ethereum - 如何将 ETH 合约输出十六进制解码为字符串？

Question

当我对 Eth 主网上的 Usdt 智能合约进行 eth_call 时，我得到一个 96 字节的十六进制输出。

0000000000000000000000000000000000000000000000000000000000000020 // What is this?
000000000000000000000000000000000000000000000000000000000000000a // Size of the output
5465746865722055534400000000000000000000000000000000000000000000 // Output ("Tether USD")

据我所知，第三个 32 字节包含带有右填充的实际字符串输出，第二个 32 字节包含带有左填充的输出大小(以字节为单位)。第一个 32 字节包含什么？

RPC 调用

{"jsonrpc":"2.0","method":"eth_call","params":[{"To":"0xdAC17F958D2ee523a2206206994597C13D831ec7","Data":"0x06fdde03"},"latest"],"id":1}

Answer 1

第一个 32 字节槽是指向长度槽的偏移量，紧随其后的是包含实际参数值的槽。

当函数返回多个动态长度数组(字符串表示为动态长度字节数组)时，偏移量非常有用，如下例所示:

pragma solidity ^0.8;

contract MyContract {
    function foo() external pure returns (string memory, string memory) {
        return ("Tether USD", "Ethereum");
    }
}

返回数据:

# offset pointing to the length of the 1st param
0x0000000000000000000000000000000000000000000000000000000000000040

# offset pointing to the length of the 2nd param
0x0000000000000000000000000000000000000000000000000000000000000080

# 1st param length
0x000000000000000000000000000000000000000000000000000000000000000a

# followed by 1st param value
0x5465746865722055534400000000000000000000000000000000000000000000

# 2nd param length
0x0000000000000000000000000000000000000000000000000000000000000008

# followed by 2nd param value
0x457468657265756d000000000000000000000000000000000000000000000000

如果这两者之间有一个固定长度的参数，则返回的数据结构将如下所示:

1. 第一个参数长度的偏移量
2. (固定长度)第二个参数实际值
3. 第三个参数长度的偏移量
4. 其余同上

文档:https://docs.soliditylang.org/en/latest/abi-spec.html#use-of-dynamic-types