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I am facing issues with the transferFrom function of smart contract address 0x6560a9b6436e129cf835ab9da96201207facd4f8:

  1. I copied the smart contract code from etherscan.io and deployed on ropsten network. When I deployed it I found that the generated tokens are with the address which deployed it. It should be with the address of the smart contract.

  2. transferFrom function is not working and throwing exception error when i am trying to transfer tokens from one address to another even tough both the accounts are allowed to use given number of token.


/**
 *Submitted for verification at Etherscan.io on 2018-10-11
*/

pragma solidity ^0.4.24;

// openzeppelin-solidity: 1.12.0-rc.2

/**
 * @title Ownable
 * @dev The Ownable contract has an owner address, and provides basic authorization control
 * functions, this simplifies the implementation of "user permissions".
 */
contract Ownable {
  address public owner;


  event OwnershipRenounced(address indexed previousOwner);
  event OwnershipTransferred(
    address indexed previousOwner,
    address indexed newOwner
  );


  /**
   * @dev The Ownable constructor sets the original `owner` of the contract to the sender
   * account.
   */
  constructor() public {
    owner = msg.sender;
  }

  /**
   * @dev Throws if called by any account other than the owner.
   */
  modifier onlyOwner() {
    require(msg.sender == owner);
    _;
  }

  /**
   * @dev Allows the current owner to relinquish control of the contract.
   * @notice Renouncing to ownership will leave the contract without an owner.
   * It will not be possible to call the functions with the `onlyOwner`
   * modifier anymore.
   */
  function renounceOwnership() public onlyOwner {
    emit OwnershipRenounced(owner);
    owner = address(0);
  }

  /**
   * @dev Allows the current owner to transfer control of the contract to a newOwner.
   * @param _newOwner The address to transfer ownership to.
   */
  function transferOwnership(address _newOwner) public onlyOwner {
    _transferOwnership(_newOwner);
  }

  /**
   * @dev Transfers control of the contract to a newOwner.
   * @param _newOwner The address to transfer ownership to.
   */
  function _transferOwnership(address _newOwner) internal {
    require(_newOwner != address(0));
    emit OwnershipTransferred(owner, _newOwner);
    owner = _newOwner;
  }
}

/**
 * @title Pausable
 * @dev Base contract which allows children to implement an emergency stop mechanism.
 */
contract Pausable is Ownable {
  event Pause();
  event Unpause();

  bool public paused = false;


  /**
   * @dev Modifier to make a function callable only when the contract is not paused.
   */
  modifier whenNotPaused() {
    require(!paused);
    _;
  }

  /**
   * @dev Modifier to make a function callable only when the contract is paused.
   */
  modifier whenPaused() {
    require(paused);
    _;
  }

  /**
   * @dev called by the owner to pause, triggers stopped state
   */
  function pause() public onlyOwner whenNotPaused {
    paused = true;
    emit Pause();
  }

  /**
   * @dev called by the owner to unpause, returns to normal state
   */
  function unpause() public onlyOwner whenPaused {
    paused = false;
    emit Unpause();
  }
}

/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
library SafeMath {

  /**
  * @dev Multiplies two numbers, throws on overflow.
  */
  function mul(uint256 _a, uint256 _b) internal pure returns (uint256 c) {
    // Gas optimization: this is cheaper than asserting 'a' not being zero, but the
    // benefit is lost if 'b' is also tested.
    // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
    if (_a == 0) {
      return 0;
    }

    c = _a * _b;
    assert(c / _a == _b);
    return c;
  }

  /**
  * @dev Integer division of two numbers, truncating the quotient.
  */
  function div(uint256 _a, uint256 _b) internal pure returns (uint256) {
    // assert(_b > 0); // Solidity automatically throws when dividing by 0
    // uint256 c = _a / _b;
    // assert(_a == _b * c + _a % _b); // There is no case in which this doesn't hold
    return _a / _b;
  }

  /**
  * @dev Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
  */
  function sub(uint256 _a, uint256 _b) internal pure returns (uint256) {
    assert(_b <= _a);
    return _a - _b;
  }

  /**
  * @dev Adds two numbers, throws on overflow.
  */
  function add(uint256 _a, uint256 _b) internal pure returns (uint256 c) {
    c = _a + _b;
    assert(c >= _a);
    return c;
  }
}

/**
 * @title ERC20Basic
 * @dev Simpler version of ERC20 interface
 * See https://github.com/ethereum/EIPs/issues/179
 */
contract ERC20Basic {
  function totalSupply() public view returns (uint256);
  function balanceOf(address _who) public view returns (uint256);
  function transfer(address _to, uint256 _value) public returns (bool);
  event Transfer(address indexed from, address indexed to, uint256 value);
}

/**
 * @title ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */
contract ERC20 is ERC20Basic {
  function allowance(address _owner, address _spender)
    public view returns (uint256);

  function transferFrom(address _from, address _to, uint256 _value)
    public returns (bool);

  function approve(address _spender, uint256 _value) public returns (bool);
  event Approval(
    address indexed owner,
    address indexed spender,
    uint256 value
  );
}

/**
 * @title Basic token
 * @dev Basic version of StandardToken, with no allowances.
 */
contract BasicToken is ERC20Basic {
  using SafeMath for uint256;

  mapping(address => uint256) internal balances;

  uint256 internal totalSupply_;

  /**
  * @dev Total number of tokens in existence
  */
  function totalSupply() public view returns (uint256) {
    return totalSupply_;
  }

  /**
  * @dev Transfer token for a specified address
  * @param _to The address to transfer to.
  * @param _value The amount to be transferred.
  */
  function transfer(address _to, uint256 _value) public returns (bool) {
    require(_value <= balances[msg.sender]);
    require(_to != address(0));

    balances[msg.sender] = balances[msg.sender].sub(_value);
    balances[_to] = balances[_to].add(_value);
    emit Transfer(msg.sender, _to, _value);
    return true;
  }

  /**
  * @dev Gets the balance of the specified address.
  * @param _owner The address to query the the balance of.
  * @return An uint256 representing the amount owned by the passed address.
  */
  function balanceOf(address _owner) public view returns (uint256) {
    return balances[_owner];
  }

}

/**
 * @title Standard ERC20 token
 *
 * @dev Implementation of the basic standard token.
 * https://github.com/ethereum/EIPs/issues/20
 * Based on code by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
 */
contract StandardToken is ERC20, BasicToken {

  mapping (address => mapping (address => uint256)) internal allowed;


  /**
   * @dev Transfer tokens from one address to another
   * @param _from address The address which you want to send tokens from
   * @param _to address The address which you want to transfer to
   * @param _value uint256 the amount of tokens to be transferred
   */
  function transferFrom(
    address _from,
    address _to,
    uint256 _value
  )
    public
    returns (bool)
  {
    require(_value <= balances[_from]);
    require(_value <= allowed[_from][msg.sender]);
    require(_to != address(0));

    balances[_from] = balances[_from].sub(_value);
    balances[_to] = balances[_to].add(_value);
    allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
    emit Transfer(_from, _to, _value);
    return true;
  }

  /**
   * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
   * Beware that changing an allowance with this method brings the risk that someone may use both the old
   * and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
   * race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
   * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
   * @param _spender The address which will spend the funds.
   * @param _value The amount of tokens to be spent.
   */
  function approve(address _spender, uint256 _value) public returns (bool) {
    allowed[msg.sender][_spender] = _value;
    emit Approval(msg.sender, _spender, _value);
    return true;
  }

  /**
   * @dev Function to check the amount of tokens that an owner allowed to a spender.
   * @param _owner address The address which owns the funds.
   * @param _spender address The address which will spend the funds.
   * @return A uint256 specifying the amount of tokens still available for the spender.
   */
  function allowance(
    address _owner,
    address _spender
   )
    public
    view
    returns (uint256)
  {
    return allowed[_owner][_spender];
  }

  /**
   * @dev Increase the amount of tokens that an owner allowed to a spender.
   * approve should be called when allowed[_spender] == 0. To increment
   * allowed value is better to use this function to avoid 2 calls (and wait until
   * the first transaction is mined)
   * From MonolithDAO Token.sol
   * @param _spender The address which will spend the funds.
   * @param _addedValue The amount of tokens to increase the allowance by.
   */
  function increaseApproval(
    address _spender,
    uint256 _addedValue
  )
    public
    returns (bool)
  {
    allowed[msg.sender][_spender] = (
      allowed[msg.sender][_spender].add(_addedValue));
    emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
    return true;
  }

  /**
   * @dev Decrease the amount of tokens that an owner allowed to a spender.
   * approve should be called when allowed[_spender] == 0. To decrement
   * allowed value is better to use this function to avoid 2 calls (and wait until
   * the first transaction is mined)
   * From MonolithDAO Token.sol
   * @param _spender The address which will spend the funds.
   * @param _subtractedValue The amount of tokens to decrease the allowance by.
   */
  function decreaseApproval(
    address _spender,
    uint256 _subtractedValue
  )
    public
    returns (bool)
  {
    uint256 oldValue = allowed[msg.sender][_spender];
    if (_subtractedValue >= oldValue) {
      allowed[msg.sender][_spender] = 0;
    } else {
      allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
    }
    emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
    return true;
  }

}

/**
 * @title Pausable token
 * @dev StandardToken modified with pausable transfers.
 **/
contract PausableToken is StandardToken, Pausable {

  function transfer(
    address _to,
    uint256 _value
  )
    public
    whenNotPaused
    returns (bool)
  {
    return super.transfer(_to, _value);
  }

  function transferFrom(
    address _from,
    address _to,
    uint256 _value
  )
    public
    whenNotPaused
    returns (bool)
  {
    return super.transferFrom(_from, _to, _value);
  }

  function approve(
    address _spender,
    uint256 _value
  )
    public
    whenNotPaused
    returns (bool)
  {
    return super.approve(_spender, _value);
  }

  function increaseApproval(
    address _spender,
    uint _addedValue
  )
    public
    whenNotPaused
    returns (bool success)
  {
    return super.increaseApproval(_spender, _addedValue);
  }

  function decreaseApproval(
    address _spender,
    uint _subtractedValue
  )
    public
    whenNotPaused
    returns (bool success)
  {
    return super.decreaseApproval(_spender, _subtractedValue);
  }
}

/**
 * @title Burnable Token
 * @dev Token that can be irreversibly burned (destroyed).
 */
contract BurnableToken is BasicToken {

  event Burn(address indexed burner, uint256 value);

  /**
   * @dev Burns a specific amount of tokens.
   * @param _value The amount of token to be burned.
   */
  function burn(uint256 _value) public {
    _burn(msg.sender, _value);
  }

  function _burn(address _who, uint256 _value) internal {
    require(_value <= balances[_who]);
    // no need to require value <= totalSupply, since that would imply the
    // sender's balance is greater than the totalSupply, which *should* be an assertion failure

    balances[_who] = balances[_who].sub(_value);
    totalSupply_ = totalSupply_.sub(_value);
    emit Burn(_who, _value);
    emit Transfer(_who, address(0), _value);
  }
}

/**
 * @title Standard Burnable Token
 * @dev Adds burnFrom method to ERC20 implementations
 */
contract StandardBurnableToken is BurnableToken, StandardToken {

  /**
   * @dev Burns a specific amount of tokens from the target address and decrements allowance
   * @param _from address The address which you want to send tokens from
   * @param _value uint256 The amount of token to be burned
   */
  function burnFrom(address _from, uint256 _value) public {
    require(_value <= allowed[_from][msg.sender]);
    // Should https://github.com/OpenZeppelin/zeppelin-solidity/issues/707 be accepted,
    // this function needs to emit an event with the updated approval.
    allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
    _burn(_from, _value);
  }
}

contract KratosToken is StandardBurnableToken, PausableToken {

    string constant public name = "KRATOS";
    string constant public symbol = "TOS";
    uint8 constant public decimals = 18;

    uint256 public timelockTimestamp = 0;
    mapping(address => uint256) public timelock;

    constructor(uint256 _totalSupply) public {
        // constructor
        totalSupply_ = _totalSupply;
        balances[msg.sender] = _totalSupply;
    }

    event TimeLocked(address indexed _beneficary, uint256 _timestamp);
    event TimeUnlocked(address indexed _beneficary);

    /**
    * @dev Modifier to make a function callable only when the contract is not timelocked or timelock expired.
    */
    modifier whenNotTimelocked(address _beneficary) {
        require(timelock[_beneficary] <= block.timestamp);
        _;
    }

    /**
    * @dev Modifier to make a function callable only when the contract is timelocked and not expired.
    */
    modifier whenTimelocked(address _beneficary) {
        require(timelock[_beneficary] > block.timestamp);
        _;
    }

    function enableTimelock(uint256 _timelockTimestamp) onlyOwner public {
        require(timelockTimestamp == 0 || _timelockTimestamp > block.timestamp);
        timelockTimestamp = _timelockTimestamp;
    }

    function disableTimelock() onlyOwner public {
        timelockTimestamp = 0;
    }

    /**
    * @dev called by the owner to timelock token belonging to beneficary
    */
    function addTimelock(address _beneficary, uint256 _timestamp) public onlyOwner {
        _addTimelock(_beneficary, _timestamp);
    }

    function _addTimelock(address _beneficary, uint256 _timestamp) internal whenNotTimelocked(_beneficary) {
        require(_timestamp > block.timestamp);
        timelock[_beneficary] = _timestamp;
        emit TimeLocked(_beneficary, _timestamp);
    }

    /**
    * @dev called by the owner to timeunlock token belonging to beneficary
    */
    function removeTimelock(address _beneficary) onlyOwner whenTimelocked(_beneficary) public {
        timelock[_beneficary] = 0;
        emit TimeUnlocked(_beneficary);
    }

    function transfer(address _to, uint256 _value) public whenNotTimelocked(msg.sender) returns (bool) {
        if (timelockTimestamp > block.timestamp)
            _addTimelock(_to, timelockTimestamp);
        return super.transfer(_to, _value);
    }

    function transferFrom(address _from, address _to, uint256 _value) public whenNotTimelocked(_from) returns (bool) {
        if (timelockTimestamp > block.timestamp)
            _addTimelock(_to, timelockTimestamp);
        return super.transferFrom(_from, _to, _value);
    }

    function approve(address _spender, uint256 _value) public whenNotTimelocked(_spender) returns (bool) {
        return super.approve(_spender, _value);
    }

    function increaseApproval(address _spender, uint _addedValue) public whenNotTimelocked(_spender) returns (bool success) {
        return super.increaseApproval(_spender, _addedValue);
    }

    function decreaseApproval(address _spender, uint _subtractedValue) public whenNotTimelocked(_spender) returns (bool success) {
        return super.decreaseApproval(_spender, _subtractedValue);
    }
}
  • when i am trying to transfer tokens from one address to another - how exactly are you trying that (or are we supposed to guess)? – goodvibration Aug 8 at 14:50
2

The transferFrom is failing, I suppose, because of the require(_value <= allowed[_from][msg.sender]);

Did you first approve the msg.sender to transfer tokens from _from address? In the cases like if third party needs to transfer your tokens to someone else, you have to first allow the third party to your certain amount of tokens so that he can use it on behalf of you. For this case, there is transferFrom. That means, you have to first approve your certain amount of tokens to the third party spender.

But, if you are the first party willing to transfer the tokens directly to second party then you need to call transfer(address _to, uint256 _value).

  • i tried after calling approve function for a particular address which contained my crypto token.still it failed ..for example there were 500 TOS(crypto currency name) in one adress-xyz..i approved it to use those 500 tokens using approve(spender address,value)..now when i am transferring from address xyz to address abc ..its still giving the exception error..even i approved address abc to use the 500 tokens ..still it didnt work..if u try deploying the smart contract(in ropsten netwrk) by copying it from the address in the question..u will b able to know the scenario better.. – Ashadipta Aug 9 at 13:51
2

I found that the generated tokens are with the address which deployed it

Token contracts typically assign the total supply to the contract deployer, so this is normal. A token contract is a ledger, not typically an owner of assets, so it is sensible to assign the created supply to the deployer who should know what the next step is.

transferFrom function is not working

It probably does. More likely, what you are trying to do with it is disallowed, by design. That could be evidence of the contract working correctly. transferFrom() is often misunderstood, so consider posting your testing methodology.

https://ethereum.stackexchange.com/search?tab=votes&q=transferFrom

Hope it helps.

  • i tried after calling approve function for a particular address which contained my crypto token.still it failed ..for example there were 500 TOS(crypto currency name) in one adress-xyz..i approved it to use those 500 tokens using approve(spender address,value)..now when i am transferring from address xyz to address abc ..its still giving the exception error..even i approved address abc to use the 500 tokens ..still it didnt work..if u try deploying the smart contract(in ropsten netwrk) by copying it from the address in the question..u will b able to know the scenario better – Ashadipta Aug 13 at 9:10
0

As per the other answers, it is likely that no allowance has been approved. When using transferFrom it is a two stage process:
1. approve an allowance.
2. transferFrom using the allowance.

If you are creating ERC20 tokens you may want to look at the OpenZeppelin Contracts implementation to see if this meets your needs. See the documentation for details: https://docs.openzeppelin.com/contracts/2.x/tokens#ERC20

Alternatively you could look at creating ERC777 tokens (no need to do approve and transferFrom in two separate transactions). See the documentation for details: https://docs.openzeppelin.com/contracts/2.x/tokens#ERC777

If you have questions on using OpenZeppelin you can ask in the Community Forum: https://forum.openzeppelin.com/

Disclosure: I am the Community Manager at OpenZeppelin

  • i tried after calling approve function for a particular address which contained my crypto token.still it failed ..for example there were 500 TOS(crypto currency name) in one adress-xyz..i approved it to use those 500 tokens using approve(spender address,value)..now when i am transferring from address xyz to address abc ..its still giving the exception error..even i approved address abc to use the 500 tokens ..still it didnt work..if u try deploying the smart contract(in ropsten netwrk) by copying it from the address in the question..u will b able to know the scenario better – Ashadipta Aug 13 at 9:10
  • For this contract, the deployer of the contract receives the total supply of tokens. If you are transferring between external accounts then you can just use transfer when called by the deployer account. – abcoathup Aug 13 at 10:24

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