In parallel computing, a barrier is a type of synchronization method. A barrier for a group of threads or processes in the source code means any thread/process must stop at this point and cannot proceed until all other threads/processes reach this barrier.

Many collective routines and directive-based parallel languages impose implicit barriers. For example, a parallel do loop in Fortran with OpenMP will not be allowed to continue on any thread until the last iteration is completed.^{[citation needed]} This is in case the program relies on the result of the loop immediately after its completion. In message passing, any global communication (such as reduction or scatter) may imply a barrier.

In concurrent computing, a barrier may be in a raised or lowered state. The term latch is sometimes used to refer to a barrier that starts in the raised state and cannot be re-raised once it is in the lowered state. The term count-down latch is sometimes used to refer to a latch that is automatically lowered once a predetermined number of threads/processes have arrived.

Take an example for thread, known as the thread barrier. The thread barrier needs a variable to keep track of the total number of threads that have entered the barrier. Whenever there are enough threads enter the barrier, it will be lifted. A synchronization primitive like mutex is also needed when implementing the thread barrier.

This thread barrier method is also known as Centralized Barrier as the threads have to wait in front of a "central barrier" until the expected number of threads have reached the barrier before it is lifted.

The following C code, which implemented thread barrier by using POSIX Threads will demonstrate this procedure:

In this program, the thread barrier is defined as a struct, struct _thread_barrier, which include:

Based on the definition of barrier, we need to implement a function like thread_barrier_wait() in this program which will "monitor" the total number of thread in the program in order to life the barrier.