Question

I was reading this: http://en.wikipedia.org/wiki/Thread_safety

Is the following function thread-safe?

void foo(int y){
    int * x = new int[50];
    /*...do some stuff with the allocated memory...*/
    delete [] x;
}

In the article it says that to be thread-safe you can only use variables from the stack. Really? Why? Wouldn’t subsequent calls of the above function allocate memory elsewhere?

Edit: Ah. Looks like I misread this part of the article:

A subroutine is reentrant, and thus thread-safe, if

• the only variables it uses are from the stack

(I took it to mean

A subroutine is reentrant, and thus thread-safe, if and only if

• the only variables it uses are from the stack

, which according to the answers below, is not the case)

Answer 1

If you are coding in an environment that supports multi-threading, then you can be pretty sure new is thread safe.

Although the memory is on the heap, the pointer to it is on the stack. Only your thread has the pointer to this memory, and so there is no risk of concurrent modification – no other thread knows where the memory is to modify it.

You would only get a problem with thread safety if you were to pass this pointer to another thread that would then concurrently modify this memory at the same time as your original (or another) thread.