Different OS, Different Support

You have thus far seen one type of support that an OS can provide in order to build a more efficient lock in a thread library. Other OS’s provide similar support; the details vary.

Futex

For example, Linux provides a futex which is similar to the Solaris interface but provides more in-kernel functionality. Specifically, each futex has associated with it a specific physical memory location, as well as a per-futex in-kernel queue. Callers can use futex calls (described below) to sleep and wake as need be.

Specifically, two calls are available. The call to futex_wait(address, expected) puts the calling thread to sleep, assuming the value at address is equal to expected. If it is not equal, the call returns immediately. The call to the routine futex_wake(address) wakes one thread that is waiting on the queue. The usage of these calls in a Linux mutex is shown below.