André Almeida <andrealmeid@collabora.com>

futex, or fast user mutex, is a set of syscalls to allow userspace to create performant synchronization mechanisms, such as mutexes, semaphores and conditional variables in userspace. C standard libraries, like glibc, uses it as a means to implement more high level interfaces like pthreads.

futex2 is a followup version of the initial futex syscall, designed to overcome limitations of the original interface.

User API


Wait on an array of futexes, wake on any:

futex_waitv(struct futex_waitv *waiters, unsigned int nr_futexes,
            unsigned int flags, struct timespec *timeout, clockid_t clockid)

struct futex_waitv {
      __u64 val;
      __u64 uaddr;
      __u32 flags;
      __u32 __reserved;

Userspace sets an array of struct futex_waitv (up to a max of 128 entries), using uaddr for the address to wait for, val for the expected value and flags to specify the type (e.g. private) and size of futex. __reserved needs to be 0, but it can be used for future extension. The pointer for the first item of the array is passed as waiters. An invalid address for waiters or for any uaddr returns -EFAULT.

If userspace has 32-bit pointers, it should do a explicit cast to make sure the upper bits are zeroed. uintptr_t does the tricky and it works for both 32/64-bit pointers.

nr_futexes specifies the size of the array. Numbers out of [1, 128] interval will make the syscall return -EINVAL.

The flags argument of the syscall needs to be 0, but it can be used for future extension.

For each entry in waiters array, the current value at uaddr is compared to val. If it's different, the syscall undo all the work done so far and return -EAGAIN. If all tests and verifications succeeds, syscall waits until one of the following happens:

  • The timeout expires, returning -ETIMEOUT.

  • A signal was sent to the sleeping task, returning -ERESTARTSYS.

  • Some futex at the list was woken, returning the index of some waked futex.

An example of how to use the interface can be found at tools/testing/selftests/futex/functional/futex_waitv.c.


struct timespec *timeout argument is an optional argument that points to an absolute timeout. You need to specify the type of clock being used at clockid argument. CLOCK_MONOTONIC and CLOCK_REALTIME are supported. This syscall accepts only 64bit timespec structs.

Types of futex

A futex can be either private or shared. Private is used for processes that shares the same memory space and the virtual address of the futex will be the same for all processes. This allows for optimizations in the kernel. To use private futexes, it's necessary to specify FUTEX_PRIVATE_FLAG in the futex flag. For processes that doesn't share the same memory space and therefore can have different virtual addresses for the same futex (using, for instance, a file-backed shared memory) requires different internal mechanisms to be get properly enqueued. This is the default behavior, and it works with both private and shared futexes.

Futexes can be of different sizes: 8, 16, 32 or 64 bits. Currently, the only supported one is 32 bit sized futex, and it need to be specified using FUTEX_32 flag.