ext-call.md

Cross-contract invocations

Configuration

• detector id: ext-call
• severity: info

Description

Find all cross-contract invocations.

We provide this detector to help users quickly locate all cross-contract invocations for further analysis.

Sample code

// https://docs.near.org/sdk/rust/cross-contract/callbacks#calculator-example
#[ext_contract(ext_calculator)]
trait Calculator {
    fn sum(&self, a: U128, b: U128) -> U128;
}

#[near_bindgen]
impl Contract {
    pub fn sum_a_b(&mut self, a: U128, b: U128) -> Promise {
        let calculator_account_id: AccountId = "calc.near".parse().unwrap();
        ext_calculator::ext(calculator_account_id).sum(a, b)  // run sum(a, b) on remote
    }
}

Macro #[ext_contract(...)] will convert a Rust trait to a module with static methods, which allows users to make cross-contract invocation. Each of these static methods takes positional arguments defined by the Trait, then the receiver_id, the attached deposit, and the amount of gas and return a new Promise.

In this sample, the contract can get the sum of a and b with the cross-contract invocation (i.e., ext_calculator::ext(calculator_account_id).sum(a, b)).