How pthread_mutex_lock is implemented

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It is both complicated and differs from Unix to Unix variant.

In Linux, for example, a system called Futex (Short for Fast Userspace Mutex) is used.

In this system an atomic increment and test operation is performed on the mutex variable in user space.

If the result of the operation indicates that there was no contention on the lock, the call to pthread_mutex_lock returns without ever context switching into the kernel, so the operation of taking a mutex can be very fast.

Only if contention was detected does a system call (called futex) and context switch into the kernel occurs that puts the calling process to sleep until the mutex is released.

There are many many more details, especially for reliable and/or priority inhertience mutexes, but this is the essence of it.

For more details see: http://linux.die.net/man/2/futex and http://en.wikipedia.org/wiki/Futex

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