pthreadbarriert

pthread_barrier_t, commonly referred to as a barrier in the POSIX threads (pthreads) library, is a synchronization primitive that coordinates a fixed set of threads. When a barrier is used, each participating thread must reach the barrier point and wait until all designated threads have arrived, at which point they all proceed. Barriers are reusable: after release, the barrier can be used again for subsequent rendezvous.

The typical usage involves creating a barrier with pthread_barrier_init, providing the number of threads that must reach the barrier. An optional barrier attribute object (pthread_barrierattr_t) can be supplied to control sharing semantics, such as whether the barrier can be shared across processes (using PTHREAD_PROCESS_SHARED) or remains private to the creating process. After initialization, threads call pthread_barrier_wait on the barrier. When the required count arrives, all threads are unblocked. Exactly one thread may receive the return value PTHREAD_BARRIER_SERIAL_THREAD; the remaining threads receive 0. The serial-thread can be used to perform a small amount of work once per barrier cycle if needed, but this is optional.

Destruction of the barrier is performed with pthread_barrier_destroy. It returns 0 on success. It may fail with EINVAL if the barrier is invalid or not properly initialized, or with EBUSY if the barrier is currently in use by threads (for example, if some threads are still waiting at the barrier).

Portability and notes: Barriers are defined by POSIX, but availability and exact error semantics can vary by implementation. Cross-process sharing requires suitable support and shared memory layout. In some environments, barriers are not available or behave differently from other synchronization primitives such as mutexes and condition variables.