What is the meaning of the term "thread-safe"?

Question

Does it mean that two threads can't change the underlying data simultaneously? Or does it mean that the given code segment will run with predictable results when multiple threads are executing that code segment?

Answer 1

Thread-safe code is code that will work even if many Threads are executing it simultaneously.

http://mindprod.com/jgloss/threadsafe.html

Answer 2

A more informative question is what makes code not thread safe- and the answer is that there are four conditions that must be true... Imagine the following code (and it's machine language translation)

totalRequests = totalRequests + 1 MOV EAX, [totalRequests]   // load memory for tot Requests into register INC EAX                    // update register MOV [totalRequests], EAX   // store updated value back to memory 

1. The first condition is that there are memory locations that are accessible from more than one thread. Typically, these locations are global/static variables or are heap memory reachable from global/static variables. Each thread gets it's own stack frame for function/method scoped local variables, so these local function/method variables, otoh, (which are on the stack) are accessible only from the one thread that owns that stack.
2. The second condition is that there is a property (often called an invariant), which is associated with these shared memory locations, that must be true, or valid, for the program to function correctly. In the above example, the property is that “totalRequests must accurately represent the total number of times any thread has executed any part of the increment statement”. Typically, this invariant property needs to hold true (in this case, totalRequests must hold an accurate count) before an update occurs for the update to be correct.
3. The third condition is that the invariant property does NOT hold during some part of the actual update. (It is transiently invalid or false during some portion of the processing). In this particular case, from the time totalRequests is fetched until the time the updated value is stored, totalRequests does not satisfy the invariant.
4. The fourth and final condition that must occur for a race to happen (and for the code to therefore NOT be "thread-safe") is that another thread must be able to access the shared memory while the invariant is broken, thereby causing inconsistent or incorrect behavior.