System Calls

Starknet smart contracts are written in Cairo. However, Cairo is a general purpose language that you can use for much more than just contract language. For details, see Cairo – a Turing-complete STARK-friendly CPU architecture.

So writing smart contracts requires some 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.

getters

In Cairo 1.0, all block/transaction/execution context getters are batched into a single system call: get_execution_info.

`get_execution_info`

Syntax

extern fn get_execution_info_syscall() -> SyscallResult<Box<starknet::info::ExecutionInfo>> implicits(
    GasBuiltin, System
) nopanic;

Description

Gets information about the original transaction.

Arguments

None.

Return values

ExecutionInfo

A struct containing the execution info

`call_contract`

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.

Syntax

extern fn call_contract_syscall(
    address: ContractAddress, entry_point_selector: felt252, calldata: Span<felt252>
) -> SyscallResult<Span<felt252>> implicits(GasBuiltin, System) nopanic;

Arguments

`address`	The address of the contract you want to call.
`entry_point_selector`	A selector for a function within that contract.
`calldata`	The calldata array

Return values

the call response, of type SyscallResult<Span<felt252>>

Common library

syscalls.cairo

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`

Syntax

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`	The class hash of the contract to be deployed
`contract_address_salt`	The salt, an arbitrary value provided by the sender, used in the computation of the contract’s address.
`calldata`	The constructor’s calldata. An array of felts.
`deploy_from_zero`	A flag used for the contract address computation. If not set, the caller address will be used as the new contract’s deployer address, otherwise 0 is used.

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

syscalls.cairo

`emit_event`

Syntax

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`	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`	The event’s data

Return values

None.

Common library

syscalls.cairo

Example

The following example emits an event with two keys, the strings status and deposit and three data elements: 1, 2, and 3.

let keys = ArrayTrait::new();
keys.append('key');
keys.append('deposit');
let values = ArrayTrait::new();
values.append(1);
values.append(2);
values.append(3);
emit_event_syscall(keys, values).unwrap_syscall();

`library_call`

Syntax

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.

This system call replaces the known delegate call functionality from Ethereum, with the important difference that there is only one contract involved.

The class is only used for its logic.

Arguments

`class_hash`	The hash of the class you want to use.
`function_selector`	A selector for a function within that class.
`calldata`	The calldata.

Return values

The call response, of type SyscallResult<Span<felt252>>.

Common library

syscalls.cairo

`send_message_to_L1`

Syntax

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`	The recipient’s L1 address.
`payload`	The array containing the message payload

Return values

None.

Common library

syscalls.cairo

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`

Syntax

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 replace_class, the code currently executing from the old class will finish running.

The new class will be used from the next transaction onwards or if the contract is called via the call_contract syscall in the same transaction (after the replacement).

Arguments

class_hash

The hash of the class you want to use as a replacement.

Return values

None

Common library

syscalls.cairo

`storage_read`

Syntax

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 balance.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`	The domain of the key, used to separate between different DA modes. Currently only `0` is supported.
`address`	The requested storage address

Address domains

One can think about the state of Starknet as a collection of address→value mappings. Each mapping is independent of the rest, and is considered a different address domain. This separation is used in Starknet to offer different data availability modes. In the on-chain mode (indicated by domain 0), all updates go to L1 to ensure data availability. Other address domains which will be introduced in the future will behave differently in terms of publication (in particular, they will not be posted on L1, creating a tradeoff between cost and security).

Return values

The value of the key, of type SyscallResult<felt252>

Common library

syscalls.cairo

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`

Sets the value of a key in the storage of the calling contract.

Syntax

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 balance.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`	The domain of the key, used to separate between different DA modes. Currently only `0` is supported.
.`value`	The value to write to the key.

Return values

None.

Common library

syscalls.cairo