---
name: "FUTEX_CMP_REQUEUE_PI"
section: "2const"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/FUTEX_CMP_REQUEUE_PI"
---

# FUTEX\_CMP\_REQUEUE\_PI(2const)

compare a priority-inheritance futex, wake a waiter, and requeue others

## NAME

FUTEX\_CMP\_REQUEUE\_PI - compare a priority-inheritance futex, wake a waiter, and requeue others

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#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\_CMP\_REQUEUE\_PI, 1,**              **uint32\_t** *val2***, uint32\_t \****uaddr2***,**              **uint32\_t** *val3***);**

## DESCRIPTION

This operation is a PI-aware variant of [**FUTEX\_CMP\_REQUEUE**(2const)](/FUTEX_CMP_REQUEUE/2const.md). It requeues waiters that are blocked via [**FUTEX\_WAIT\_REQUEUE\_PI**(2const)](/FUTEX_WAIT_REQUEUE_PI/2const.md) on *uaddr* from a non-PI source futex (*uaddr*) to a PI target futex (*uaddr2*).

Unlike with [**FUTEX\_CMP\_REQUEUE**(2const)](/FUTEX_CMP_REQUEUE/2const.md), this operation wakes up a maximum of 1 waiter that is waiting on the futex at *uaddr* (since the main point is to avoid a thundering herd). The remaining waiters are removed from the wait queue of the source futex at *uaddr* and added to the wait queue of the target futex at *uaddr2*.

The *val2* and *val3* arguments serve the same purposes as for [**FUTEX\_CMP\_REQUEUE**(2const)](/FUTEX_CMP_REQUEUE/2const.md).

## RETURN VALUE

On error, -1 is returned, and *errno* is set to indicate the error.

On success, **FUTEX\_CMP\_REQUEUE\_PI** Returns the total number of waiters that were woken up or requeued to the futex for the futex word at *uaddr2*. If this value is greater than 1, then difference is the number of waiters requeued to the futex for the futex word at *uaddr2*.

## ERRORS

See [**futex**(2)](/futex/2.md).

- **EAGAIN**

  The value pointed to by *uaddr* is not equal to the expected value *val3*.

- **EAGAIN**

  The futex owner thread ID of *uaddr2* is about to exit, but has not yet handled the internal state cleanup. Try again.

- **EDEADLK**

  The futex word at *uaddr* is already locked by the caller.

- **EDEADLK**

  While requeueing a waiter to the PI futex for the futex word at *uaddr2*, the kernel detected a deadlock.

- **EFAULT**

  *uaddr2* did not point to a valid user-space address.

- **EINVAL**

  *uaddr2* does not point to a valid object—that is, the address is not four-byte-aligned.

- **EINVAL**

  *uaddr* equals *uaddr2* (i.e., an attempt was made to requeue to the same futex).

- **EINVAL**

  The kernel detected an inconsistency between the user-space state at *uaddr2* and the kernel state; that is, the kernel detected a waiter which waits via [**FUTEX\_WAIT**(2const)](/FUTEX_WAIT/2const.md) or [**FUTEX\_WAIT\_BITSET**(2const)](/FUTEX_WAIT_BITSET/2const.md) on *uaddr2*.

- **EINVAL**

  The kernel detected an inconsistency between the user-space state at *uaddr* and the kernel state; that is, the kernel detected a waiter which waits via [**FUTEX\_WAIT**(2const)](/FUTEX_WAIT/2const.md) or [**FUTEX\_WAIT\_BITSET**(2const)](/FUTEX_WAIT_BITSET/2const.md) on *uaddr*.

- **EINVAL**

  The kernel detected an inconsistency between the user-space state at *uaddr* and the kernel state; that is, the kernel detected a waiter which waits on *uaddr* via [**FUTEX\_LOCK\_PI**(2const)](/FUTEX_LOCK_PI/2const.md) or [**FUTEX\_LOCK\_PI2**(2const)](/FUTEX_LOCK_PI2/2const.md) (instead of **FUTEX\_WAIT\_REQUEUE\_PI**).

- **EINVAL**

  An attempt was made to requeue a waiter to a futex other than that specified by the matching **FUTEX\_WAIT\_REQUEUE\_PI** call for that waiter.

- **EINVAL**

  The fourth argument is not 1.

- **ENOMEM**

  The kernel could not allocate memory to hold state information.

- **ENOSYS**

  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.

- **EPERM**

  The caller is not allowed to attach itself to the futex at *uaddr2*. (This may be caused by a state corruption in user space.)

- **ESRCH**

  The thread ID in the futex word at *uaddr* does not exist.

- **ESRCH**

  The thread ID in the futex word at *uaddr2* does not exist.

## STANDARDS

Linux.

## HISTORY

Linux 2.6.31.

## SEE ALSO

[**futex**(2)](/futex/2.md)