atomaticity/thread safeness of apc shared memory functions + mutexes

Couldn't find it in the docs, so I hope you know it: Are the userland apc cache functions (apc_add, apc_fetch, apc_delete etc) *thread-safe/atomic*? Also: I need to ensure that conncurrent access to shared memory doesn't get me in race conditions. Wouldn't it be nice if apc could offer mutex functions and the like? There is a semaphore extension for php, but it is *nix-only according to the docs. The need for synchronization mechanisms arise almost allways when using shared memory.