Is NULL checking across ONLY 2 threads thread-safe?

Question

Given the following scenario:

• 2 Threads (no more, no less)
• A shared variable (let's say a void *)
• One thread writes to the variable ONLY once (this is guaranteed)
• And the other thread is responsible for reading the variable

Is it thread safe to have the reader thread check for null of the variable? Explicitly in a C program?

Example code:

Thread 1:

void initOnStartup()
{
   ptr = malloc(10);
}

Thread 2:

void waitingForValue()
{
  while(!ptr);
}

Answer 1

It is not atomic. Newer versions of C (and c++) provide the following. http://en.cppreference.com/w/c/atomic/atomic_store

void atomic_store( volatile A* obj , C desired); // (since C11)
void atomic_store_explicit( volatile A* obj, C desired, memory_order order ); // (since C11)

Answer 2

The answer is there: Is changing a pointer considered an atomic action in C? Unfortunately it isn't. Think of far pointers on 16 bit x86 platforms.

Answer 3

No, as the assignment = operation is not atomic.