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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | CAVEATS | SEE ALSO | COLOPHON |
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FUTEX_WAIT(2const) FUTEX_WAIT(2const)
FUTEX_WAIT - sleep waiting for a FUTEX_WAKE operation
Standard C library (libc, -lc)
#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, FUTEX_WAIT, uint32_t val,
const struct timespec *_Nullable timeout);
This operation tests that the value at the futex word pointed to
by the address uaddr still contains the expected value val, and if
so, then sleeps waiting for a FUTEX_WAKE(2const) operation 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 val, then the call
fails immediately with the error 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 FUTEX_WAKE(2const)
operation (or similar wake-up) after the value change and before
this FUTEX_WAIT operation, then the calling thread will observe
the value change and will not start to sleep.
If the timeout is not NULL, the structure it points to specifies a
timeout for the wait. (This interval will be rounded up to the
system clock granularity, and is guaranteed not to expire early.)
If timeout is NULL, the call blocks indefinitely.
On error, -1 is returned, and errno is set to indicate the error.
On success, 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.
See futex(2).
EAGAIN The value pointed to by uaddr was not equal to the expected
value val at the time of the call.
Note: on Linux, the symbolic names EAGAIN and EWOULDBLOCK
(both of which appear in different parts of the kernel
futex code) have the same value.
EFAULT timeout did not point to a valid user-space address.
EINTR The operation was interrupted by a signal (see signal(7)).
Before Linux 2.6.22, this error could also be returned for
a spurious wakeup; since Linux 2.6.22, this no longer
happens.
EINVAL The supplied timeout argument was invalid (tv_sec was less
than zero, or tv_nsec was not less than 1,000,000,000).
ETIMEDOUT
The timeout expired before the operation completed.
Linux.
Linux 2.6.0.
timeout is interpreted as a relative value. This differs from
other futex operations, where timeout is interpreted as an
absolute value. To obtain the equivalent of FUTEX_WAIT with an
absolute timeout, employ FUTEX_WAIT_BITSET(2const) with val3
specified as FUTEX_BITSET_MATCH_ANY.
futex(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-30 FUTEX_WAIT(2const)
Pages that refer to this page: futex(2), FUTEX_CMP_REQUEUE_PI(2const), FUTEX_LOCK_PI(2const), FUTEX_TRYLOCK_PI(2const), FUTEX_UNLOCK_PI(2const), FUTEX_WAIT_BITSET(2const), FUTEX_WAIT_REQUEUE_PI(2const), FUTEX_WAKE(2const)
Copyright and license for this manual page
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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