Analysing the Compiler Output

When you compile a Solidity contract using neo-solc, the output differs significantly from standard solc.

💡 NeoVM Difference

Standard Solidity outputs an EVM binary (often represented as a hex string) and an Ethereum ABI JSON. Neo DevPack for Solidity outputs a .nef (Neo Executable Format) binary file and a .manifest.json file.

The NEF File (.nef)

The NEF file is the compiled bytecode executable deployed to the Neo blockchain. It is a strictly formatted binary file containing:

1. Magic Header: Identifies the file as a Neo N3 executable (0x3346454E).
2. Compiler Name: A 64-byte string identifying the compiler (for example, neo-devpack-solidity-0.18.0).
3. Source/Version Context: Metadata tracking the compiler version used.
4. Method Tokens: A table mapping external contract calls to optimize runtime execution (if --callt is used).
5. Script: The actual NeoVM opcodes representing your contract logic.
6. Checksum: A SHA-256 checksum to ensure file integrity.

You can disassemble the NEF file during compilation by passing the -f assembly flag:

neo-solc MyContract.sol -O2 -f assembly -o build/MyContract

The Manifest File (.manifest.json)

The manifest file is a comprehensive JSON document describing how the Neo blockchain and other contracts can interact with your executable.

Key sections include:

• abi: Describes the methods and events the contract exposes.
  • methods: Arrays of functions, their parameters (using Neo types like Hash160, Integer), and safe flags (equivalent to view/pure).
  • events: Notifications the contract emits.
• permissions: Defines which external contracts and methods this contract is allowed to call. This is critical for security.
• supportedstandards: Lists the NEP standards the contract implements (e.g., NEP-17, NEP-11), which are automatically inferred by the compiler.
• trusts: Specifies which contracts are allowed to invoke this contract without user warnings.
• extra: Custom metadata like author, email, or description.

Manifest Example

{
  "name": "MyToken",
  "groups": [],
  "features": {},
  "supportedstandards": ["NEP-17"],
  "abi": {
    "methods": [
      {
        "name": "balanceOf",
        "parameters": [{ "name": "account", "type": "Hash160" }],
        "returntype": "Integer",
        "offset": 45,
        "safe": true
      }
    ],
    "events": []
  },
  "permissions": [
    {
      "contract": "0xd2a4cff31913016155e38e474a2c06d08be276cf",
      "methods": ["transfer"]
    }
  ]
}