System Calls
Writing smart contracts requires various associated operations, such as calling another contract or accessing the contract’s storage, that standalone programs do not require. The Starknet contract language supports these operations by using system calls. System calls enable a contract to require services from the Starknet OS. You can use system calls in a function to get information that depends on the broader state of Starknet, which would otherwise be inaccessible, rather than local variables that appear in the function’s scope.
get_block_hash
Function signature
extern fn get_block_hash_syscall(
block_number: u64
) -> SyscallResult<felt252> implicits(GasBuiltin, System) nopanic;
Description
Gets the hash of a specific Starknet block within the range of [first_v0_12_0_block, current_block - 10]
.
Arguments
block_number: u64
|
The number of the block whose hash you want to get. |
Return values
The hash of the specified block.
Common library
Error messages
Block number out of range
|
|
0
|
|
get_execution_info
Function signature
extern fn get_execution_info_syscall() -> SyscallResult<Box<starknet::info::ExecutionInfo>> implicits(
GasBuiltin, System
) nopanic;
Description
Gets information about the currently executing block and the transactions in the block. For a complete description of this information, see Execution information
This single system call contains all information for a block, transaction, and execution context.
When an account’s __validate__
, __validate_deploy__
, or __validate_declare__
function calls get_execution_info
, the return values for block_timestamp
and block_number
are modified as follows:
-
block_timestamp
returns the hour, rounded down to the nearest hour. -
block_number
returns the block number, rounded down to the nearest multiple of 100.
Arguments
None.
Return values
ExecutionInfo
|
A struct that contains information about the currently executing function, transaction, and block. |
Common library
Example
This example shows how to pull the block number from the ExecutionInfo
struct.
let execution_info = get_execution_info().unbox();
let block_info = execution_info.block_info.unbox();
let block number = block_info.block_number;
call_contract
Function signature
extern fn call_contract_syscall(
address: ContractAddress, entry_point_selector: felt252, calldata: Span<felt252>
) -> SyscallResult<Span<felt252>> implicits(GasBuiltin, System) nopanic;
Description
Calls a given contract. This system call expects the address of the called contract, a selector for a function within that contract, and call arguments.
An internal call can’t return If |
Arguments
address: ContractAddress
|
The address of the contract you want to call. |
entry_point_selector: felt252
|
A selector for a function within that contract. |
calldata: Span<felt252>
|
The calldata array. |
Return values
-
The call response, of type
SyscallResult<Span<felt252>>
.
Common library
This is considered a lower-level syntax for calling contracts. If the interface of the called contract is available, then you can use a more straightforward syntax. |
deploy
Function signature
extern fn deploy_syscall(
class_hash: ClassHash,
contract_address_salt: felt252,
calldata: Span<felt252>,
deploy_from_zero: bool,
) -> SyscallResult<(ContractAddress, Span::<felt252>)> implicits(GasBuiltin, System) nopanic;
Description
Deploys a new instance of a previously declared class.
Arguments
class_hash: ClassHash
|
The class hash of the contract to be deployed. |
contract_address_salt: felt252
|
The salt, an arbitrary value provided by the sender, used in the computation of the contract’s address. |
calldata: Span<felt252>
|
The constructor’s calldata. An array of felts. |
deploy_from_zero: bool
|
A flag that determines whether the deployer’s address affects the computation of the contract address. When not set, or when set to |
Return values
-
A tuple wrapped with
SyscallResult
where:-
The first element is the address of the deployed contract, of type
ContractAddress
. -
The second element is the response array from the contract’s constructor, of type
Span::<felt252>
.
-
Common library
emit_event
Function signature
extern fn emit_event_syscall(
keys: Span<felt252>, data: Span<felt252>
) -> SyscallResult<()> implicits(GasBuiltin, System) nopanic;
Description
Emits an event with a given set of keys and data.
For more information, and for a higher-level syntax for emitting events, see Starknet events.
Arguments
keys: Span<felt252>
|
The event’s keys. These are analogous to Ethereum’s event topics, you can use the starknet_getEvents method to filter by these keys. |
data: Span<felt252>
|
The event’s data. |
Return values
None.
Common library
Example
The following example emits an event with two keys, the strings key
and deposit
and three data elements: 1
, 2
, and 3
.
let keys = array!['key', 'deposit'];
let values = array![1, 2, 3];
emit_event_syscall(keys, values).unwrap_syscall();
library_call
Function signature
extern fn library_call_syscall(
class_hash: ClassHash, function_selector: felt252, calldata: Span<felt252>
) -> SyscallResult<Span<felt252>> implicits(GasBuiltin, System) nopanic;
Description
Calls the requested function in any previously declared class. The class is only used for its logic.
This system call replaces the known delegate call functionality from Ethereum, with the important difference that there is only one contract involved.
Arguments
class_hash: ClassHash
|
The hash of the class you want to use. |
function_selector: felt252
|
A selector for a function within that class. |
calldata: Span<felt252>
|
The calldata. |
Return values
-
The call response, of type
SyscallResult<Span<felt252>>
.
Common library
send_message_to_L1
Function signature
extern fn send_message_to_l1_syscall(
to_address: felt252, payload: Span<felt252>
) -> SyscallResult<()> implicits(GasBuiltin, System) nopanic;
Description
Sends a message to L1.
This system call includes the message parameters as part of the proof’s output and exposes these parameters to the Starknet Core Contract on L1 once the state update, including the transaction, is received.
For more information, see Starknet’s messaging mechanism.
Arguments
to_address: felt252
|
The recipient’s L1 address. |
payload: Span<felt252>
|
The array containing the message payload |
Return values
None.
Common library
Example
The following example sends a message whose content is (1,2)
to the L1 contract whose address is 3423542542364363
.
let payload = ArrayTrait::new();
payload.append(1);
payload.append(2);
send_message_to_l1_syscall(payload).unwrap_syscall();
replace_class
Function signature
extern fn replace_class_syscall(
class_hash: ClassHash
) -> SyscallResult<()> implicits(GasBuiltin, System) nopanic;
Description
Once replace_class
is called, the class of the calling contract (i.e. the contract whose address is returned by get_contract_address
at the time the syscall is called) will be replaced
by the class whose hash is given by the class_hash argument.
After calling The new class will be used from the next transaction onwards or if the contract is called via
the |
Arguments
class_hash_: ClassHash
|
The hash of the class you want to use as a replacement. |
Return values
None.
Common library
storage_read
Function signature
extern fn storage_read_syscall(
address_domain: u32, address: StorageAddress
) -> SyscallResult<felt252> implicits(GasBuiltin, System) nopanic;
Description
Gets the value of a key in the storage of the calling contract.
This system call provides direct access to any possible key in storage, in contrast with var.read()
, which enables you to read storage variables that are defined explicitly in the contract.
For information on accessing storage by using the storage variables, see storage variables.
Arguments
address_domain: u32
|
The domain of the key, used to separate between different data availability modes. This separation is used in Starknet to offer different data availability modes. Currently, only the onchain mode (where all updates go to L1), indicated by domain |
address: StorageAddress
|
The requested storage address. |
Return values
-
The value of the key, of type
SyscallResult<felt252>
.
Common library
Example
use starknet::storage_access::storage_base_address_from_felt252;
...
let storage_address = storage_base_address_from_felt252(3534535754756246375475423547453)
storage_read_syscall(0, storage_address).unwrap_syscall()
storage_write
Function signature
extern fn storage_write_syscall(
address_domain: u32, address: StorageAddress, value: felt252
) -> SyscallResult<()> implicits(GasBuiltin, System) nopanic;
Description
Sets the value of a key in the storage of the calling contract.
This system call provides direct access to any possible key in storage, in contrast with var.write()
, which enables you to write to storage variables that are defined explicitly in the contract.
For information on accessing storage by using the storage variables, see storage variables.
Arguments
address_domain: u32
|
The domain of the key, used to separate between different data availability modes. This separation is used in Starknet to offer different data availability modes. Currently, only the onchain mode (where all updates go to L1), indicated by domain |
address: StorageAddress
|
The requested storage address. |
value: felt252
|
The value to write to the key. |
Return values
None.
keccak
Function signature
extern fn keccak_syscall(
data: Span<felt252>
) -> SyscallResult<u256> implicits(GasBuiltin, System) nopanic;
Description
Computes the Keccak-256 hash of the input data.
This system call is particularly useful when interacting with Ethereum contracts or implementing Ethereum-compatible functionality, as Keccak-256 is widely used in the Ethereum ecosystem.
Instead of using this syscall directly, it is recommended to use the functions provided in |
Arguments
data: Span<felt252>
|
The input data to be hashed, represented as an array of field elements. |
Return values
-
The Keccak-256 hash of the input data as a
u256
value.
Common library
Example
The following example demonstrates how to compute a Keccak-256 hash using the recommended functions from keccak.cairo
:
use array::ArrayTrait;
use keccak::keccak_u256s_le_inputs;
// Create input data
let mut data = ArrayTrait::new();
data.append(1_u256);
data.append(2_u256);
// Compute Keccak hash using the recommended function
let hash = keccak_u256s_le_inputs(data.span());
secp256r1
Various systems calls for computations over the secp256r1 curve (also known as NIST P-256).
These system calls are particularly useful when implementing traditional PKI or secure communication functionality, as secp256r1 is widely used in TLS, secure messaging, and government standards.
Instead of using these syscalls directly, it is recommended to use the traits and implementations of the corelib’s secp256r1 module which provides a more convenient interface and handles them for you.
|
secp256r1_new
Function signature
extern fn secp256r1_new_syscall(
x: u256, y: u256
) -> SyscallResult<Option<Secp256r1Point>> implicits(GasBuiltin, System) nopanic;
Return values
Returns Some(point)
if the coordinates represent a valid point on the curve, None
otherwise.
Example
The following example demonstrates how to create a new point on the secp256r1 curve:
use starknet::secp256r1::Secp256r1Point;
// Known valid point coordinates on secp256r1 curve (generator point)
let x = 0x6b17d1f2e12c4247f8bce6e563a440f277037d812deb33a0f4a13945d898c296_u256;
let y = 0x4fe342e2fe1a7f9b8ee7eb4a7c0f9e162bce33576b315ececbb6406837bf51f5_u256;
// Create a new point using the recommended function
let point = secp256r1_new_syscall(x, y).unwrap_syscall();
match point {
Option::Some(p) => {
// Point is valid and can be used for further operations
// For example, you could get its coordinates back:
let (px, py) = secp256r1_get_xy_syscall(p).unwrap_syscall();
},
Option::None => {
// Point was not on the curve
}
}
secp256r1_add
Function signature
extern fn secp256r1_add_syscall(
p0: Secp256r1Point, p1: Secp256r1Point,
) -> SyscallResult<Secp256r1Point> implicits(GasBuiltin, System) nopanic;
Arguments
p0: Secp256r1Point
|
The first point on the curve. |
p1: Secp256r1Point
|
The second point on the curve. |
Example
The following example demonstrates how to add two points on the secp256r1 curve:
use starknet::secp256r1::Secp256r1Point;
// Create two points to add (using generator point)
let x = 0x6b17d1f2e12c4247f8bce6e563a440f277037d812deb33a0f4a13945d898c296_u256;
let y = 0x4fe342e2fe1a7f9b8ee7eb4a7c0f9e162bce33576b315ececbb6406837bf51f5_u256;
// Create first point
let point1 = secp256r1_new_syscall(x, y).unwrap_syscall().expect('Invalid point1');
// Create second point (using same point for example)
let point2 = secp256r1_new_syscall(x, y).unwrap_syscall().expect('Invalid point2');
// Add the points
let sum = secp256r1_add_syscall(point1, point2).unwrap_syscall();
// Get the resulting coordinates
let (sum_x, sum_y) = secp256r1_get_xy_syscall(sum).unwrap_syscall();
secp256r1_mul
Function signature
extern fn secp256r1_mul_syscall(
p: Secp256r1Point, scalar: u256,
) -> SyscallResult<Secp256r1Point> implicits(GasBuiltin, System) nopanic;
Arguments
p: Secp256r1Point
|
The point to be multiplied. |
scalar: u256
|
The scalar value to multiply the point by. |
Example
The following example demonstrates how to multiply a point by a scalar on the secp256r1 curve:
use starknet::secp256r1::Secp256r1Point;
// Create a point to multiply (using generator point)
let x = 0x6b17d1f2e12c4247f8bce6e563a440f277037d812deb33a0f4a13945d898c296_u256;
let y = 0x4fe342e2fe1a7f9b8ee7eb4a7c0f9e162bce33576b315ececbb6406837bf51f5_u256;
// Create the point
let point = secp256r1_new_syscall(x, y).unwrap_syscall().expect('Invalid point');
// Multiply point by scalar
let scalar = 0x2_u256; // Example scalar value of 2
let product = secp256r1_mul_syscall(point, scalar).unwrap_syscall();
// Get the resulting coordinates
let (product_x, product_y) = secp256r1_get_xy_syscall(product).unwrap_syscall();
secp256r1_get_point_from_x
Function signature
extern fn secp256r1_get_point_from_x_syscall(
x: u256, y_parity: bool,
) -> SyscallResult<Option<Secp256r1Point>> implicits(GasBuiltin, System) nopanic;
Description
Recovers a point on the curve given its x-coordinate and y-parity. Since the secp256r1 curve has an even and an odd solution for y given x, the y_parity parameter determines which y value to use.
Arguments
x: u256
|
The x-coordinate of the point. |
y_parity: bool
|
If true, choose the odd y value; if false, choose the even y value. |
Example
The following example demonstrates how to recover a point from its x-coordinate and y-parity:
use starknet::secp256r1::Secp256r1Point;
// Known x-coordinate of a valid point (generator point x-coordinate)
let x = 0x6b17d1f2e12c4247f8bce6e563a440f277037d812deb33a0f4a13945d898c296_u256;
let y_is_odd = false;
// Recover the point
let point = secp256r1_get_point_from_x_syscall(x, y_is_odd).unwrap_syscall();
match point {
Option::Some(p) => {
// Point was successfully recovered
let (recovered_x, recovered_y) = secp256r1_get_xy_syscall(p).unwrap_syscall();
assert(recovered_x == x, 'x coordinate mismatch');
},
Option::None => {
// No point exists with this x-coordinate
panic!('Point recovery failed')
}
}
secp256r1_get_xy
Function signature
extern fn secp256r1_get_xy_syscall(
p: Secp256r1Point,
) -> SyscallResult<(u256, u256)> implicits(GasBuiltin, System) nopanic;
Example
The following example demonstrates how to get the coordinates of a point on the secp256r1 curve:
use starknet::secp256r1::Secp256r1Point;
// Create a point to extract coordinates from (using generator point)
let x = 0x6b17d1f2e12c4247f8bce6e563a440f277037d812deb33a0f4a13945d898c296_u256;
let y = 0x4fe342e2fe1a7f9b8ee7eb4a7c0f9e162bce33576b315ececbb6406837bf51f5_u256;
// Create the point
let point = secp256r1_new_syscall(x, y).unwrap_syscall().expect('Invalid point');
// Get the coordinates
let (point_x, point_y) = secp256r1_get_xy_syscall(point).unwrap_syscall();
// Verify the coordinates match the original values
assert(point_x == x, 'x coordinate mismatch');
assert(point_y == y, 'y coordinate mismatch');