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.TH FUTEX_WAKE_OP 2const 2025-05-30 "Linux man-pages 6.15"
.SH NAME
FUTEX_WAKE_OP \- 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 ", FUTEX_WAKE_OP, uint32_t " val ,
.BI "             uint32_t " val2 ", uint32_t *" uaddr2 ,
.BI "             uint32_t " val3 );
.fi
.SH DESCRIPTION
.\" commit 4732efbeb997189d9f9b04708dc26bf8613ed721
.\"	Author: Jakub Jelinek <jakub@redhat.com>
.\"	Date:   Tue Sep 6 15:16:25 2005 -0700
.\" FIXME. (Torvald) 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.
.\" [Torvald, do you have an update here?]
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.
.B FUTEX_WAKE_OP
allows such cases to be implemented without leading to
high rates of contention and context switching.
.P
The
.B FUTEX_WAKE_OP
operation is equivalent to executing the following code atomically
and totally ordered with respect to other futex operations on
any of the two supplied futex words:
.P
.in +4n
.EX
uint32_t oldval = *(uint32_t *) uaddr2;
*(uint32_t *) uaddr2 = oldval \f[I]op\f[] \f[I]oparg\f[];
futex(uaddr, FUTEX_WAKE, val, 0, 0, 0);
if (oldval \f[I]cmp\f[] \f[I]cmparg\f[])
    futex(uaddr2, FUTEX_WAKE, val2, 0, 0, 0);
.EE
.in
.P
In other words,
.B FUTEX_WAKE_OP
does the following:
.IP \[bu] 3
saves the original value of the futex word at
.I 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 \[bu]
wakes up a maximum of
.I val
waiters on the futex for the futex word at
.IR uaddr ;
and
.IP \[bu]
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 .
.P
The operation and comparison that are to be performed are encoded
in the bits of the argument
.IR val3 .
Pictorially, the encoding is:
.P
.in +4n
.EX
+---+---+-----------+-----------+
|op |cmp|   oparg   |  cmparg   |
+---+---+-----------+-----------+
  4   4       12          12    <== # of bits
.EE
.in
.P
Expressed in code, the encoding is:
.P
.in +4n
.EX
#define FUTEX_OP(op, oparg, cmp, cmparg) \[rs]
                (((op & 0xf) << 28) | \[rs]
                ((cmp & 0xf) << 24) | \[rs]
                ((oparg & 0xfff) << 12) | \[rs]
                (cmparg & 0xfff))
.EE
.in
.P
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.
.P
The
.I op
component has one of the following values:
.P
.in +4n
.EX
FUTEX_OP_SET        0  /* uaddr2 = oparg; */
FUTEX_OP_ADD        1  /* uaddr2 += oparg; */
FUTEX_OP_OR         2  /* uaddr2 |= oparg; */
FUTEX_OP_ANDN       3  /* uaddr2 &= \[ti]oparg; */
FUTEX_OP_XOR        4  /* uaddr2 \[ha]= oparg; */
.EE
.in
.P
In addition, bitwise ORing the following value into
.I op
causes
.I (1\~<<\~oparg)
to be used as the operand:
.P
.in +4n
.EX
FUTEX_OP_ARG_SHIFT  8  /* Use (1 << oparg) as operand */
.EE
.in
.P
The
.I cmp
field is one of the following:
.P
.in +4n
.EX
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 */
.EE
.in
.P
The return value of
.B FUTEX_WAKE_OP
is the sum of the number of waiters woken on the futex
.I uaddr
plus the number of waiters woken on the futex
.IR uaddr2 .
.\"
.SH RETURN VALUE
On error,
\-1 is returned,
and
.I errno
is set to indicate the error.
.P
On success,
.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 .
.SH ERRORS
See
.BR futex (2).
.TP
.B EFAULT
.I uaddr2
did not point to a valid user-space address.
.TP
.B EINVAL
.I uaddr2
does not point to a valid object\[em]that is,
the address is not four-byte-aligned.
.TP
.B EINVAL
The kernel detected an inconsistency between the user-space state at
.I uaddr
and the kernel state\[em]that is, it detected a waiter which waits in
.BR FUTEX_LOCK_PI (2const)
or
.BR FUTEX_LOCK_PI2 (2const)
on
.IR uaddr .
.SH STANDARDS
Linux.
.SH HISTORY
Linux 2.6.14.
.SH SEE ALSO
.BR futex (2)