Atomic Services

An atomic variable is one that can be read and modified by threads and ISRs in an uninterruptible manner. It is a 32-bit variable on 32-bit machines and a 64-bit variable on 64-bit machines.

Concepts 

Any number of atomic variables can be defined (limited only by available RAM).

Using the kernel’s atomic APIs to manipulate an atomic variable guarantees that the desired operation occurs correctly, even if higher priority contexts also manipulate the same variable.

The kernel also supports the atomic manipulation of a single bit in an array of atomic variables.

Implementation 

Defining an Atomic Variable 

An atomic variable is defined using a variable of type atomic_t.

By default an atomic variable is initialized to zero. However, it can be given a different value using ATOMIC_INIT:

atomic_t flags = ATOMIC_INIT(0xFF);

Manipulating an Atomic Variable 

An atomic variable is manipulated using the APIs listed at the end of this section.

The following code shows how an atomic variable can be used to keep track of the number of times a function has been invoked. Since the count is incremented atomically, there is no risk that it will become corrupted in mid-increment if a thread calling the function is interrupted if by a higher priority context that also calls the routine.

atomic_t call_count;

int call_counting_routine(void)
{
    /* increment invocation counter */
    atomic_inc(&call_count);

    /* do rest of routine's processing */
    ...
}

Manipulating an Array of Atomic Variables 

An array of 32-bit atomic variables can be defined in the conventional manner. However, you can also define an N-bit array of atomic variables using ATOMIC_DEFINE.

A single bit in array of atomic variables can be manipulated using the APIs listed at the end of this section that end with _bit().

The following code shows how a set of 200 flag bits can be implemented using an array of atomic variables.

#define NUM_FLAG_BITS 200

ATOMIC_DEFINE(flag_bits, NUM_FLAG_BITS);

/* set specified flag bit & return its previous value */
int set_flag_bit(int bit_position)
{
    return (int)atomic_set_bit(flag_bits, bit_position);
}

Memory Ordering 

For consistency and correctness, all Zephyr atomic APIs are expected to include a full memory barrier (in the sense of e.g. “serializing” instructions on x86, “DMB” on ARM, or a “sequentially consistent” operation as defined by the C++ memory model) where needed by hardware to guarantee a reliable picture across contexts. Any architecture-specific implementations are responsible for ensuring this behavior.

Suggested Uses 

Use an atomic variable to implement critical section processing that only requires the manipulation of a single 32-bit value.

Use multiple atomic variables to implement critical section processing on a set of flag bits in a bit array longer than 32 bits.

Note

Using atomic variables is typically far more efficient than using other techniques to implement critical sections such as using a mutex or locking interrupts.

Configuration Options 

Related configuration options:

API Reference 

Important

All atomic services APIs can be used by both threads and ISRs.

Atomic Services APIs