# solc：solidity的编译器
solidity编写的以太坊智能合约可通过命令行编译工具solc来进行编译，成为以太坊虚拟机中的代码。solc编译后最终部署到链上形成我们所见到的各种智能合约。
作为一个solidity命令行编译工具，我们来看看官网都怎么说solc。
solc的安装很简单：
```
npm install -g solc
//或者
npm install -g solc-cli
//或者
sudo apt-get install solc
```
安装完成后我们来看，```solc --help```，solc --help命令显示所有的solc命令选项。编译器可以生成各种输出，比如最终的二进制合约文件、语法树的汇编或者需要预计的要花费的gas等。```solc --bin sourceFile.sol```，可以编译后输出一个名为sourceFile.sol的智能合约文件。如果你想从```solc```获得更丰富的一些输出变量，你可以使用```solc -o outputDirectory --bin --ast --asm sourceFile.sol```。
你在部署[以太坊智能合约](http://xc.hubwiz.com/course/5a952991adb3847553d205d1)之前可以用```solc --optimize --bin sourceFile.sol```优化一下。默认情况下solc编译器会帮你优化200次。你也可以设置 ```--runs=1```，这样就按照最小化的方式进行编译，如果你希望多次交易不太在乎成本，那你可以设置成你想要的次数:)。
命令行编译器会自动读取需要导入的文件，也可以通过使用```prefix = path```来指定路径，例如：
>```solc github.com/ethereum/dapp-bin/=/usr/local/lib/dapp-bin/ =/usr/local/lib/fallback file.sol```
这样编译器就会从指定目录```github.com/ethereum/dapp-bin/```下的```/usr/local/lib/dapp-bin/```目录开始搜索，如果没有找到文件，它将查看```/usr/local/lib/fallback```。solc将只读取你指定的这两个路径的，因此像```import "/etc/passwd";```必须要通过```/=```重新映射才起作用。如果有多个匹配，则选择具有最长公共前缀的进行匹配。
出于安全上的考虑，编译器限制了它可以访问的一些目录。在命令行中指定的源文件的路径（及其子目录）和命令行指定的路径外其他所有内容都会被拒绝。```--allow-paths /sample/path,/another/sample/path```来切换。
如果智能合约使用了[libraries](https://solidity.readthedocs.io/en/v0.4.24/contracts.html#libraries)，你会注意到字节码包含了```__LibraryName______```的子字符串。您可以使用solc作为链接器，这意味着它将在这些点为您插入库地址。
可以通过添加库```--libraries "Math:0x12345678901234567890 Heap:0xabcdef0123456"```到您的命令，以提供每个库的地址，或者使用文件中的说明字符串（每行一个库），并使用```--libraries fileName```运行solc。
如果用选项```--link```调用Solc，则所有输入文件都被解释为未链接的二进制文件（HEX编码），在上面给出的```__LibraryName____```格式中，将其链接到适当地址（如果从stdin读取输入，则将其写入stdout）。在这种情况下，除了库外，所有选项都被忽略（包括```-o```）。
如果用```--standard-json```调用SOLC，它就将标准的JSON输入（如下所述），并返回JSON输出。
#solc编译器输入输出JSON描述
这些JSON格式通过编译器API使用，可以通过SOLC获得。内容都是可以修改的，一些对象是可选的（如前所述），其目的是向后兼容。
编译器的API需要一个JSON格式的输入，然后以JSON格式输出编译结果。
**注意不允许注释**。下面示例中的注释，是官网为了学习者更好的理解标注的。
* 输入格式说明：
```
{
// Required: Source code language, such as "Solidity", "serpent", "lll", "assembly", etc.
language: "Solidity",
// Required
sources:
{
// The keys here are the "global" names of the source files,
// imports can use other files via remappings (see below).
"myFile.sol":
{
// Optional: keccak256 hash of the source file
// It is used to verify the retrieved content if imported via URLs.
"keccak256": "0x123...",
// Required (unless "content" is used, see below): URL(s) to the source file.
// URL(s) should be imported in this order and the result checked against the
// keccak256 hash (if available). If the hash doesn't match or none of the
// URL(s) result in success, an error should be raised.
"urls":
[
"bzzr://56ab...",
"ipfs://Qma...",
"file:///tmp/path/to/file.sol"
]
},
"mortal":
{
// Optional: keccak256 hash of the source file
"keccak256": "0x234...",
// Required (unless "urls" is used): literal contents of the source file
"content": "contract mortal is owned { function kill() { if (msg.sender == owner) selfdestruct(owner); } }"
}
},
// Optional
settings:
{
// Optional: Sorted list of remappings
remappings: [ ":g/dir" ],
// Optional: Optimizer settings
optimizer: {
// disabled by default
enabled: true,
// Optimize for how many times you intend to run the code.
// Lower values will optimize more for initial deployment cost, higher values will optimize more for high-frequency usage.
runs: 200
},
evmVersion: "byzantium", // Version of the EVM to compile for. Affects type checking and code generation. Can be homestead, tangerineWhistle, spuriousDragon, byzantium or constantinople
// Metadata settings (optional)
metadata: {
// Use only literal content and not URLs (false by default)
useLiteralContent: true
},
// Addresses of the libraries. If not all libraries are given here, it can result in unlinked objects whose output data is different.
libraries: {
// The top level key is the the name of the source file where the library is used.
// If remappings are used, this source file should match the global path after remappings were applied.
// If this key is an empty string, that refers to a global level.
"myFile.sol": {
"MyLib": "0x123123..."
}
}
// The following can be used to select desired outputs.
// If this field is omitted, then the compiler loads and does type checking, but will not generate any outputs apart from errors.
// The first level key is the file name and the second is the contract name, where empty contract name refers to the file itself,
// while the star refers to all of the contracts.
//
// The available output types are as follows:
// abi - ABI
// ast - AST of all source files
// legacyAST - legacy AST of all source files
// devdoc - Developer documentation (natspec)
// userdoc - User documentation (natspec)
// metadata - Metadata
// ir - New assembly format before desugaring
// evm.assembly - New assembly format after desugaring
// evm.legacyAssembly - Old-style assembly format in JSON
// evm.bytecode.object - Bytecode object
// evm.bytecode.opcodes - Opcodes list
// evm.bytecode.sourceMap - Source mapping (useful for debugging)
// evm.bytecode.linkReferences - Link references (if unlinked object)
// evm.deployedBytecode* - Deployed bytecode (has the same options as evm.bytecode)
// evm.methodIdentifiers - The list of function hashes
// evm.gasEstimates - Function gas estimates
// ewasm.wast - eWASM S-expressions format (not supported atm)
// ewasm.wasm - eWASM binary format (not supported atm)
//
// Note that using a using `evm`, `evm.bytecode`, `ewasm`, etc. will select every
// target part of that output. Additionally, `*` can be used as a wildcard to request everything.
//
outputSelection: {
// Enable the metadata and bytecode outputs of every single contract.
"*": {
"*": [ "metadata", "evm.bytecode" ]
},
// Enable the abi and opcodes output of MyContract defined in file def.
"def": {
"MyContract": [ "abi", "evm.bytecode.opcodes" ]
},
// Enable the source map output of every single contract.
"*": {
"*": [ "evm.bytecode.sourceMap" ]
},
// Enable the legacy AST output of every single file.
"*": {
"": [ "legacyAST" ]
}
}
}
}
```
* 输出格式说明
```
{
// Optional: not present if no errors/warnings were encountered
errors: [
{
// Optional: Location within the source file.
sourceLocation: {
file: "sourceFile.sol",
start: 0,
end: 100
],
// Mandatory: Error type, such as "TypeError", "InternalCompilerError", "Exception", etc.
// See below for complete list of types.
type: "TypeError",
// Mandatory: Component where the error originated, such as "general", "ewasm", etc.
component: "general",
// Mandatory ("error" or "warning")
severity: "error",
// Mandatory
message: "Invalid keyword"
// Optional: the message formatted with source location
formattedMessage: "sourceFile.sol:100: Invalid keyword"
}
],
// This contains the file-level outputs. In can be limited/filtered by the outputSelection settings.
sources: {
"sourceFile.sol": {
// Identifier (used in source maps)
id: 1,
// The AST object
ast: {},
// The legacy AST object
legacyAST: {}
}
},
// This contains the contract-level outputs. It can be limited/filtered by the outputSelection settings.
contracts: {
"sourceFile.sol": {
// If the language used has no contract names, this field should equal to an empty string.
"ContractName": {
// The Ethereum Contract ABI. If empty, it is represented as an empty array.
// See https://github.com/ethereum/wiki/wiki/Ethereum-Contract-ABI
abi: [],
// See the Metadata Output documentation (serialised JSON string)
metadata: "{...}",
// User documentation (natspec)
userdoc: {},
// Developer documentation (natspec)
devdoc: {},
// Intermediate representation (string)
ir: "",
// EVM-related outputs
evm: {
// Assembly (string)
assembly: "",
// Old-style assembly (object)
legacyAssembly: {},
// Bytecode and related details.
bytecode: {
// The bytecode as a hex string.
object: "00fe",
// Opcodes list (string)
opcodes: "",
// The source mapping as a string. See the source mapping definition.
sourceMap: "",
// If given, this is an unlinked object.
linkReferences: {
"libraryFile.sol": {
// Byte offsets into the bytecode. Linking replaces the 20 bytes located there.
"Library1": [
{ start: 0, length: 20 },
{ start: 200, length: 20 }
]
}
}
},
// The same layout as above.
deployedBytecode: { },
// The list of function hashes
methodIdentifiers: {
"delegate(address)": "5c19a95c"
},
// Function gas estimates
gasEstimates: {
creation: {
codeDepositCost: "420000",
executionCost: "infinite",
totalCost: "infinite"
},
external: {
"delegate(address)": "25000"
},
internal: {
"heavyLifting()": "infinite"
}
}
},
// eWASM related outputs
ewasm: {
// S-expressions format
wast: "",
// Binary format (hex string)
wasm: ""
}
}
}
}
}
```
* 错误类型说明：
1. JSONError：JSON错误，JSON输入不符合要求的格式，例如输入不是JSON对象，不支持语言，等等。
2. IOError：IO错误，IO和导入处理错误，如提供的源中的不可解析URL或hash不匹配。
3. ParserError：语法f分析错误，源代码不符合语言规则。
4. DocstringParsingError：文档解析错误，无法解析注释块中的NATSPEC标记。
5. SytRealError：语法错误，如```continue ```在```for```循环之外使用。
6. DeclarationError：声明错误，无效、不可解析或冲突的标识符名称。例如未找到标识符
7. TypeError：类型错误，如无效类型转换、无效赋值等。
8. UnimplementedFeatureError：编译器不支持该特性，但希望在将来的版本中得到支持。
9. InternalCompilerError：编译器中触发内部错误，这应该作为一个问题来反馈。
10. Exception：例外，编译过程中未知的故障，这应该作为一个问题反馈。
11. CompilerError：编译错误，编译器堆栈的使用无效，这应该作为一个问题来反馈。
12. FatalError：致命错误，这应该作为一个问题来反馈。
13. Warning：警告并没有停止编译，但如果可能的话，应该加以处理。
原文请访问：[solc](http://blog.hubwiz.com/2018/06/14/solc-compiler/)
如果你希望马上开始学习以太坊DApp开发，推荐访问一个在线教程：
* [以太坊智能合约](http://xc.hubwiz.com/course/5a952991adb3847553d205d1?affid=studygolang616)，主要介绍智能合约与dapp应用开发，适合入门。
* [以太坊开发](http://xc.hubwiz.com/course/5abbb7acc02e6b6a59171dd6?affid=studygolang616)，主要是介绍使用node.js、mongodb、区块链、ipfs实现去中心化电商DApp实战，适合进阶。
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