2

Is it possible in Ethereum blockchain to have two or more events simultaneously occurred?

Regarding the following solidity code:

    function _transfer(address _from, address _to, uint _value) internal {
    // Prevent transfer to 0x0 address. Use burn() instead
    require(_to != 0x0);
    // Check if the sender has enough
    require(balanceOf[_from] >= _value);
    // Check for overflows
    require(balanceOf[_to] + _value > balanceOf[_to]);
    // Save this for an assertion in the future
    uint previousBalances = balanceOf[_from] + balanceOf[_to];
    // Subtract from the sender
    balanceOf[_from] -= _value;
    // Add the same to the recipient
    balanceOf[_to] += _value;
    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);
}

What if some burning or minting operations have occurred and changed the balance of _to or _from after balances update but before assertion.

For example: Balance of (_from) = 10 , Balance of (_to) = 0 uint previousBalances = 10

function _transfered triggered and _value = 5 so after updating balances we will have Balance of (_from) = 5 , Balance of (_to) = 5 uint previousBalances = 10

What if after balances update but before assertion 3 tokens were minted to the address of (_to) Balance of (_from) = 5 , Balance of (_to) = 5+3 therefor assertion would be failed uint previousBalances = 5 + 5 + 3 = 13

IS THIS ASSERTION A RIGHT WAY TO CONTROL UPDATING BALANCES??

0

Concurrency is not possible.

Transactions are not accepted until mined. Mined blocks are well-ordered transaction lists. Each transaction executes in the context of the state following the transactions that came before it.

Your use of assert looks appropriate.

Hope it helps.

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