libzthread-dev 2.3.2-8build1 (riscv64 binary) in ubuntu focal

 Zthreads is an advanced platform-independent, object-oriented
 threading and synchronization library. Designed and tested under
 POSIX & Win32 systems.
 .
 It provides several structures for concurrent programming like
 PoolExecutor, MonitoredQueue, Barriers and much more. Furthermore,
 structures like Task and Thread are provided for creating threading
 applications in C++ easier.
 .
 This library wraps and interfaces with pthreads, so it is fully
 compliant.