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Overview

Opcode: 0x3d Introduced: Byzantium (EIP-211) RETURNDATASIZE returns the size in bytes of the return data from the most recent external call.

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Specification

Stack Input: 
[]
Stack Output: 
size (uint256, in bytes)
Gas Cost: 2 (GasQuickStep)

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Behavior

Returns the length of the return data buffer populated by the last CALL, STATICCALL, DELEGATECALL, or CALLCODE. Key characteristics:
  • Introduced in Byzantium hardfork
  • Updated after each external call
  • 0 if no call made yet or call failed
  • Persists until next call

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Examples

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Basic Usage

function checkReturnSize(address target) public returns (uint256 size) {
    target.call("");
    assembly {
        size := returndatasize()
    }
}

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Efficient Return Data Handling

function efficientCall(address target) public returns (bytes memory) {
    (bool success,) = target.call("");
    require(success);

    bytes memory data;
    assembly {
        let size := returndatasize()
        data := mload(0x40)
        mstore(data, size)
        returndatacopy(add(data, 0x20), 0, size)
        mstore(0x40, add(add(data, 0x20), size))
    }
    return data;
}

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Gas Cost

Cost: 2 gas (GasQuickStep)

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Common Usage

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Proxy Pattern

fallback() external payable {
    address impl = implementation;
    assembly {
        calldatacopy(0, 0, calldatasize())
        let result := delegatecall(gas(), impl, 0, calldatasize(), 0, 0)

        let size := returndatasize()
        returndatacopy(0, 0, size)

        switch result
        case 0 { revert(0, size) }
        default { return(0, size) }
    }
}

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Dynamic Return Handling

function dynamicCall(address target, bytes memory data) public returns (bytes memory) {
    (bool success,) = target.call(data);

    assembly {
        let size := returndatasize()
        let result := mload(0x40)
        mstore(result, size)
        returndatacopy(add(result, 0x20), 0, size)

        switch success
        case 0 { revert(add(result, 0x20), size) }
        default { return(add(result, 0x20), size) }
    }
}

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Security

Safe opcode - just returns buffer size.

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Implementation

export function returndatasize(frame: FrameType): EvmError | null {
  const gasErr = consumeGas(frame, 2n);
  if (gasErr) return gasErr;

  const pushErr = pushStack(frame, BigInt(frame.returnData.length));
  if (pushErr) return pushErr;

  frame.pc += 1;
  return null;
}

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References