I am using following code for custom token on ethereum.

pragma solidity >=0.4.22 <0.6.0;

contract owned {
    address public owner;

    constructor() public {
        owner = msg.sender;

    modifier onlyOwner {
        require(msg.sender == owner);

    function transferOwnership(address newOwner) onlyOwner public {
        owner = newOwner;

contract GenericToken is owned{
    // Public variables of the token
    string public name;
    string public symbol;
    uint8 public decimals = 18;
    // 18 decimals is the strongly suggested default, avoid changing it
    uint256 public totalSupply;

    uint256 public sellPrice;
    uint256 public buyPrice;

    mapping (address => bool) public frozenAccount;

    /* This generates a public event on the blockchain that will notify clients */
    event FrozenFunds(address target, bool frozen);

    // This creates an array with all balances
    mapping (address => uint256) public balanceOf;
    mapping (address => mapping (address => uint256)) public allowance;

    // This generates a public event on the blockchain that will notify clients
    event Transfer(address indexed from, address indexed to, uint256 value);

    // This generates a public event on the blockchain that will notify clients
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);

    // This notifies clients about the amount burnt
    event Burn(address indexed from, uint256 value);

     * Constrctor function
     * Initializes contract with initial supply tokens to the creator of the contract
        uint256 initialSupply,
        string memory tokenName,
        string memory tokenSymbol
    ) public {
        totalSupply = initialSupply * 10 ** uint256(decimals);  // Update total supply with the decimal amount
        balanceOf[msg.sender] = totalSupply;                    // Give the creator all initial tokens
        name = tokenName;                                       // Set the name for display purposes
        symbol = tokenSymbol;                                   // Set the symbol for display purposes

     * Internal transfer, only can be called by this contract
    function _transfer(address _from, address _to, uint _value) internal {

        require(_to != address(0x0));                               // Prevent transfer to 0x0 address. Use burn() instead
        require(balanceOf[_from] >= _value);                        // Check if the sender has enough
        require(balanceOf[_to] + _value > balanceOf[_to]);          // Check for overflows
        require(!frozenAccount[_from]);                             // Check if sender is frozen
        require(!frozenAccount[_to]);                               // Check if recipient is frozen
        uint previousBalances = balanceOf[_from] + balanceOf[_to];  // Save this for an assertion in the future
        balanceOf[_from] -= _value;                                 // Subtract from the sender
        balanceOf[_to] += _value;                                   // Add the same to the recipient
        emit Transfer(_from, _to, _value);                          // Asserts are used to use static analysis to find bugs in your code. They should never fail
        assert(balanceOf[_from] + balanceOf[_to] == previousBalances);

     * Transfer tokens
     * Send `_value` tokens to `_to` from your account
     * @param _to The address of the recipient
     * @param _value the amount to send
    function transfer(address _to, uint256 _value) public returns (bool success) {
        _transfer(msg.sender, _to, _value);
        return true;

     * Transfer tokens from other address
     * Send `_value` tokens to `_to` in behalf of `_from`
     * @param _from The address of the sender
     * @param _to The address of the recipient
     * @param _value the amount to send
    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        require(_value <= allowance[_from][msg.sender]);     // Check allowance
        allowance[_from][msg.sender] -= _value;
        _transfer(_from, _to, _value);
        return true;

    /// @notice Create `mintedAmount` tokens and send it to `target`
    /// @param target Address to receive the tokens
    /// @param mintedAmount the amount of tokens it will receive
    function mintToken(address target, uint256 mintedAmount) onlyOwner public {
        balanceOf[target] += mintedAmount;
        totalSupply += mintedAmount;
        emit Transfer(address(0), address(this), mintedAmount);
        emit Transfer(address(this), target, mintedAmount);

    /// @notice `freeze? Prevent | Allow` `target` from sending & receiving tokens
    /// @param target Address to be frozen
    /// @param freeze either to freeze it or not
    function freezeAccount(address target, bool freeze) onlyOwner public {
        frozenAccount[target] = freeze;
        emit FrozenFunds(target, freeze);

     * Destroy tokens
     * Remove `_value` tokens from the system irreversibly
     * @param _value the amount of money to burn
    function burn(uint256 _value) public returns (bool success) {
        require(balanceOf[msg.sender] >= _value);   // Check if the sender has enough
        balanceOf[msg.sender] -= _value;            // Subtract from the sender
        totalSupply -= _value;                      // Updates totalSupply
        emit Burn(msg.sender, _value);
        return true;

    /// @notice Allow users to buy tokens for `newBuyPrice` eth and sell tokens for `newSellPrice` eth
    /// @param newSellPrice Price the users can sell to the contract
    /// @param newBuyPrice Price users can buy from the contract
    function setPrices(uint256 newSellPrice, uint256 newBuyPrice) onlyOwner public {
        sellPrice = newSellPrice;
        buyPrice = newBuyPrice;

    /// @notice Buy tokens from contract by sending ether
    function buy() payable public {
        uint amount = msg.value / buyPrice;                 // calculates the amount
        _transfer(address(this), msg.sender, amount);       // makes the transfers

    /// @notice Sell `amount` tokens to contract
    /// @param amount amount of tokens to be sold
    function sell(uint256 amount) public {
        address myAddress = address(this);
        require(myAddress.balance >= amount * sellPrice);   // checks if the contract has enough ether to buy
        _transfer(msg.sender, address(this), amount);       // makes the transfers
        msg.sender.transfer(amount * sellPrice);            // sends ether to the seller. It's important to do this last to avoid recursion attacks

     * Destroy tokens from other account
     * Remove `_value` tokens from the system irreversibly on behalf of `_from`.
     * @param _from the address of the sender
     * @param _value the amount of money to burn
    function burnFrom(address _from, uint256 _value) public returns (bool success) {
        require(balanceOf[_from] >= _value);                // Check if the targeted balance is enough
        require(_value <= allowance[_from][msg.sender]);    // Check allowance
        balanceOf[_from] -= _value;                         // Subtract from the targeted balance
        allowance[_from][msg.sender] -= _value;             // Subtract from the sender's allowance
        totalSupply -= _value;                              // Update totalSupply
        emit Burn(_from, _value);
        return true;

I am receiving following error when try to execute buy

transact to GenericToken.buy errored: VM error: invalid opcode. invalid opcode The execution might have thrown. Debug the transaction to get more information

And i am also confused about the use of allowance, is it necessary to use or should be a use case specific ?

1 Answer 1


Most likely, the contract itself doesn't enough tokens to satisfy this requirement at line 73.

require(balanceOf[_from] >= _value); // Check if the sender has enough

Hope it helps.

  • Thanks it helped but now the error is changed to transact to GenericToken.buy errored: VM error: revert. revert The transaction has been reverted to the initial state. Note: The constructor should be payable if you send value
    – Saeedi
    Feb 22, 2019 at 9:14

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