# os_sync_wake_by_address_all An atomic operation that wakes all threads blocked on a futex wait, used to implement higher-level synchronization primitives. ``` extern int os_sync_wake_by_address_all(void *addr, size_t size, os_sync_wake_by_address_flags_t flags); ``` ## Parameters `addr` The user-space address waited on to wake. This address must be aligned to `size`. `size` The size of the wait value, in bytes. Values can be 4 or 8 bytes, where 4-byte values use the lower bytes of `value`. `flags` Flags for the operation. ## Return Value Returns 0 on success, and -1 on error with `errno` set. ## Discussion This function wakes all threads waiting on `addr`. There’s no guarantee on the order threads wake. Pass consistent values across wait and wake APIs for `addr`, `size`, and `flags`. See [`os_sync_wake_by_address_flags_t`](/documentation/os/os_sync_wake_by_address_flags_t) for details. > Important: Use this function only for implementing synchronization primitives that don’t have a sense of ownership, > such as condition variables or semaphores. In particular, this function doesn’t provide priority inversion avoidance. > For locking APIs, use existing OS primitives, such as `pthread` threads or > APIs. For synchronization and threading managed by the OS, use a higher-level API, such > as . ### Error Codes When this function returns `-1`, the global variable `errno` contains one of these values, indicating the corresponding error. |Error code|Description | |----------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| |`EINVAL` |Invalid flags or size.The `addr` passed is NULL.The operation associated with the existing kernel state at `addr` is inconsistent with other function arguments.| |`ENOENT` |There are no compare-and-wait operations on `addr`. | |`EDOM` |The `addr` is in use for non-compare-and-wait synchronization. | --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)