaddr2 .
.TP
.B EINVAL
.RB ( FUTEX_CMP_REQUEUE_PI )
The kernel detected an inconsistency between the user-space state at
.I uaddr
and the kernel state;
that is, the kernel detected a waiter which waits via
.BR FUTEX_WAIT
or
.BR FUTEX_WAIT_BITESET
on
.IR uaddr .
.TP
.B EINVAL
.RB ( FUTEX_CMP_REQUEUE_PI )
The kernel detected an inconsistency between the user-space state at
.I uaddr
and the kernel state;
that is, the kernel detected a waiter which waits on
.I uaddr
via
.BR FUTEX_LOCK_PI
(instead of
.BR FUTEX_WAIT_REQUEUE_PI ).
.TP
.B EINVAL
.RB ( FUTEX_CMP_REQUEUE_PI )
.\" FIXME XXX The following is a reworded version of Darren Hart's text.
.\"       Please check that I did not introduce any errors.
An attempt was made to requeue a waiter to a futex other than that
specified by the matching
.B FUTEX_WAIT_REQUEUE_PI
call for that waiter.
.TP
.B EINVAL
.RB ( FUTEX_CMP_REQUEUE_PI )
The
.I val
argument is not 1.
.TP
.B EINVAL
Invalid argument.
.TP
.BR ENOMEM
.RB ( FUTEX_LOCK_PI ,
.BR FUTEX_TRYLOCK_PI ,
.BR FUTEX_CMP_REQUEUE_PI )
The kernel could not allocate memory to hold state information.
.TP
.B ENFILE
.RB ( FUTEX_FD )
The system limit on the total number of open files has been reached.
.TP
.B ENOSYS
Invalid operation specified in
.IR futex_op .
.TP
.B ENOSYS
The
.BR FUTEX_CLOCK_REALTIME
option was specified in
.IR futex_op ,
but the accompanying operation was neither
.BR FUTEX_WAIT_BITSET
nor
.BR FUTEX_WAIT_REQUEUE_PI .
.TP
.BR ENOSYS
.RB ( FUTEX_LOCK_PI ,
.BR FUTEX_TRYLOCK_PI ,
.BR FUTEX_UNLOCK_PI ,
.BR FUTEX_CMP_REQUEUE_PI ,
.BR FUTEX_WAIT_REQUEUE_PI )
A run-time check determined that the operation is not available.
The PI futex operations are not implemented on all architectures and
are not supported on some CPU variants.  
.TP
.BR EPERM
.RB ( FUTEX_LOCK_PI ,
.BR FUTEX_TRYLOCK_PI ,
.BR FUTEX_CMP_REQUEUE_PI )
The caller is not allowed to attach itself to the futex at
.I uaddr
(for
.BR FUTEX_CMP_REQUEUE_PI :
the futex at
.IR uaddr2 ).
(This may be caused by a state corruption in user space.)
.TP
.BR EPERM
.RB ( FUTEX_UNLOCK_PI )
The caller does not own the lock represented by the futex word.
.TP
.BR ESRCH
.RB ( FUTEX_LOCK_PI ,
.BR FUTEX_TRYLOCK_PI ,
.BR FUTEX_CMP_REQUEUE_PI )
.\" FIXME I reworded the following sentence a bit differently from
.\"       tglx's formulation. Is it okay?
The thread ID in the futex word at
.I uaddr
does not exist.
.TP
.BR ESRCH
.RB ( FUTEX_CMP_REQUEUE_PI )
.\" FIXME I reworded the following sentence a bit differently from
.\"       tglx's formulation. Is it okay?
The thread ID in the futex word at
.I uaddr2
does not exist.
.TP
.B ETIMEDOUT
The operation in
.IR futex_op
employed the timeout specified in
.IR timeout ,
and the timeout expired before the operation completed.
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.SH VERSIONS
.PP
Futexes were first made available in a stable kernel release
with Linux 2.6.0.

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.
In Linux 2.5.70, one argument
was added.
In Linux 2.6.7, a sixth argument was added\(emmessy, especially
on the s390 architecture.
.SH CONFORMING TO
This system call is Linux-specific.
.SH NOTES
Glibc does not provide a wrapper for this system call; call it using
.BR syscall (2).
.\" TODO FIXME Above, we cite this section and claim it contains details on
.\" the synchronization semantics; add the C11 equivalents here (or whatever
.\" we find consensus for).
.\"
.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.\"
.SH EXAMPLE
.\" FIXME Is it worth having an example program?
.\" FIXME Anything obviously broken in the example program?
.\"
The program below demonstrates 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
.IR 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:

.in +4n
.nf
$ \fB./futex_demo\fP
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
.fi
.in
.SS Program source
\&
.nf
/* 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 \(aqnum\-loops\(aq messages to the terminal and employ
   a synchronization protocol that ensures that they alternate in
   writing messages.
*/
#define _GNU_SOURCE
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <linux/futex.h>
#include <sys/time.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \\
                        } while (0)

static int *futex1, *futex2, *iaddr;

static int
futex(int *uaddr, int futex_op, int val,
      const struct timespec *timeout, int *uaddr2, int val3)
{
    return syscall(SYS_futex, uaddr, futex_op, val,
                   timeout, uaddr, val3);
}

/* Acquire the futex pointed to by \(aqfutexp\(aq: wait for its value to
   become 1, and then set the value to 0. */

static void
fwait(int *futexp)
{
    int s;

    /* __sync_bool_compare_and_swap(ptr, oldval, newval) is a gcc
       built\-in function.  It atomically performs the equivalent of:

           if (*ptr == oldval)
               *ptr = newval;

       It returns true if the test yielded true and *ptr was updated.
       The alternative here would be to employ the equivalent atomic
       machine\-language instructions.  For further information, see
       the GCC Manual. */

    /* Maybe the futex is already available: */

    if (__sync_bool_compare_and_swap(futexp, 1, 0))
        return;

    /* No; we must wait for the futex value to be changed */

    while (1) {
        s = futex(futexp, FUTEX_WAIT, 0, NULL, NULL, 0);
        if (s == \-1 && errno != EAGAIN)
            errExit("futex\-FUTEX_WAIT");

        /* Is the futex now available? */

        if (__sync_bool_compare_and_swap(futexp, 1, 0))
            break;      /* Yes */

        /* Futex is still not available; wait again */
    }
}

/* Release the futex pointed to by \(aqfutexp\(aq: if the futex currently
   has the value 0, set its value to 1 and the wake any futex waiters,
   so that if the peer is blocked in fpost(), it can proceed. */

static void
fpost(int *futexp)
{
    int s;

    /* __sync_bool_compare_and_swap() was described in comments above */

    if (__sync_bool_compare_and_swap(futexp, 0, 1)) {

        s = futex(futexp, FUTEX_WAKE, 1, NULL, NULL, 0);
        if (s  == \-1)
            errExit("futex\-FUTEX_WAKE");
    }
}

int
main(int argc, char *argv[])
{
    pid_t childPid;
    int j, 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(int) * 2, PROT_READ | PROT_WRITE,
                MAP_ANONYMOUS | MAP_SHARED, \-1, 0);
    if (iaddr == MAP_FAILED)
        errExit("mmap");

    futex1 = &iaddr[0];
    futex2 = &iaddr[1];

    *futex1 = 0;        /* State: unavailable */
    *futex2 = 1;        /* State: available */

    /* Create a child process that inherits the shared anonymous
       mappping */

    childPid = fork();
    if (childPid == 1)
        errExit("fork");

    if (childPid == 0) {        /* Child */
        for (j = 0; j < nloops; j++) {
            fwait(futex1);
            printf("Child  (%ld) %d\\n", (long) getpid(), j);
            fpost(futex2);
        }

        exit(EXIT_SUCCESS);
    }

    /* Parent falls through to here */

    for (j = 0; j < nloops; j++) {
        fwait(futex2);
        printf("Parent (%ld) %d\\n", (long) getpid(), j);
        fpost(futex1);
    }

    wait(NULL);

    exit(EXIT_SUCCESS);
}
.fi
.SH SEE ALSO
.ad l
.BR get_robust_list (2),
.BR restart_syscall (2),
.BR futex (7)
.PP
The following kernel source files:
.IP * 2
.I Documentation/pi-futex.txt
.IP *
.I Documentation/futex-requeue-pi.txt
.IP *
.I Documentation/locking/rt-mutex.txt
.IP *
.I Documentation/locking/rt-mutex-design.txt
.IP *
.I Documentation/robust-futex-ABI.txt
.PP
Franke, H., Russell, R., and Kirwood, M., 2002.
\fIFuss, Futexes and Furwocks: Fast Userlevel Locking in Linux\fP
(from proceedings of the Ottawa Linux Symposium 2002),
.br
.UR http://kernel.org\:/doc\:/ols\:/2002\:/ols2002-pages-479-495.pdf
.UE

Hart, D., 2009. \fIA futex overview and update\fP,
.UR http://lwn.net/Articles/360699/
.UE

Hart, D. and Guniguntala, D., 2009.
\fIRequeue-PI: Making Glibc Condvars PI-Aware\fP
(from proceedings of the 2009 Real-Time Linux Workshop),
.UR http://lwn.net/images/conf/rtlws11/papers/proc/p10.pdf
.UE

Drepper, U., 2011. \fIFutexes Are Tricky\fP,
.UR http://www.akkadia.org/drepper/futex.pdf
.UE
.PP
Futex example library, futex-*.tar.bz2 at
.br
.UR ftp://ftp.kernel.org\:/pub\:/linux\:/kernel\:/people\:/rusty/
.UE
.\"
.\" FIXME Are there any other resources that should be listed
.\"       in the SEE ALSO section?
.\" FIXME 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.