Pthread-barrier implementation for macOS using a pthread mutex and a condition variable.
The implementation consists of 1 header and 1 source file and can be included directly in other projects. Additionally, the project contains a CMake file to build as a library.
The sources files use #if guards to hide their contents if not used on macOS. Therefore, it is possible to #include "pthread_barrier.h" on all systems which just results in #include <pthread.h> when not on macOS.
In this implementation, barriers among processes (in shared memory) work only if they are at the same address in all processes. This works out-of-the box for scenarios where the processes are forked from a common parent but requires specifying a dedicated address if shm_open is used.
For barriers among processes call pthread_barrierattr_setpshared with PTHREAD_PROCESS_SHARED (as described in the pthread specification).
The implementation uses the corresponding attributes for the mutex and condition variable. However, there seems to be a macOS limitation that requires the mutex and/or the condition variable to be at the same address in all processes as described above.
The barrier implementation imposes the usage restrictions as described by the Posix standard. Most importantly calling pthread_barrier_destroy while any thread is waiting on the barrier results in undefined behavior.
While it is possible to use the barrier with more threads/processes than specified by the count parameter, the implementation does not attempt to provide any guarantees regarding to fairness.