futex(2)

💖 futex(2) 💖 — the System Calls Manual page, bestie edition

okay so. settle in, grab a snack, because we're doing fast user-space locking and it's lowkey iconic 🦋

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✨ NAME ✨

futex — fast user-space locking 🔒

📚 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, int op, ...);

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🌸 DESCRIPTION 🌸

So the futex() system call hands you a way to wait until a certain condition becomes true. It's typically used as a blocking construct in the context of shared-memory synchronization. Here's the vibe: when you're using futexes, the majority of the synchronization operations happen in user space. Your user-space program only calls futex() when it looks like it's gonna have to block for a while until the condition becomes true. And the other futex() operations? Those are for waking up any processes or threads that are waiting on a particular condition. 💫

A futex is a 32-bit value — we call it a futex word below — and its address is what you hand to the futex() system call. (Futexes are 32 bits in size on all platforms, including 64-bit systems. no exceptions, periodt.) All futex operations are governed by this value. To share a futex between processes, you drop it in a region of shared memory, created using (for example) mmap(2) or shmat(2). (So yes, the futex word may live at different virtual addresses in different processes, but those addresses all point at the same location in physical memory. cute, right?) In a multithreaded program, it's enough to put the futex word in a global variable that all the threads share. 🫶

When you execute a futex operation that asks to block a thread, the kernel will block only if the futex word has the value that the calling thread supplied (as one of the arguments of the futex() call) as the expected value of the futex word. The loading of the futex word's value, the comparison of that value with the expected value, and the actual blocking all happen atomically, and they're totally ordered with respect to concurrent operations performed by other threads on the same futex word. So the futex word is literally the bridge connecting your user-space synchronization to the kernel's blocking implementation. Analogously to an atomic compare-and-exchange operation that potentially changes shared memory, blocking via a futex is an atomic compare-and-block operation. the way that works… chef's kiss 💅

One use of futexes is implementing locks. The state of the lock (acquired or not acquired) can be represented as an atomically accessed flag in shared memory. In the uncontended case, a thread can access or modify the lock state with atomic instructions — for example, atomically changing it from not acquired to acquired using an atomic compare-and-exchange instruction. (Those instructions run entirely in user mode, and the kernel keeps no information about the lock state. main character energy, honestly.) On the other hand, a thread may be unable to acquire a lock because another thread already has it. In that case it can pass the lock's flag as a futex word, and the value representing the acquired state as the expected value, to a futex() wait operation. This futex() operation will block if and only if the lock is still acquired (i.e., the value in the futex word still matches the "acquired state"). When releasing the lock, a thread has to first reset the lock state to not acquired, and then execute a futex operation that wakes threads blocked on the lock flag used as a futex word (this can be further optimized to avoid unnecessary wake-ups). For more detail on how to use futexes, see futex(7). 📖

Besides the basic wait and wake-up futex functionality, there are further futex operations aimed at supporting more complex use cases. 🔥

Note that no explicit initialization or destruction is necessary to use futexes; the kernel maintains a futex (i.e., the kernel-internal implementation artifact) only while operations such as FUTEX_WAIT(2const) are being performed on a particular futex word. so chill, no setup era required ✨

🧵 Arguments 🧵

The uaddr argument points to the futex word. On all platforms, futexes are four-byte integers that must be aligned on a four-byte boundary. The operation to perform on the futex is specified in the op argument.

💫 Futex operations 💫

The op argument is made of two parts: a command specifying the operation to perform, bitwise ORed with zero or more options that modify the behavior of the operation. The options that may be included in op are:

• FUTEX_PRIVATE_FLAG (since Linux 2.6.22) — This option bit can be employed with all futex operations. It tells the kernel that the futex is process-private and not shared with another process (i.e., it's being used for synchronization only between threads of the same process). This lets the kernel make some additional performance optimizations. 🏎️ As a convenience, <linux/futex.h> defines a set of constants with the suffix _PRIVATE that are equivalents of all of the operations listed below, but with the FUTEX_PRIVATE_FLAG ORed into the constant value. Thus, there are FUTEX_WAIT_PRIVATE, FUTEX_WAKE_PRIVATE, and so on. so convenient, obsessed 💖
• FUTEX_CLOCK_REALTIME (since Linux 2.6.28) — This option bit can be employed only with the FUTEX_WAIT_BITSET(2const), FUTEX_WAIT_REQUEUE_PI(2const), (since Linux 4.5) FUTEX_WAIT(2const), and (since Linux 5.14) FUTEX_LOCK_PI2(2const) operations. If this option is set, the kernel measures the timeout against the CLOCK_REALTIME clock. If this option is not set, the kernel measures the timeout against the CLOCK_MONOTONIC clock. ⏰

The operation specified in op is one of the following:

• FUTEX_WAIT(2const)
• FUTEX_WAKE(2const)
• FUTEX_FD(2const)
• FUTEX_REQUEUE(2const)
• FUTEX_CMP_REQUEUE(2const)
• FUTEX_WAKE_OP(2const)
• FUTEX_WAIT_BITSET(2const)
• FUTEX_WAKE_BITSET(2const)

👑 Priority-inheritance futexes 👑

Linux supports priority-inheritance (PI) futexes in order to handle priority-inversion problems that can be encountered with normal futex locks. Priority inversion is the problem that occurs when a high-priority task is blocked waiting to acquire a lock held by a low-priority task, while tasks at an intermediate priority continuously preempt the low-priority task off the CPU. Consequently, the low-priority task makes no progress toward releasing the lock, and the high-priority task just… stays blocked. 😭 a whole tragedy.

Priority inheritance is a mechanism for dealing with this problem. With this mechanism, when a high-priority task becomes blocked by a lock held by a low-priority task, the priority of the low-priority task gets temporarily raised to that of the high-priority task, so it doesn't get preempted by any intermediate level tasks, and can thus make progress toward releasing the lock. To be effective, priority inheritance must be transitive — meaning that if a high-priority task blocks on a lock held by a lower-priority task that is itself blocked by a lock held by another intermediate-priority task (and so on, for chains of arbitrary length), then both of those tasks (or more generally, all of the tasks in a lock chain) have their priorities raised to be the same as the high-priority task. it's giving ✨solidarity✨

From a user-space perspective, what makes a futex PI-aware is a policy agreement (described below) between user space and the kernel about the value of the futex word, coupled with the use of the PI-futex operations described below. (Unlike the other futex operations described above, the PI-futex operations are designed for the implementation of very specific IPC mechanisms.) 🛠️

The PI-futex operations described below differ from the other futex operations in that they impose policy on the use of the value of the futex word:

• If the lock is not acquired, the futex word's value shall be 0.
• If the lock is acquired, the futex word's value shall be the thread ID (TID; see gettid(2)) of the owning thread.
• If the lock is owned and there are threads contending for the lock, then the FUTEX_WAITERS bit shall be set in the futex word's value; in other words, this value is:

FUTEX_WAITERS | TID

(Note that it is invalid for a PI futex word to have no owner and FUTEX_WAITERS set. 🚫)

With this policy in place, a user-space application can acquire an unacquired lock or release a lock using atomic instructions executed in user mode (e.g., a compare-and-swap operation such as cmpxchg on the x86 architecture). Acquiring a lock simply consists of using compare-and-swap to atomically set the futex word's value to the caller's TID if its previous value was 0. Releasing a lock requires using compare-and-swap to set the futex word's value to 0 if the previous value was the expected TID. slay, no kernel needed 💅

If a futex is already acquired (i.e., has a nonzero value), waiters must employ the FUTEX_LOCK_PI(2const) or FUTEX_LOCK_PI2(2const) operations to acquire the lock. If other threads are waiting for the lock, then the FUTEX_WAITERS bit is set in the futex value; in this case, the lock owner must employ the FUTEX_UNLOCK_PI(2const) operation to release the lock.

In the cases where callers are forced into the kernel (i.e., required to perform a futex() call), they then deal directly with a so-called RT-mutex — a kernel locking mechanism which implements the required priority-inheritance semantics. After the RT-mutex is acquired, the futex value is updated accordingly, before the calling thread returns to user space.

It is important to note that the kernel will update the futex word's value prior to returning to user space. (This prevents the possibility of the futex word's value ending up in an invalid state — such as having an owner but the value being 0, or having waiters but not having the FUTEX_WAITERS bit set.) 🧐

If a futex has an associated RT-mutex in the kernel (i.e., there are blocked waiters) and the owner of the futex/RT-mutex dies unexpectedly, then the kernel cleans up the RT-mutex and hands it over to the next waiter. This in turn requires that the user-space value is updated accordingly. To indicate that this is required, the kernel sets the FUTEX_OWNER_DIED bit in the futex word along with the thread ID of the new owner. User space can detect this situation via the presence of the FUTEX_OWNER_DIED bit and is then responsible for cleaning up the stale state left over by the dead owner. 💀 rip but the kernel handled the handoff

PI futexes are operated on by specifying one of the values listed below in op. Note that the PI futex operations must be used as paired operations and are subject to some additional requirements:

• FUTEX_LOCK_PI(2const), FUTEX_LOCK_PI2(2const), and FUTEX_TRYLOCK_PI(2const) — okay so these three pair up with FUTEX_UNLOCK_PI(2const). And bestie, listen: FUTEX_UNLOCK_PI(2const) must be called only on a futex owned by the calling thread, as defined by the value policy. Otherwise? EPERM enters the chat. 💀
• FUTEX_WAIT_REQUEUE_PI(2const) — this one pairs with FUTEX_CMP_REQUEUE_PI(2const). And it must be performed from a non-PI futex to a distinct PI futex, or the error EINVAL shows up. Plus, the number of waiters to be woken must be 1, or again EINVAL says hi. 🙈

The PI futex operations are as follows:

• FUTEX_LOCK_PI(2const)
• FUTEX_LOCK_PI2(2const)
• FUTEX_TRYLOCK_PI(2const)
• FUTEX_UNLOCK_PI(2const)
• FUTEX_CMP_REQUEUE_PI(2const)
• FUTEX_WAIT_REQUEUE_PI(2const)

The FUTEX_WAIT_REQUEUE_PI(2const) and FUTEX_CMP_REQUEUE_PI(2const) were added to support a fairly specific use case: support for priority-inheritance-aware POSIX threads condition variables. The idea is that these operations should always be paired, in order to ensure that user space and the kernel remain in sync. Thus, in the FUTEX_WAIT_REQUEUE_PI(2const) operation, the user-space application pre-specifies the target of the requeue that takes place in the FUTEX_CMP_REQUEUE_PI(2const) operation. 💅

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✨ RETURN VALUE ✨

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

The return value on success depends on the operation. (Like, it's giving "it depends" energy. 🙃)

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💥 ERRORS 💥

• EACCES — No read access to the memory of a futex word. Rude. 🙄
• EFAULT — uaddr did not point to a valid user-space address. Oops. 🫠
• EINVAL — uaddr does not point to a valid object—that is, the address is not four-byte-aligned. Alignment matters, bestie. 📐
• EINVAL — Invalid argument. Classic. 💀
• ENOSYS — Invalid operation specified in op. Not a vibe. 🚫
• ENOSYS — The FUTEX_CLOCK_REALTIME option was specified in op, but the accompanying operation was neither FUTEX_WAIT_BITSET(2const), FUTEX_WAIT_REQUEUE_PI(2const), nor FUTEX_LOCK_PI2(2const). So pick the right bestie next time. 🙈

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🌈 STANDARDS 🌈

Linux. That's it. That's the standard. 👑

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📜 HISTORY 📜

Linux 2.6.0. ✨

Initial futex support was merged in Linux 2.5.7 but with different semantics from what was described above. A four-argument system call with the semantics described in this page was introduced in Linux 2.5.40. A fifth argument was added in Linux 2.5.70, and a sixth argument was added in Linux 2.6.7. So yeah, it evolved. 🦋

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🌸 EXAMPLES 🌸

The program below demonstrates use of futexes in a program where a parent process and a child process use a pair of futexes located inside a shared anonymous mapping to synchronize access to a shared resource: the terminal. The two processes each write nloops (a command-line argument that defaults to 5 if omitted) messages to the terminal and employ a synchronization protocol that ensures that they alternate in writing messages. Upon running this program we see output such as the following:

           $ ./futex_demo;
           Parent (18534) 0
           Child  (18535) 0
           Parent (18534) 1
           Child  (18535) 1
           Parent (18534) 2
           Child  (18535) 2
           Parent (18534) 3
           Child  (18535) 3
           Parent (18534) 4
           Child  (18535) 4

🎀 Program source 🎀

       /* futex_demo.c

          Usage: futex_demo [nloops]
                           (Default: 5)

          Demonstrate the use of futexes in a program where parent and child
          use a pair of futexes located inside a shared anonymous mapping to
          synchronize access to a shared resource: the terminal.  The two
          processes each write 'num-loops' messages to the terminal and employ
          a synchronization protocol that ensures that they alternate in
          writing messages.
       */
       #define _GNU_SOURCE
       #include <err.h>
       #include <errno.h>
       #include <linux/futex.h>
       #include <stdatomic.h>
       #include <stdint.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/mman.h>
       #include <sys/syscall.h>
       #include <sys/time.h>
       #include <sys/wait.h>
       #include <unistd.h>

       static uint32_t *futex1, *futex2, *iaddr;

       static int
       futex(uint32_t *uaddr, int op, uint32_t val,
             const struct timespec *timeout, uint32_t *uaddr2, uint32_t val3)
       {
           return syscall(SYS_futex, uaddr, op, val,
                          timeout, uaddr2, val3);
       }

       /* Acquire the futex pointed to by 'futexp': wait for its value to
          become 1, and then set the value to 0.  */

       static void
       fwait(uint32_t *futexp)
       {
           long            s;
           const uint32_t  one = 1;

           /* atomic_compare_exchange_strong(ptr, oldval, newval)
              atomically performs the equivalent of:

                  if (*ptr == *oldval)
                      *ptr = newval;

              It returns true if the test yielded true and *ptr was updated.  */

           for (;;) {

               /* Is the futex available? */
               if (atomic_compare_exchange_strong(futexp, &one, 0))
                   break;      /* Yes */

               /* Futex is not available; wait.  */

               s = futex(futexp, FUTEX_WAIT, 0, NULL, NULL, 0);
               if (s == -1 && errno != EAGAIN)
                   err(EXIT_FAILURE, "futex-FUTEX_WAIT");
           }
       }

       /* Release the futex pointed to by 'futexp': if the futex currently
          has the value 0, set its value to 1 and then wake any futex waiters,
          so that if the peer is blocked in fwait(), it can proceed.  */

       static void
       fpost(uint32_t *futexp)
       {
           long            s;
           const uint32_t  zero = 0;

           /* atomic_compare_exchange_strong() was described
              in comments above.  */

           if (atomic_compare_exchange_strong(futexp, &zero, 1)) {
               s = futex(futexp, FUTEX_WAKE, 1, NULL, NULL, 0);
               if (s  == -1)
                   err(EXIT_FAILURE, "futex-FUTEX_WAKE");
           }
       }

       int
       main(int argc, char *argv[])
       {
           pid_t         childPid;
           unsigned int  nloops;

           setbuf(stdout, NULL);

           nloops = (argc > 1) ? atoi(argv[1]) : 5;

           /* Create a shared anonymous mapping that will hold the futexes.
              Since the futexes are being shared between processes, we
              subsequently use the "shared" futex operations (i.e., not the
              ones suffixed "_PRIVATE").  */

           iaddr = mmap(NULL, sizeof(*iaddr) * 2, PROT_READ | PROT_WRITE,
                        MAP_ANONYMOUS | MAP_SHARED, -1, 0);
           if (iaddr == MAP_FAILED)
               err(EXIT_FAILURE, "mmap");

           futex1 = &iaddr[0];
           futex2 = &iaddr[1];

           *futex1 = 0;        /* State: unavailable */
           *futex2 = 1;        /* State: available */

           /* Create a child process that inherits the shared anonymous
              mapping.  */

           childPid = fork();
           if (childPid == -1)
               err(EXIT_FAILURE, "fork");

           if (childPid == 0) {        /* Child */
               for (unsigned int j = 0; j < nloops; j++) {
                   fwait(futex1);
                   printf("Child  (%jd) %u\n", (intmax_t) getpid(), j);
                   fpost(futex2);
               }

               exit(EXIT_SUCCESS);
           }

           /* Parent falls through to here.  */

           for (unsigned int j = 0; j < nloops; j++) {
               fwait(futex2);
               printf("Parent (%jd) %u\n", (intmax_t) getpid(), j);
               fpost(futex1);
           }

           wait(NULL);

           exit(EXIT_SUCCESS);
       }

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👀 SEE ALSO 👀

• get_robust_list(2)
• restart_syscall(2)
• pthread_mutexattr_getprotocol(3)
• futex(7)
• sched(7)

The following kernel source files:

• Documentation/locking/pi-futex.rst
• Documentation/locking/futex-requeue-pi.rst
• Documentation/locking/rt-mutex.rst
• Documentation/locking/rt-mutex-design.rst
• Documentation/robust-futex-ABI.rst
• Franke, H., Russell, R., and Kirwood, M., 2002. Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (from proceedings of the Ottawa Linux Symposium 2002).
• Hart, D., 2009. A futex overview and update.
• Hart, D. and Guniguntala, D., 2009. Requeue-PI: Making glibc Condvars PI-Aware (from proceedings of the 2009 Real-Time Linux Workshop).
• Drepper, U., 2011. Futexes Are Tricky.
• Futex example library, futex-*.tar.bz2.

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This page is from Linux man-pages 6.20, dated 2026-02-14, all about futex(2). Stay futexy, bestie! 💅✨🫶