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Hello, cycles!

View this sample's code on GitHub

Overview​

The hello_cycles sample project provides a simple example to illustrate how you might add functions to receive cycles, transfer cycles, and check your cycle balance with a simple Motoko actor (canister).

This sample project assumes that you are using the default cycles wallet canister that is created for you.

This example consists of the following functions (see src/hello_cycles/main.mo):

  • The wallet_balance : () -> async Nat: enables you to check the current cycle balance for the canister.

  • The wallet_receive : () -> { amount : Nat64 }: enables the program to accept cycles that are sent to the canister from a wallet. Both the name and type of this function are dictated by the wallet's implementation (so don't mess with them).

  • The transfer : (shared () -> (), Nat) -> async { refunded : Nat }: enables the program to transfer cycles to any shared function with candid signature "() -> ()" (assuming it accepts cycles). One example is the wallet's own wallet_receive : () -> () function.

The wallet's wallet_receive return type differs from hello_cycle's wallet_receive.

This is a Motoko example that does not currently have a Rust variant.

Prerequisites​

This example requires an installation of:

  • Install the IC SDK.
  • Download the following project files from GitHub: git clone https://github.com/dfinity/examples

Begin by opening a terminal window.

Step 1: Navigate into the folder containing the project's files and start a local instance of the replica with the command:​

cd examples/motoko/hello_cycles
dfx start --background

Step 2: Deploy the canister:​

dfx deploy

Step 3: Check that the current cycles balance of canister hello_cycles by running the following command:​

dfx canister call hello_cycles wallet_balance

The output should resemble the following:

(3_091_662_816_985 : nat)

You can also see the cycles balance of hello_cycles (or any canister you control) by calling:

dfx canister status hello_cycles

The output of this command will be similar to:

Canister status call result for hello_cycles.
Status: Running
Controllers: 2vxsx-fae b77ix-eeaaa-aaaaa-qaada-cai
Memory allocation: 0
Compute allocation: 0
Freezing threshold: 2_592_000
Memory Size: Nat(2371252)
Balance: 3_092_278_099_590 Cycles
Module hash: 0x09198be65e161bdb5c75c705dfec4b707a8091ac5d1a095dd45c025142a1fc43

Step 4: To display the default wallet and hello_cycles canister principals, run the commands:​

dfx identity get-wallet
dfx canister id hello_cycles

The output should resemble the following:

b77ix-eeaaa-aaaaa-qaada-cai
dzh22-nuaaa-aaaaa-qaaoa-cai

Below, we'll frequently use $(dfx identity get-wallet) and $(dfx canister id hello_cycles) to splice canister principals into longer bash commands.

Step 5: Attempt to send 2 trillion cycles from the default wallet to the hello_cycles canister by running the following command:​

dfx canister call $(dfx identity get-wallet) wallet_send "(record { canister = principal \"$(dfx canister id hello_cycles)\"; amount = (2000000000000:nat64); } )"

The wallet's wallet_send function transfers the amount to the argument canister's wallet_receive function (see above) and returns a result signaling success or failure.

If successful, the output will look similar to:

(variant { 17_724 })

Step 6: Verify that the cycles balance for the hello_cycles canister has increased by 10_000_000 by running the following command:​

dfx canister call hello_cycles wallet_balance

Output:

(5_091_662_569_379 : nat)

The amount is only increased by 10_000_000 because the implementation of wallet_receive is coded to accept at most 10_000_000 cycles, even when more cycles were transferred with the call. The unaccepted cycles are not lost but implicitly refunded to the caller (in this case, the wallet).

Step 7: Send some cycles from the hello_cycles canister back to the wallet by running the command:​

dfx canister call hello_cycles transfer "(func \"$(dfx identity get-wallet)\".\"wallet_receive\", 5000000)"

Output: 

(record { refunded = 0 : nat })

Step 8: Verify that the cycles balance of the hello_cycles canister has decreased with:​

dfx canister call hello_cycles wallet_balance

Output:

(5_091_657_208_987 : nat)

In this step, we are passing our own wallet's wallet_receive function as the first argument, followed by the amount. We, or a third party, could also pass any other function of the same signature, belonging to any other canister or wallet.

Without some additional access control checks (omitted here), a malicious client could abuse our naive transfer function to drain the canister of all of its cycles.

Security considerations and best practices​

If you base your application on this example, we recommend you familiarize yourself with and adhere to the security best practices for developing on the Internet Computer. This example may not implement all the best practices.

For example, the following aspect is particularly relevant for this app: