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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).

The value pointed to by uaddr was not equal to the expected value val at the time of the call.
timeout did not point to a valid user-space address.
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.
The supplied timeout argument was invalid (tv_sec was less than zero, or tv_nsec was not less than 1,000,000,000).
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)