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This primer illustrates Chapel's atomic variables. For more information on Chapel's atomics, see the Chapel Language Specification.

The atomic keyword is a type qualifiers that can be applied to the following Chapel primitive types to declare atomic variables:

  • bool
  • int (all supported sizes)
  • uint (all supported sizes)
  • real (all supported sizes)

Atomic variables support a limited number of operations that are currently implemented as methods.

Atomic variables can only be used or assigned via the read() and write() methods, respectively.

config const n = 31;
const R = 1..n;

var x: atomic int;

if != n then
  halt("Error: x (",, ") != n (", n, ")");

All atomic types support atomic exchange(), compareExchangeStrong(), compareExchangeWeak(), and compareExchange() (same as compareExchangeStrong()).

The exchange() method atomically swaps the old value with the given argument and returns the old value. The compareExchange(), compareExchangeStrong() and compareExchangeWeak() methods only perform the swap if the current value is equal to the first argument, returning true if the exchange was performed See the Chapel Language Specification for the difference between the weak and strong varieties.

In the following example, n parallel tasks are created via a coforall statement. Each task tries to set the current value of x to id-1, but only one will succeed.

var numFound: atomic int;
coforall id in R {
  numFound.add(x.compareExchangeWeak(n, id-1));
if != 1 then
  halt("Error: numFound != 1 (",, ")");
if == n then
  halt("Error: x == n");

As a convenience, atomic bools also support testAndSet() and clear(). The testAndSet() method atomically reads the value of the variable, sets it to true, then returns the original value. The clear() method atomically sets the value to false.

In the following example, we create n tasks and each task calls testAndSet() on the atomic variable named flag and saves the result in a unique element of the result array.

var flag: atomic bool;
var result: [R] bool;
coforall r in result do
  r = flag.testAndSet();

When the coforall is complete, only one of the above testAndSet() calls should have returned false.

var found = false;
for r in result {
  if !r then
    if found then
      halt("Error: found multiple times!");
      found = true;
if !found then
  halt("Error: not found!");

The integral and real atomic types also support the following atomic fetch and non-fetching operations:

  • fetchAdd() and add()
  • fetchSub() and sub()
  • fetchOr() and or() (bit-wise) (integral types only)
  • fetchAnd() and and() (bit-wise) (integral types only)
  • fetchXor() and xor() (bit-wise) (integral types only)

Each of the above atomically reads the variable, stores the result of the operation (+, -, |, &, ^) using the value and the method argument, then, for the fetchOps functions, returns the original value.

In the following example, we create n tasks to atomically increment the atomic variable a with the square of the task's given id.

var a: atomic int;
coforall id in R do a.add(id*id);

The sum of this finite series should be n(n+1)*(2n+1)/6

var expected = n*(n+1)*(2*n+1)/6;
if != expected then
  halt("Error: a=",, " (should be ", expected, ")");

In the following example, we create n tasks to atomically increment the atomic variable a with the square of the task's given id. The sum of this finite series is n(n+1)*(2n+1)/6. If the returned value is the expected value less id*id (last task to arrive), check the results.

const sumOfSq = n*(n+1)*(2*n+1)/6;
coforall id in R {
  const mySq = id*id;
  const last = a.fetchAdd(mySq);
  if sumOfSq-mySq == last {
    const t =;
    if t != n*(n+1)*(2*n+1)/6 then
      halt("Error: a=", t, " (should be ", sumOfSq, ") id=", id);