411 REFERENCE · Wikibooks
C++ Language/Std/Multithreading/Semaphores/ReadersAndWriters
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
Semaphores provide a solution to the classic "Readers and Writers Problem", where threads are either trying to read or write a file (multiple reads can occur concurrently, but writes require exclusive access).
Additional information about the Readers and Writers problem