Mastering C++ Multithreading

Memory order

This property is defined as an enumeration in the <atomic> header:

enum memory_order { 
    memory_order_relaxed, 
    memory_order_consume, 
    memory_order_acquire, 
    memory_order_release, 
    memory_order_acq_rel, 
    memory_order_seq_cst 
}; 

In the GCC section, we already touched briefly on the topic of memory order. As mentioned there, this is one of the parts where the characteristics of the underlying hardware architecture surface somewhat.

Basically, memory order determines how non-atomic memory accesses are to be ordered (memory access order) around an atomic operation. What this affects is how different threads will see the data in memory as they're executing their instructions:

Enum	Description
memory_order_relaxed	Relaxed operation: ...