What's the difference between an exclusive lock and a shared lock?

Question

According to wikipedia,

Shared locks are sometimes called "read locks" and exclusive locks are sometimes called "write locks".

Can you explain the reasoning behind the terms "shared" and "exclusive"?

Answer 1

I wrote this answer down because I thought this would be a fun (and fitting) analogy:

Think of a lockable object as a blackboard (lockable) in a class room containing a teacher (writer) and many students (readers).

While a teacher is writing something (exclusive lock) on the board:

1. Nobody can read it, because it's still being written, and she's blocking your view => If an object is exclusively locked, shared locks cannot be obtained.

2. Other teachers won't come up and start writing either, or the board becomes unreadable, and confuses students => If an object is exclusively locked, other exclusive locks cannot be obtained.

When the students are reading (shared locks) what is on the board:

1. They all can read what is on it, together => Multiple shared locks can co-exist.

2. The teacher waits for them to finish reading before she clears the board to write more => If one or more shared locks already exist, exclusive locks cannot be obtained.