Erlang Run-Time System Application (ERTS)

Reference Manual

Version 10.3

Table of Contents




Module Summary

Atomic Functions


Module atomics was introduced in OTP 21.2.


This module provides a set of functions to do atomic operations towards mutable atomic variables. The implementation utilizes only atomic hardware instructions without any software level locking, which makes it very efficient for concurrent access. The atomics are organized into arrays with the following semantics:

  • Atomics are 64 bit integers.

  • Atomics can be represented as either signed or unsigned.

  • Atomics wrap around at overflow and underflow operations.

  • All operations guarantee atomicity. No intermediate results can be seen. The result of one mutation can only be the input to one following mutation.

  • All atomic operations are mutually ordered. If atomic B is updated after atomic A, then that is how it will appear to any concurrent readers. No one can read the new value of B and then read the old value of A.

  • Indexes into atomic arrays are one-based. An atomic array of arity N contains N atomics with index from 1 to N.

Data Types

Identifies an atomic array returned from new/2.


new(Arity, Opts) -> atomics_ref()
OTP 21.2


Arity = integer() >= 1
Opts = [Opt]
Opt = {signed, boolean()}

Create a new atomic array of Arity atomics.

Argument Opts is a list of the following possible options:

{signed, boolean()}

Indicate if the elements of the array will be treated as signed or unsigned integers. Default is true (signed).

The integer interval for signed atomics are from -(1 bsl 63) to (1 bsl 63)-1 and for unsigned atomics from 0 to (1 bsl 64)-1.

Atomics are not tied to the current process and are automatically garbage collected when they are no longer referenced.

put(Ref, Ix, Value) -> ok
OTP 21.2


Ix = Value = integer()

Set atomic to Value.

get(Ref, Ix) -> integer()
OTP 21.2


Ix = integer()

Read atomic value.

add(Ref, Ix, Incr) -> ok
OTP 21.2


Ix = Incr = integer()

Add Incr to atomic.

add_get(Ref, Ix, Incr) -> integer()
OTP 21.2


Ix = Incr = integer()

Atomic addition and return of the result.

sub(Ref, Ix, Decr) -> ok
OTP 21.2


Ix = Decr = integer()

Subtract Decr from atomic.

sub_get(Ref, Ix, Decr) -> integer()
OTP 21.2


Ix = Decr = integer()

Atomic subtraction and return of the result.

exchange(Ref, Ix, Desired) -> integer()
OTP 21.2


Ix = Desired = integer()

Atomically replaces the value of the atomic with Desired and returns the value it held previously.

compare_exchange(Ref, Ix, Expected, Desired) -> ok | integer()
OTP 21.2


Ix = Expected = Desired = integer()

Atomically compares the atomic with Expected, and if those are equal, set atomic to Desired. Returns ok if Desired was written. Returns the actual atomic value if not equal to Expected.

info(Ref) -> Info
OTP 21.2


Info =
    #{size := Size, max := Max, min := Min, memory := Memory}
Size = integer() >= 0
Max = Min = integer()
Memory = integer() >= 0

Return information about an atomic array in a map. The map has the following keys:


The number of atomics in the array.


The highest possible value an atomic in this array can hold.


The lowest possible value an atomic in this array can hold.


Approximate memory consumption for the array in bytes.