I work in Truffle development environment and use Windows PowerShell. My Solidity code is a test code and is simple. There is a library called Alireza that just compares two uint values. There is a contract called B that just uses the library function to compare two uint values. Now, i need to link Alireza to B. For that

  1. I embedded using-for command in B.
  2. I deployed Alireza and got its address.
  3. I tried to deploy B to create its bytecode. As one expects, the deplyoment failed.
  4. I embedded Alireza's address to B's bytecode. Because, as i searched, it is a way to link.
  5. I tried to deploy B again and it failed again.

Is this work flow correct? i don't know. So, i intent to try another way that here mentioned:

  1. I tried to embed the hash of Alireza's title or directory to B's bytecode. Just like:

  2. I tried to deploy B again and again the deployment failed.

Even, before all ways i mentioned, i deployed B without deployment of Alireza before that. Because i thought maybe, by this way, the compiler would create the link. But, the below error emerges:

Error: B contains unresolved libraries. You must deploy and link the following libraries before you can deploy a new version of B: Alireza

Am i in a correct way? Where is my problem? How can i link a library to a contract just by Solidity and contract's bytecode and without node-js? How should i alter my work flow to link?

My whole code:

pragma solidity ^0.5.12;

library Alireza{

    function comparison(uint a, uint b) view public returns(bool){  
            return true;
            return false;

contract B{

    using Alireza for uint;

    function assessment(uint m, uint n) view public returns(bool){
        return m.comparison(n);

Thank you.

2 Answers 2


I face-planted on these sorts of errors recently too! Turns out there be additional setup required known as migrations...

Truffle -- Running Migrations -- deployer.link(library, destinations) section covers how to link libraries and contracts.



const Alireza = artifacts.require("Alireza");
const B = artifacts.require("B");

module.exports = (deployer, _network, accounts) => {
  deployer.link(Alireza, B);

The deployer.link bit is the magic line that allows B to make use of Alireza

Bonus Tips

  • if ya have a contract requiring parameters within it's constructor, then it is possible to define 'em during the .deploy phase

  • the accounts parameter holds an array of test account addresses that may be useful too


contract Name {
    address public owner;

    function constructor(address _owner) {
        owner = _owner;


const Name = artifacts.require("Name");

module.exports = (deployer, _network, accounts) => {
  deployer.deploy(Name, accounts[0]);

Just compile the code and deploy contract B, no need to do the link thing. The library code will copy to contract B automatically by the compiler.

  • I tried that but the followed error emerges: Error: B contains unresolved libraries. You must deploy and link the following libraries before you can deploy a new version of B: Alireza. It says first a library must be deployed before a contract deployment.
    – Alireza
    Commented May 11, 2021 at 12:17

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