As per my question here:

I need to compare msg.data (specifically, numbers that are sent with the sendTransaction data object) to a data type of uint256 within my contract.

How is this possible?

  • msg.data is of type calldata Feb 5, 2016 at 6:11
  • I am sorry, but that does not answer my question... If someone sends a number between 1-100 to my contract using the data object in sendTransaction, how can I compare it to a uint256 within my contract? Feb 5, 2016 at 6:15
  • Welcome to the SE :) I don't know the answer, but I'm sure someone will come along who does. Just trying to be precise so it's easier for that person. Feb 5, 2016 at 6:17
  • Thank you I do appreciate it. I do have another account but I have lost my logins deets lol. Feb 5, 2016 at 7:05
  • hmm there is no "calldata" type. msg.data type is bytes See global variables solidity.readthedocs.org/en/latest/miscellaneous.html
    – eth
    Feb 5, 2016 at 20:11

3 Answers 3


Byte-per-byte comparisons. Conversions needed. reddit link is dead. Why do you need to access msg.data directly?

EDIT: Convert a uint256 from calldata bytes to a proper uint256 in Solidity: https://gist.github.com/anonymous/a734a5d299ffa7b5a834

EDIT: The typing system and posting rules on this site are super weird. Anyways that works. It should always return true.

  • Hi and welcome to Ethereum SE, your answer appears to be a comment. Could you add some more information about the conversions? I fixed the link by the way.
    – q9f
    Feb 5, 2016 at 11:39
  • I misunderstood. I thought the only way to send data to a contract was with the data object in the sendTransaction. Why does this contract not send 1 Finney back to me when I send it this transaction, I cannot for the life of me work out why not: random.rand.sendTransaction(1,{from: eth.accounts[0], value: web3.toWei(.0001, 'ether'), to: '0x4abc14efad9a7a84a0a9ef6e1a782c43a9e3f550', gas: 500000, data: web3.fromAscii('message')}) Contract code: gist.github.com/anonymous/e210a052b6b4c86337c2 Great, no paragraph breaks in comments. Feb 5, 2016 at 13:41
  • I upvoted this answer and suggestions for next time: include the actual code itself (since it's not that long) and provide a little more explanation.
    – eth
    Feb 5, 2016 at 19:59

The specification for msg.data is the Ethereum Contract ABI.

Here's the code from the answer provided by Andreas:

contract ReadConvertUint256Bytes {

    function equal(uint a) constant returns (bool) {
        uint x = 0;
        for (uint i = 0; i < 32; i++) {
            uint b = uint(msg.data[35 - i]);
            x += b * 256**i;
        return a == x;


It starts reading from the byte indexed at 35, because the first 4 bytes (at index 0 to 3) is the Method ID (see ABI above), and everything in Ethereum internally is big-endian.


Whilst other answers may provide the tools required to answer this question as verbatim, my problem was solved because I was not aware of other ways to send parameters to contracts on Ethereum. Namely: eth.sendTransaction(2{from: eth.accounts[0], value: web3.toWei(5, 'ether'), to: '0xXXXX', gas: 200000, data: web3.fromAscii('MinedBlock')})

With '2' being the parameter being passed which can then easily be compared inside the contract to other variables. Thanks for the input.

  • Since you're passing in value (5 ether), you can check for it with msg.value. ("value" is not part of msg.data)
    – eth
    Feb 5, 2016 at 22:50
  • Also, using Contract Methods like myContract.myMethod(5, {from: eth.accounts[0]}) is usually simpler to pass a data (not value) of 5, than direct eth.sendTransaction.
    – eth
    Feb 5, 2016 at 22:56
  • @eth sorry, I updated my answer, I am passing in a parameter in my answer case - 2. Just to confirm, this is not accessible via msg.value. Correct? Feb 6, 2016 at 2:30
  • Correct, the "2" is not accessible via msg.value. But it is simpler to access the "2": if your function is like myMethod(int8 num) then num will get the value of 2.
    – eth
    Feb 8, 2016 at 20:22

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