ExperimentalStableMemory

Byte-level access to (virtual) stable memory.

[Experimental module]

As the name suggests, this library is experimental, subject to change, and may be replaced by safer alternatives in later versions of Motoko. Use at your own risk and discretion.

[Deprecation notice]

Use of ExperimentalStableMemory may be deprecated in the future. Consider using Region.mo for isolated memory regions. Isolated regions ensure that writing to one region does not affect unrelated state elsewhere.

This is a lightweight abstraction over IC stable memory and supports persisting raw binary data across Motoko upgrades. It is fully compatible with Motoko's stable variables, which also use IC stable memory internally, but do not interfere with this API.

Memory is allocated using grow(pages), sequentially and on demand, in units of 64KiB pages, starting with 0 allocated pages. New pages are zero-initialised. Growth is capped by a soft page limit set with the compile-time flag --max-stable-pages <n> (default: 65536, or 4GiB).

Each load reads from byte address offset in little-endian format using the natural bit-width of the type. Traps if reading beyond the allocated size.

Each store writes to byte address offset in little-endian format using the natural bit-width of the type. Traps if writing beyond the allocated size.

Text can be handled using Text.decodeUtf8 and Text.encodeUtf8 in combination with loadBlob and storeBlob.

The current page allocation and contents are preserved across upgrades.

[IC stable memory discrepancy]

The IC’s reported stable memory size (ic0.stable_size) may exceed what Motoko’s size() returns. This and the growth cap exist to protect Motoko’s internal use of stable variables. If you're not using stable variables (or using them sparingly), you may increase --max-stable-pages toward the IC maximum (currently 64GiB). Even if not using stable variables, always reserve at least one page.

Usage:

import StableMemory "mo:base/ExperimentalStableMemory";

Function size

func size() : (pages : Nat64)

Current size of the stable memory, in pages. Each page is 64KiB (65536 bytes). Initially 0. Preserved across upgrades, together with contents of allocated stable memory.

Example:

let beforeSize = StableMemory.size();
ignore StableMemory.grow(10);
let afterSize = StableMemory.size();
afterSize - beforeSize // => 10

Function grow

func grow(newPages : Nat64) : (oldPages : Nat64)

Grow current size of stable memory by the given number of pages. Each page is 64KiB (65536 bytes). Returns the previous size when able to grow. Returns 0xFFFF_FFFF_FFFF_FFFF if remaining pages insufficient. Every new page is zero-initialized, containing byte 0x00 at every offset. Function grow is capped by a soft limit on size controlled by compile-time flag --max-stable-pages <n> (the default is 65536, or 4GiB).

Example:

import Error "mo:base/Error";

let beforeSize = StableMemory.grow(10);
if (beforeSize == 0xFFFF_FFFF_FFFF_FFFF) {
  throw Error.reject("Out of memory");
};
let afterSize = StableMemory.size();
afterSize - beforeSize // => 10

Function stableVarQuery

func stableVarQuery() : (shared query () -> async { size : Nat64 })

Returns a query that, when called, returns the number of bytes of (real) IC stable memory that would be occupied by persisting its current stable variables before an upgrade. This function may be used to monitor or limit real stable memory usage. The query computes the estimate by running the first half of an upgrade, including any preupgrade system method. Like any other query, its state changes are discarded so no actual upgrade (or other state change) takes place. The query can only be called by the enclosing actor and will trap for other callers.

Example:

actor {
  stable var state = "";
  public func example() : async Text {
    let memoryUsage = StableMemory.stableVarQuery();
    let beforeSize = (await memoryUsage()).size;
    state #= "abcdefghijklmnopqrstuvwxyz";
    let afterSize = (await memoryUsage()).size;
    debug_show (afterSize - beforeSize)
  };
};

Function loadNat32

func loadNat32(offset : Nat64) : Nat32

Loads a Nat32 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat32(offset, value);
StableMemory.loadNat32(offset) // => 123

Function storeNat32

func storeNat32(offset : Nat64, value : Nat32) : ()

Stores a Nat32 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat32(offset, value);
StableMemory.loadNat32(offset) // => 123

Function loadNat8

func loadNat8(offset : Nat64) : Nat8

Loads a Nat8 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat8(offset, value);
StableMemory.loadNat8(offset) // => 123

Function storeNat8

func storeNat8(offset : Nat64, value : Nat8) : ()

Stores a Nat8 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat8(offset, value);
StableMemory.loadNat8(offset) // => 123

Function loadNat16

func loadNat16(offset : Nat64) : Nat16

Loads a Nat16 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat16(offset, value);
StableMemory.loadNat16(offset) // => 123

Function storeNat16

func storeNat16(offset : Nat64, value : Nat16) : ()

Stores a Nat16 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat16(offset, value);
StableMemory.loadNat16(offset) // => 123

Function loadNat64

func loadNat64(offset : Nat64) : Nat64

Loads a Nat64 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat64(offset, value);
StableMemory.loadNat64(offset) // => 123

Function storeNat64

func storeNat64(offset : Nat64, value : Nat64) : ()

Stores a Nat64 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat64(offset, value);
StableMemory.loadNat64(offset) // => 123

Function loadInt32

func loadInt32(offset : Nat64) : Int32

Loads an Int32 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt32(offset, value);
StableMemory.loadInt32(offset) // => 123

Function storeInt32

func storeInt32(offset : Nat64, value : Int32) : ()

Stores an Int32 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt32(offset, value);
StableMemory.loadInt32(offset) // => 123

Function loadInt8

func loadInt8(offset : Nat64) : Int8

Loads an Int8 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt8(offset, value);
StableMemory.loadInt8(offset) // => 123

Function storeInt8

func storeInt8(offset : Nat64, value : Int8) : ()

Stores an Int8 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt8(offset, value);
StableMemory.loadInt8(offset) // => 123

Function loadInt16

func loadInt16(offset : Nat64) : Int16

Loads an Int16 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt16(offset, value);
StableMemory.loadInt16(offset) // => 123

Function storeInt16

func storeInt16(offset : Nat64, value : Int16) : ()

Stores an Int16 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt16(offset, value);
StableMemory.loadInt16(offset) // => 123

Function loadInt64

func loadInt64(offset : Nat64) : Int64

Loads an Int64 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt64(offset, value);
StableMemory.loadInt64(offset) // => 123

Function storeInt64

func storeInt64(offset : Nat64, value : Int64) : ()

Stores an Int64 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt64(offset, value);
StableMemory.loadInt64(offset) // => 123

Function loadFloat

func loadFloat(offset : Nat64) : Float

Loads a Float value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 1.25;
StableMemory.storeFloat(offset, value);
StableMemory.loadFloat(offset) // => 1.25

Function storeFloat

func storeFloat(offset : Nat64, value : Float) : ()

Stores a Float value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 1.25;
StableMemory.storeFloat(offset, value);
StableMemory.loadFloat(offset) // => 1.25

Function loadBlob

func loadBlob(offset : Nat64, size : Nat) : Blob

Load size bytes starting from offset as a Blob. Traps on an out-of-bounds access.

Example:

import Blob "mo:base/Blob";

let offset = 0;
let value = Blob.fromArray([1, 2, 3]);
let size = value.size();
StableMemory.storeBlob(offset, value);
Blob.toArray(StableMemory.loadBlob(offset, size)) // => [1, 2, 3]

Function storeBlob

func storeBlob(offset : Nat64, value : Blob) : ()

Write bytes of blob beginning at offset. Traps on an out-of-bounds access.

Example:

import Blob "mo:base/Blob";

let offset = 0;
let value = Blob.fromArray([1, 2, 3]);
let size = value.size();
StableMemory.storeBlob(offset, value);
Blob.toArray(StableMemory.loadBlob(offset, size)) // => [1, 2, 3]