| SEM(4) | Device Drivers Manual | SEM(4) | 
sem —
sem facility provides system calls used by the
  standard C library (libc) to implement POSIX semaphores.
sem facility appeared as a kernel option in
  NetBSD 2.0 and became non-optional in
  NetBSD 7.0.
| August 20, 2015 | NetBSD 10.1 |