在学习区块链时，我们按照某些文章的教程，使用 Browser-solidity 在 Go-Ethereum上进行智能合约部署时，可能会出现Error: exceeds block gas limit undefined的报错信息，表示当前合约所需的gas超过了区块的最大gas。这可能是由于创世区块的配置文件genesis.json中的参数gasLimit设置过小有关，以下是可能的解决办法。
关于区块gasLimit的介绍
查询原因
这些文章中提供的创世区块配置文件genesis.json中的参数gasLimit的值为0x2fefd8，这是一个16进制的数字，转换为十进制为3141592。进制在线转换工具。
我们使用此配置初始化，然后输入miner.start()挖矿，等待一段时间后输入miner.stop()停止挖矿。
先查看区块数：
> eth.blockNumber
132
随便查看一个区块的信息：
> eth.getBlock(132)
{
difficulty: 138787,
extraData: "0xd783010801846765746886676f312e31308664617277696e",
gasLimit: 3573388,
gasUsed: 0,
hash: "0x6f7d84c21967eafe67f13f6d7abc2143ddee006e93999c890990aad63e9a4fc5",
logsBloom: "0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
miner: "0xf2e4cf8222e79b579543a5708ff0dfd1e6e37cfe",
mixHash: "0x76d1f5e34735baa2491664025fadaa8376780df3e67125eff281736a27d57e8f",
nonce: "0x090925fd3caf31cd",
number: 132,
parentHash: "0x92462eaf6fbbd80445724d5fde82794e3c0af56d348b2edfb7ce1e46ecefae39",
receiptsRoot: "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421",
sha3Uncles: "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
size: 536,
stateRoot: "0xfa9ea92f0ad0545079e4f0bed40a08632a24c2693ac57b6b2242ae122b6356b0",
timestamp: 1519481595,
totalDifficulty: 17930276,
transactions: [],
transactionsRoot: "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421",
uncles: []
}
可以看到此区块的gasLimit为3573388。
然后再通过Browser-solidity编写智能合约，然后编译生成web3代码：
var testcontractContract = web3.eth.contract([{"constant":false,"inputs":[{"name":"a","type":"uint256"}],"name":"add","outputs":[{"name":"b","type":"uint256"}],"payable":false,"stateMutability":"nonpayable","type":"function"}]);
var testcontract = testcontractContract.new(
{
from: web3.eth.accounts[0],
data: '0x6060604052341561000f57600080fd5b60b68061001d6000396000f300606060405260043610603f576000357c0100000000000000000000000000000000000000000000000000000000900463ffffffff1680631003e2d2146044575b600080fd5b3415604e57600080fd5b606260048080359060200190919050506078565b6040518082815260200191505060405180910390f35b600080606483019050809150509190505600a165627a7a723058208f5727572e0f6113593417a5211692c99ed69158e62764f4e3f76c0c5846afc30029',
gas: '4700000'
}, function (e, contract){
console.log(e, contract);
if (typeof contract.address !== 'undefined') {
console.log('Contract mined! address: ' + contract.address + ' transactionHash: ' + contract.transactionHash);
}
})
可以看到此合约所需的gas为4700000，已经比区块的gasLimit还高，部署时当然会失败。这可能由于之前通过Browser-solidity编译出来的gas普遍只有几十万，而现在已经需要几百万的gas。
解决办法
将创世区块的配置文件genesis.json中的参数gasLimit设置为0xffffffff（十进制值为4294967295）。重新初始化，生成区块，查看区块信息。
> eth.blockNumber
20
> eth.getBlock(20)
{
difficulty: 132288,
extraData: "0xd783010801846765746886676f312e31308664617277696e",
gasLimit: 4211854946,
gasUsed: 0,
hash: "0x98a230aa2d7aaee1b92732488aff3571d9569b44ee2c52bd2a47cc115649ba1d",
logsBloom: "0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
miner: "0xb6cd75af6594f46374378cf3a7d9cbfc06485994",
mixHash: "0xef93a1be29bbabcf7e6a9d2031b2b76b224547c77aa4f5a6d5edaa1ed538e428",
nonce: "0x756c86cc86699021",
number: 20,
parentHash: "0x62dacb00b038677688e70da047f5074038caa8ac16d161118a06b8a053789bda",
receiptsRoot: "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421",
sha3Uncles: "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
size: 536,
stateRoot: "0xc74d4094304b408ac107955a721c1a45393687672cf1ab40761c1bf37cc5efff",
timestamp: 1519531846,
totalDifficulty: 2764672,
transactions: [],
transactionsRoot: "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421",
uncles: []
}
可以看到此区块的gasLimit为4211854946。这时再部署就不会报错了。
遗留问题
我在网上搜索解决方法时，有看到类似的问题A problem about gas limit #3760，解答者表示可以使用--targetgaslimit这个指令来修改区块的gas limit，不过我试了很久一直提示指令错误，也没有搜索到这个指令应该怎么用，我是这样用的：
> eth.blockNumber
132
> --target 132 gaslimit 8888888
(anonymous): Line 1:10 Unexpected number (and 1 more errors)
提示错误。有大神如果知道的话请留言告诉我，谢谢！
已解决。使用--targetgaslimit value就可以设置最低gas限制。value传gas最低值，默认4712388。