The Counter Smart Contract

By following the tutorial on this page, you will learn how to build, deploy and interact with a basic Smart Contract (written in C).

info

This is a mere example. We do not offer support for writing contracts in C. The recommended language to write smart contracts is Rust.

Prerequisites

You need to have mxpy installed.

Create the contract

In a folder of your choice, add the files contained at this location.

The file counter.c is the implementation of the Smart Contract, which defines the following functions:

• init(): this function is executed when the contract is deployed on the Blockchain
• increment() and decrement(): these functions modify the internal state of the Smart Contract
• get(): this is a pure function (does not modify the state) which we'll use to query the value of the counter

Build the contract

In order to build the contract to WASM, run the following command:

mxpy --verbose contract build mycounter

Above, mycounter refers to the folder that holds the source code. After executing the command, you can inspect the generated files in mycounter/output.

Deploy the contract on the Testnet

In order to deploy the contract on the Testnet you need to have an account with sufficient balance (required for the deployment fee) and the associated private key in PEM format.

The deployment command is as follows:

mxpy --verbose contract deploy --project=mycounter --pem="alice.pem" --gas-limit=5000000 --proxy="https://testnet-gateway.multiversx.com" --outfile="counter.json" --recall-nonce --send

Above, mycounter refers to the same folder that contains the source code and the build artifacts. The deploy command knows to search for the WASM bytecode within this folder.

Note the last parameter of the command - this instructs mxpy to dump the output of the operation in the specified file. The output contains the address of the newly deployed contract and the hash of the deployment transaction.

counter.json
{
    "emitted_tx": {
        "tx": {
            "nonce": 0,
            "value": "0",
            "receiver": "erd1qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq6gq4hu",
            "sender": "erd1...",
            "gasPrice": 1000000000,
            "gasLimit": 5000000,
            "data": "MDA2MTczNmQwMTAwMDA...MDA=",
            "chainID": "T",
            "version": 1,
            "signature": "a0ba..."
        },
        "hash": "...",
        "data": "...",
        "address": "erd1qqqqqqqqqqqqqpgqp93y6..."
    }
}

Feel free to inspect these values in the Explorer.

Interact with the deployed contract

Let's extract the contract address from counter.json before proceeding to an actual contract execution.

export CONTRACT_ADDRESS=$(python3 -c "import json; data = json.load(open('counter.json')); print(data['emitted_tx']['address'])")

Now that we have the contract address saved in a shell variable, we can call the increment function of the contract as follows:

mxpy --verbose contract call $CONTRACT_ADDRESS --pem="alice.pem" --gas-limit=2000000 --function="increment" --proxy="https://testnet-gateway.multiversx.com" --recall-nonce --send

Execute the command above a few times, with some pause in between. Then feel free to experiment with calling the decrement function.

Then, in order to query the value of the counter - that is, to execute the get pure function of the contract - run the following:

mxpy contract query $CONTRACT_ADDRESS --function="get" --proxy="https://testnet-gateway.multiversx.com"

The output should look like this:

[{'base64': 'AQ==', 'hex': '01', 'number': 1}]

Interaction script

The previous steps can be summed up in a simple script as follows:

#!/bin/bash

# Deployment
mxpy --verbose contract deploy --project=mycounter --pem="alice.pem" --gas-limit=5000000 --proxy="https://testnet-gateway.multiversx.com" --outfile="counter.json" --recall-nonce --send
export CONTRACT_ADDRESS=$(python3 -c "import json; data = json.load(open('address.json')); print(data['emitted_tx']['contract'])")

# Interaction
mxpy --verbose contract call $CONTRACT_ADDRESS --pem="alice.pem" --gas-limit=2000000 --function="increment" --proxy="https://testnet-gateway.multiversx.com" --recall-nonce --send
sleep 10
mxpy --verbose contract call $CONTRACT_ADDRESS --pem="alice.pem" --gas-limit=2000000 --function="increment" --proxy="https://testnet-gateway.multiversx.com" --recall-nonce --send
sleep 10
mxpy --verbose contract call $CONTRACT_ADDRESS --pem="alice.pem" --gas-limit=2000000 --function="decrement" --proxy="https://testnet-gateway.multiversx.com" --recall-nonce --send
sleep 10

# Querying
mxpy contract query $CONTRACT_ADDRESS --function="get" --proxy="https://testnet-gateway.multiversx.com"