futex

Futex, short for fast userspace mutex, is a synchronization primitive used in the Linux kernel and user space to implement locks with a fast path in user space and a kernel fallback when contention occurs. It is designed to minimize kernel involvement for uncontended operations while providing a mechanism for blocking and waking threads when contention happens.

The common pattern is to acquire a lock using atomic operations in user space. If the lock is uncontended, the atomic operation succeeds and the thread proceeds. If contention occurs, a thread may call the futex system call to wait on a specific address; the kernel blocks the thread until another thread wakes it.

The futex system call accepts a pointer to a user-space address, an operation code (such as wait or wake), and a value used by the operation; additional arguments permit timeouts and cross-address requeueing. A FUTEX_PRIVATE_FLAG may be used to indicate the futex is private to the process, avoiding cross-process wakeups.

In practice, futexes underlie many user-space locking primitives, notably the pthreads mutex and condition variables on Linux. The design minimizes kernel involvement for uncontended locks and enables efficient scaling on multi-core systems; advanced variants support wake-up on specific conditions and requeueing waiters.

Limitations include the need for correct user-space synchronization and the possibility of spurious wakeups; robust usage relies on rechecking conditions after wake. Futexes are specific to Linux's API and are not portable across all operating systems.