Atomic Services

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

Concepts

Any number of atomic variables can be defined.

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);
}

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.

API Reference

Important

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

group atomic_apis

Defines

ATOMIC_INIT(i)

Initialize an atomic variable.

This macro can be used to initialize an atomic variable. For example,

atomic_t my_var = ATOMIC_INIT(75); 

Parameters
  • i: Value to assign to atomic variable.

ATOMIC_DEFINE(name, num_bits)

Define an array of atomic variables.

This macro defines an array of atomic variables containing at least num_bits bits.

Note
If used from file scope, the bits of the array are initialized to zero; if used from within a function, the bits are left uninitialized.
Parameters
  • name: Name of array of atomic variables.
  • num_bits: Number of bits needed.

Functions

int atomic_cas(atomic_t *target, atomic_val_t old_value, atomic_val_t new_value)

Atomic compare-and-set.

This routine performs an atomic compare-and-set on target. If the current value of target equals old_value, target is set to new_value. If the current value of target does not equal old_value, target is left unchanged.

Return
true if new_value is written, false otherwise.
Parameters
  • target: Address of atomic variable.
  • old_value: Original value to compare against.
  • new_value: New value to store.

atomic_val_t atomic_add(atomic_t *target, atomic_val_t value)

Atomic addition.

This routine performs an atomic addition on target.

Return
Previous value of target.
Parameters
  • target: Address of atomic variable.
  • value: Value to add.

atomic_val_t atomic_sub(atomic_t *target, atomic_val_t value)

Atomic subtraction.

This routine performs an atomic subtraction on target.

Return
Previous value of target.
Parameters
  • target: Address of atomic variable.
  • value: Value to subtract.

atomic_val_t atomic_inc(atomic_t *target)

Atomic increment.

This routine performs an atomic increment by 1 on target.

Return
Previous value of target.
Parameters
  • target: Address of atomic variable.

atomic_val_t atomic_dec(atomic_t *target)

Atomic decrement.

This routine performs an atomic decrement by 1 on target.

Return
Previous value of target.
Parameters
  • target: Address of atomic variable.

atomic_val_t atomic_get(const atomic_t *target)

Atomic get.

This routine performs an atomic read on target.

Return
Value of target.
Parameters
  • target: Address of atomic variable.

atomic_val_t atomic_set(atomic_t *target, atomic_val_t value)

Atomic get-and-set.

This routine atomically sets target to value and returns the previous value of target.

Return
Previous value of target.
Parameters
  • target: Address of atomic variable.
  • value: Value to write to target.

atomic_val_t atomic_clear(atomic_t *target)

Atomic clear.

This routine atomically sets target to zero and returns its previous value. (Hence, it is equivalent to atomic_set(target, 0).)

Return
Previous value of target.
Parameters
  • target: Address of atomic variable.

atomic_val_t atomic_or(atomic_t *target, atomic_val_t value)

Atomic bitwise inclusive OR.

This routine atomically sets target to the bitwise inclusive OR of target and value.

Return
Previous value of target.
Parameters
  • target: Address of atomic variable.
  • value: Value to OR.

atomic_val_t atomic_xor(atomic_t *target, atomic_val_t value)

Atomic bitwise exclusive OR (XOR).

This routine atomically sets target to the bitwise exclusive OR (XOR) of target and value.

Return
Previous value of target.
Parameters
  • target: Address of atomic variable.
  • value: Value to XOR

atomic_val_t atomic_and(atomic_t *target, atomic_val_t value)

Atomic bitwise AND.

This routine atomically sets target to the bitwise AND of target and value.

Return
Previous value of target.
Parameters
  • target: Address of atomic variable.
  • value: Value to AND.

atomic_val_t atomic_nand(atomic_t *target, atomic_val_t value)

Atomic bitwise NAND.

This routine atomically sets target to the bitwise NAND of target and value. (This operation is equivalent to target = ~(target & value).)

Return
Previous value of target.
Parameters
  • target: Address of atomic variable.
  • value: Value to NAND.

static bool atomic_test_bit(const atomic_t *target, int bit)

Atomically test a bit.

This routine tests whether bit number bit of target is set or not. The target may be a single atomic variable or an array of them.

Return
true if the bit was set, false if it wasn’t.
Parameters
  • target: Address of atomic variable or array.
  • bit: Bit number (starting from 0).

static bool atomic_test_and_clear_bit(atomic_t *target, int bit)

Atomically test and clear a bit.

Atomically clear bit number bit of target and return its old value. The target may be a single atomic variable or an array of them.

Return
true if the bit was set, false if it wasn’t.
Parameters
  • target: Address of atomic variable or array.
  • bit: Bit number (starting from 0).

static bool atomic_test_and_set_bit(atomic_t *target, int bit)

Atomically set a bit.

Atomically set bit number bit of target and return its old value. The target may be a single atomic variable or an array of them.

Return
true if the bit was set, false if it wasn’t.
Parameters
  • target: Address of atomic variable or array.
  • bit: Bit number (starting from 0).

static void atomic_clear_bit(atomic_t *target, int bit)

Atomically clear a bit.

Atomically clear bit number bit of target. The target may be a single atomic variable or an array of them.

Return
N/A
Parameters
  • target: Address of atomic variable or array.
  • bit: Bit number (starting from 0).

static void atomic_set_bit(atomic_t *target, int bit)

Atomically set a bit.

Atomically set bit number bit of target. The target may be a single atomic variable or an array of them.

Return
N/A
Parameters
  • target: Address of atomic variable or array.
  • bit: Bit number (starting from 0).

static void atomic_set_bit_to(atomic_t *target, int bit, bool val)

Atomically set a bit to a given value.

Atomically set bit number bit of target to value val. The target may be a single atomic variable or an array of them.

Return
N/A
Parameters
  • target: Address of atomic variable or array.
  • bit: Bit number (starting from 0).
  • val: true for 1, false for 0.