Why is mstore not working here? (assembly)

Question

function debug() returns (bytes) {
        bytes memory ret = new bytes(6);
        bytes memory word = new bytes(6);
        word = hex"61626f757400";
        assembly {
            mstore(add(ret, 32), word)
        }
        return ret;
}

returns 0x000000000000, desired outcome return 0x61626f757400

Answer 1

Notice how ret is the location in memory of the length of the bytes array ret. It's the same with word, so when you do mstore(add(ret, 32), word), you're actually storing the location in memory of the length of array word at the start of the data in the ret array. Here is the correct version of what you're trying to do:

function debug() view returns (bytes) {
    bytes memory ret = new bytes(6);
    bytes memory word = new bytes(6);
    word = hex"61626f757400";
    assembly {
        mstore(add(ret, 32), mload(add(word, 32)))
    }
    return ret;
}