Documentation ¶
Index ¶
- Constants
- Variables
- func AccumulateRewards(statedb *state.StateDB, block *types.Block)
- func AddressFromMessage(msg Message) common.Address
- func ApplyMessage(env vm.Environment, msg Message, coinbase *state.StateObject) ([]byte, *big.Int, error)
- func CalcDifficulty(block, parent *types.Header) *big.Int
- func CalcGasLimit(parent *types.Block) *big.Int
- func CalcTD(block, parent *types.Block) *big.Int
- func Disassemble(script []byte) (asm []string)
- func GenesisBlock(nonce uint64, db common.Database) *types.Block
- func IntrinsicGas(msg Message) *big.Int
- func IsBlockNonceErr(err error) bool
- func IsInvalidTxErr(err error) bool
- func IsKnownBlockErr(e error) bool
- func IsNonceErr(err error) bool
- func IsOutOfGasErr(err error) bool
- func IsParentErr(err error) bool
- func IsTDError(e error) bool
- func IsUncleErr(err error) bool
- func IsValidationErr(err error) bool
- func IsValueTransferErr(e error) bool
- func MakeBlock(bman *BlockProcessor, parent *types.Block, i int, db common.Database, seed int) *types.Block
- func MakeChain(bman *BlockProcessor, parent *types.Block, max int, db common.Database, ...) types.Blocks
- func MessageCreatesContract(msg Message) bool
- func MessageGasValue(msg Message) *big.Int
- func NewBlockFromParent(addr common.Address, parent *types.Block) *types.Block
- func ParentError(hash common.Hash) error
- func UncleError(format string, v ...interface{}) error
- func ValidateHeader(pow pow.PoW, block, parent *types.Header, checkPow bool) error
- type AccountChange
- type Backend
- type BlockCache
- type BlockNonceErr
- type BlockProcessor
- func (self *BlockProcessor) ApplyTransaction(coinbase *state.StateObject, statedb *state.StateDB, block *types.Block, ...) (*types.Receipt, *big.Int, error)
- func (self *BlockProcessor) ApplyTransactions(coinbase *state.StateObject, statedb *state.StateDB, block *types.Block, ...) (types.Receipts, error)
- func (self *BlockProcessor) ChainManager() *ChainManager
- func (sm *BlockProcessor) GetBlockReceipts(bhash common.Hash) (receipts types.Receipts, err error)
- func (sm *BlockProcessor) GetLogs(block *types.Block) (logs state.Logs, err error)
- func (sm *BlockProcessor) Process(block *types.Block) (logs state.Logs, err error)
- func (sm *BlockProcessor) RetryProcess(block *types.Block) (logs state.Logs, err error)
- func (sm *BlockProcessor) TransitionState(statedb *state.StateDB, parent, block *types.Block, transientProcess bool) (receipts types.Receipts, err error)
- func (sm *BlockProcessor) VerifyUncles(statedb *state.StateDB, block, parent *types.Block) error
- type ChainEvent
- type ChainHeadEvent
- type ChainManager
- func (self *ChainManager) CalcTotalDiff(block *types.Block) (*big.Int, error)
- func (self *ChainManager) CurrentBlock() *types.Block
- func (self *ChainManager) Export(w io.Writer) error
- func (self *ChainManager) ExportN(w io.Writer, first uint64, last uint64) error
- func (self *ChainManager) GasLimit() *big.Int
- func (bc *ChainManager) Genesis() *types.Block
- func (self *ChainManager) GetAncestors(block *types.Block, length int) (blocks []*types.Block)
- func (self *ChainManager) GetBlock(hash common.Hash) *types.Block
- func (self *ChainManager) GetBlockByNumber(num uint64) *types.Block
- func (self *ChainManager) GetBlockHashesFromHash(hash common.Hash, max uint64) (chain []common.Hash)
- func (self *ChainManager) GetUnclesInChain(block *types.Block, length int) (uncles []*types.Header)
- func (bc *ChainManager) HasBlock(hash common.Hash) bool
- func (self *ChainManager) InsertChain(chain types.Blocks) (int, error)
- func (self *ChainManager) LastBlockHash() common.Hash
- func (bc *ChainManager) NewBlock(coinbase common.Address) *types.Block
- func (bc *ChainManager) Reset()
- func (bc *ChainManager) ResetWithGenesisBlock(gb *types.Block)
- func (bc *ChainManager) SetHead(head *types.Block)
- func (self *ChainManager) SetProcessor(proc types.BlockProcessor)
- func (self *ChainManager) State() *state.StateDB
- func (self *ChainManager) Status() (td *big.Int, currentBlock common.Hash, genesisBlock common.Hash)
- func (bc *ChainManager) Stop()
- func (self *ChainManager) Td() *big.Int
- func (self *ChainManager) TransState() *state.StateDB
- type ChainSideEvent
- type ChainSplitEvent
- type ChainUncleEvent
- type Execution
- type FakePow
- type Filter
- func (self *Filter) FilterLogs(logs state.Logs) state.Logs
- func (self *Filter) Find() state.Logs
- func (self *Filter) SetAddress(addr []common.Address)
- func (self *Filter) SetEarliestBlock(earliest int64)
- func (self *Filter) SetLatestBlock(latest int64)
- func (self *Filter) SetMax(max int)
- func (self *Filter) SetSkip(skip int)
- func (self *Filter) SetTopics(topics [][]common.Hash)
- type GasPriceChanged
- type InvalidTxErr
- type KnownBlockError
- type Message
- type NewBlockEvent
- type NewMinedBlockEvent
- type NonceErr
- type OutOfGasErr
- type ParentErr
- type PendingBlockEvent
- type StartMining
- type StateTransition
- func (self *StateTransition) AddGas(amount *big.Int)
- func (self *StateTransition) BuyGas() error
- func (self *StateTransition) Coinbase() *state.StateObject
- func (self *StateTransition) From() (*state.StateObject, error)
- func (self *StateTransition) To() *state.StateObject
- func (self *StateTransition) UseGas(amount *big.Int) error
- type TDError
- type TopMining
- type TxPool
- func (self *TxPool) Add(tx *types.Transaction) error
- func (self *TxPool) AddTransactions(txs []*types.Transaction)
- func (self *TxPool) GetQueuedTransactions() types.Transactions
- func (tp *TxPool) GetTransaction(hash common.Hash) *types.Transaction
- func (self *TxPool) GetTransactions() (txs types.Transactions)
- func (self *TxPool) RemoveTransactions(txs types.Transactions)
- func (pool *TxPool) State() *state.ManagedState
- func (pool *TxPool) Stop()
- type TxPostEvent
- type TxPreEvent
- type UncleErr
- type VMEnv
- func (self *VMEnv) AddLog(log *state.Log)
- func (self *VMEnv) AddStructLog(log vm.StructLog)
- func (self *VMEnv) BlockNumber() *big.Int
- func (self *VMEnv) Call(me vm.ContextRef, addr common.Address, data []byte, gas, price, value *big.Int) ([]byte, error)
- func (self *VMEnv) CallCode(me vm.ContextRef, addr common.Address, data []byte, gas, price, value *big.Int) ([]byte, error)
- func (self *VMEnv) Coinbase() common.Address
- func (self *VMEnv) Create(me vm.ContextRef, data []byte, gas, price, value *big.Int) ([]byte, error, vm.ContextRef)
- func (self *VMEnv) Depth() int
- func (self *VMEnv) Difficulty() *big.Int
- func (self *VMEnv) GasLimit() *big.Int
- func (self *VMEnv) GetHash(n uint64) common.Hash
- func (self *VMEnv) Origin() common.Address
- func (self *VMEnv) SetDepth(i int)
- func (self *VMEnv) SetVmType(t vm.Type)
- func (self *VMEnv) State() *state.StateDB
- func (self *VMEnv) StructLogs() []vm.StructLog
- func (self *VMEnv) Time() int64
- func (self *VMEnv) Transfer(from, to vm.Account, amount *big.Int) error
- func (self *VMEnv) Value() *big.Int
- func (self *VMEnv) VmType() vm.Type
- type ValidationErr
- type ValueTransferError
Constants ¶
const ( // must be bumped when consensus algorithm is changed, this forces the upgradedb // command to be run (forces the blocks to be imported again using the new algorithm) BlockChainVersion = 3 )
Variables ¶
var ( CanonicalSeed = 1 ForkSeed = 2 )
So we can deterministically seed different blockchains
var ( BlockNumberErr = errors.New("block number invalid") BlockFutureErr = errors.New("block time is in the future") BlockEqualTSErr = errors.New("block time stamp equal to previous") )
var ( // Transaction Pool Errors ErrInvalidSender = errors.New("Invalid sender") ErrNonce = errors.New("Nonce too low") ErrCheap = errors.New("Gas price too low for acceptance") ErrBalance = errors.New("Insufficient balance") ErrNonExistentAccount = errors.New("Account does not exist or account balance too low") ErrInsufficientFunds = errors.New("Insufficient funds for gas * price + value") ErrIntrinsicGas = errors.New("Intrinsic gas too low") ErrGasLimit = errors.New("Exceeds block gas limit") ErrNegativeValue = errors.New("Negative value") )
var BadHashes = map[common.Hash]bool{ common.HexToHash("f269c503aed286caaa0d114d6a5320e70abbc2febe37953207e76a2873f2ba79"): true, common.HexToHash("38f5bbbffd74804820ffa4bab0cd540e9de229725afb98c1a7e57936f4a714bc"): true, common.HexToHash("7064455b364775a16afbdecd75370e912c6e2879f202eda85b9beae547fff3ac"): true, common.HexToHash("5b7c80070a6eff35f3eb3181edb023465c776d40af2885571e1bc4689f3a44d8"): true, }
Set of manually tracked bad hashes (usually hard forks)
var BlockReward *big.Int = big.NewInt(1.5e+18)
var GenesisAccounts = []byte(`{
"0000000000000000000000000000000000000001": {"balance": "1"},
"0000000000000000000000000000000000000002": {"balance": "1"},
"0000000000000000000000000000000000000003": {"balance": "1"},
"0000000000000000000000000000000000000004": {"balance": "1"},
"dbdbdb2cbd23b783741e8d7fcf51e459b497e4a6": {"balance": "1606938044258990275541962092341162602522202993782792835301376"},
"e4157b34ea9615cfbde6b4fda419828124b70c78": {"balance": "1606938044258990275541962092341162602522202993782792835301376"},
"b9c015918bdaba24b4ff057a92a3873d6eb201be": {"balance": "1606938044258990275541962092341162602522202993782792835301376"},
"6c386a4b26f73c802f34673f7248bb118f97424a": {"balance": "1606938044258990275541962092341162602522202993782792835301376"},
"cd2a3d9f938e13cd947ec05abc7fe734df8dd826": {"balance": "1606938044258990275541962092341162602522202993782792835301376"},
"2ef47100e0787b915105fd5e3f4ff6752079d5cb": {"balance": "1606938044258990275541962092341162602522202993782792835301376"},
"e6716f9544a56c530d868e4bfbacb172315bdead": {"balance": "1606938044258990275541962092341162602522202993782792835301376"},
"1a26338f0d905e295fccb71fa9ea849ffa12aaf4": {"balance": "1606938044258990275541962092341162602522202993782792835301376"}
}`)
var ZeroHash160 = make([]byte, 20)
var ZeroHash256 = make([]byte, 32)
var ZeroHash512 = make([]byte, 64)
Functions ¶
func AccumulateRewards ¶ added in v0.9.20
func AddressFromMessage ¶
func ApplyMessage ¶ added in v0.9.20
func ApplyMessage(env vm.Environment, msg Message, coinbase *state.StateObject) ([]byte, *big.Int, error)
func Disassemble ¶
func GenesisBlock ¶ added in v0.8.4
func IntrinsicGas ¶ added in v0.9.20
func IsBlockNonceErr ¶ added in v0.9.32
IsBlockNonceErr returns true for invalid block nonce errors.
func IsInvalidTxErr ¶ added in v0.9.20
func IsKnownBlockErr ¶
func IsNonceErr ¶
func IsOutOfGasErr ¶
func IsParentErr ¶
func IsUncleErr ¶
func IsValidationErr ¶
func IsValueTransferErr ¶ added in v0.9.20
func MessageCreatesContract ¶
func MessageGasValue ¶
func NewBlockFromParent ¶ added in v0.9.20
Utility functions for making chains on the fly Exposed for sake of testing from other packages (eg. go-ethash)
func ParentError ¶
func UncleError ¶
Types ¶
type AccountChange ¶
type AccountChange struct {
Address, StateAddress []byte
}
type Backend ¶ added in v0.8.4
type Backend interface { AccountManager() *accounts.Manager BlockProcessor() *BlockProcessor ChainManager() *ChainManager TxPool() *TxPool BlockDb() common.Database StateDb() common.Database EventMux() *event.TypeMux }
TODO move this to types?
type BlockCache ¶ added in v0.9.20
type BlockCache struct {
// contains filtered or unexported fields
}
BlockCache implements a caching mechanism specifically for blocks and uses FILO to pop
func NewBlockCache ¶ added in v0.9.20
func NewBlockCache(size int) *BlockCache
Creates and returns a `BlockCache` with `size`. If `size` is smaller than 1 it will panic
func (*BlockCache) Clear ¶ added in v0.9.20
func (bc *BlockCache) Clear()
func (*BlockCache) Delete ¶ added in v0.9.20
func (bc *BlockCache) Delete(hash common.Hash)
func (*BlockCache) Push ¶ added in v0.9.20
func (bc *BlockCache) Push(block *types.Block)
type BlockNonceErr ¶ added in v0.9.32
BlockNonceErr indicates that a block's nonce is invalid.
func (*BlockNonceErr) Error ¶ added in v0.9.32
func (err *BlockNonceErr) Error() string
type BlockProcessor ¶ added in v0.8.4
type BlockProcessor struct { // Proof of work used for validating Pow pow.PoW // contains filtered or unexported fields }
func NewBlockProc ¶ added in v0.9.20
func NewBlockProc(db common.Database, cman *ChainManager, eventMux *event.TypeMux) *BlockProcessor
func NewBlockProcessor ¶ added in v0.8.4
func NewBlockProcessor(db, extra common.Database, pow pow.PoW, chainManager *ChainManager, eventMux *event.TypeMux) *BlockProcessor
func NewCanonical ¶ added in v0.9.20
func NewCanonical(n int, db common.Database) (*BlockProcessor, error)
func (*BlockProcessor) ApplyTransaction ¶ added in v0.8.4
func (*BlockProcessor) ApplyTransactions ¶ added in v0.8.4
func (self *BlockProcessor) ApplyTransactions(coinbase *state.StateObject, statedb *state.StateDB, block *types.Block, txs types.Transactions, transientProcess bool) (types.Receipts, error)
func (*BlockProcessor) ChainManager ¶ added in v0.9.20
func (self *BlockProcessor) ChainManager() *ChainManager
func (*BlockProcessor) GetBlockReceipts ¶ added in v0.9.23
GetBlockReceipts returns the receipts beloniging to the block hash
func (*BlockProcessor) GetLogs ¶ added in v0.8.4
GetLogs returns the logs of the given block. This method is using a two step approach where it tries to get it from the (updated) method which gets them from the receipts or the depricated way by re-processing the block.
func (*BlockProcessor) Process ¶ added in v0.8.4
Process block will attempt to process the given block's transactions and applies them on top of the block's parent state (given it exists) and will return wether it was successful or not.
func (*BlockProcessor) RetryProcess ¶ added in v0.9.20
func (*BlockProcessor) TransitionState ¶ added in v0.8.4
func (*BlockProcessor) VerifyUncles ¶ added in v0.9.20
type ChainEvent ¶ added in v0.8.4
type ChainHeadEvent ¶ added in v0.9.20
type ChainManager ¶
type ChainManager struct {
// contains filtered or unexported fields
}
func NewChainMan ¶ added in v0.9.20
func NewChainManager ¶
func (*ChainManager) CalcTotalDiff ¶
func (*ChainManager) CurrentBlock ¶
func (self *ChainManager) CurrentBlock() *types.Block
func (*ChainManager) Export ¶
func (self *ChainManager) Export(w io.Writer) error
Export writes the active chain to the given writer.
func (*ChainManager) ExportN ¶ added in v0.9.32
ExportN writes a subset of the active chain to the given writer.
func (*ChainManager) GasLimit ¶ added in v0.9.20
func (self *ChainManager) GasLimit() *big.Int
func (*ChainManager) GetAncestors ¶ added in v0.8.4
func (*ChainManager) GetBlockByNumber ¶
func (self *ChainManager) GetBlockByNumber(num uint64) *types.Block
func (*ChainManager) GetBlockHashesFromHash ¶ added in v0.8.4
func (*ChainManager) GetUnclesInChain ¶ added in v0.8.4
func (*ChainManager) HasBlock ¶
func (bc *ChainManager) HasBlock(hash common.Hash) bool
Block fetching methods
func (*ChainManager) InsertChain ¶
func (self *ChainManager) InsertChain(chain types.Blocks) (int, error)
InsertChain will attempt to insert the given chain in to the canonical chain or, otherwise, create a fork. It an error is returned it will return the index number of the failing block as well an error describing what went wrong (for possible errors see core/errors.go).
func (*ChainManager) LastBlockHash ¶
func (self *ChainManager) LastBlockHash() common.Hash
func (*ChainManager) NewBlock ¶
func (bc *ChainManager) NewBlock(coinbase common.Address) *types.Block
Block creation & chain handling
func (*ChainManager) Reset ¶
func (bc *ChainManager) Reset()
func (*ChainManager) ResetWithGenesisBlock ¶ added in v0.9.20
func (bc *ChainManager) ResetWithGenesisBlock(gb *types.Block)
func (*ChainManager) SetHead ¶ added in v0.9.20
func (bc *ChainManager) SetHead(head *types.Block)
func (*ChainManager) SetProcessor ¶
func (self *ChainManager) SetProcessor(proc types.BlockProcessor)
func (*ChainManager) State ¶
func (self *ChainManager) State() *state.StateDB
func (*ChainManager) Stop ¶
func (bc *ChainManager) Stop()
func (*ChainManager) Td ¶
func (self *ChainManager) Td() *big.Int
func (*ChainManager) TransState ¶
func (self *ChainManager) TransState() *state.StateDB
type ChainSideEvent ¶ added in v0.9.20
type ChainSplitEvent ¶ added in v0.8.4
ChainSplit is posted when a new head is detected
type ChainUncleEvent ¶ added in v0.9.20
type Execution ¶
func NewExecution ¶
func (*Execution) Create ¶
func (self *Execution) Create(caller vm.ContextRef) (ret []byte, err error, account *state.StateObject)
type FakePow ¶ added in v0.9.20
type FakePow struct{}
So we can generate blocks easily
func (FakePow) GetHashrate ¶ added in v0.9.20
type Filter ¶
type Filter struct { BlockCallback func(*types.Block, state.Logs) TransactionCallback func(*types.Transaction) LogsCallback func(state.Logs) // contains filtered or unexported fields }
Filtering interface
func NewFilter ¶
Create a new filter which uses a bloom filter on blocks to figure out whether a particular block is interesting or not.
func (*Filter) FilterLogs ¶ added in v0.8.4
func (*Filter) SetAddress ¶ added in v0.8.4
func (*Filter) SetEarliestBlock ¶
Set the earliest and latest block for filtering. -1 = latest block (i.e., the current block) hash = particular hash from-to
func (*Filter) SetLatestBlock ¶
type GasPriceChanged ¶ added in v0.9.20
type InvalidTxErr ¶ added in v0.9.20
type InvalidTxErr struct {
Message string
}
func InvalidTxError ¶ added in v0.9.20
func InvalidTxError(err error) *InvalidTxErr
func (*InvalidTxErr) Error ¶ added in v0.9.20
func (err *InvalidTxErr) Error() string
type KnownBlockError ¶
type KnownBlockError struct {
// contains filtered or unexported fields
}
func (*KnownBlockError) Error ¶
func (self *KnownBlockError) Error() string
type Message ¶
type Message interface { From() (common.Address, error) To() *common.Address GasPrice() *big.Int Gas() *big.Int Value() *big.Int Nonce() uint64 Data() []byte }
Message represents a message sent to a contract.
type NewBlockEvent ¶
NewBlockEvent is posted when a block has been imported.
type NewMinedBlockEvent ¶ added in v0.8.4
NewMinedBlockEvent is posted when a block has been imported.
type NonceErr ¶
func NonceError ¶
type OutOfGasErr ¶
type OutOfGasErr struct {
Message string
}
func OutOfGasError ¶
func OutOfGasError() *OutOfGasErr
func (*OutOfGasErr) Error ¶
func (self *OutOfGasErr) Error() string
type ParentErr ¶
type ParentErr struct {
Message string
}
Parent error. In case a parent is unknown this error will be thrown by the block manager
type PendingBlockEvent ¶ added in v0.8.4
type StateTransition ¶
type StateTransition struct {
// contains filtered or unexported fields
}
* The State transitioning model * * A state transition is a change made when a transaction is applied to the current world state * The state transitioning model does all all the necessary work to work out a valid new state root. * 1) Nonce handling * 2) Pre pay / buy gas of the coinbase (miner) * 3) Create a new state object if the recipient is \0*32 * 4) Value transfer * == If contract creation == * 4a) Attempt to run transaction data * 4b) If valid, use result as code for the new state object * == end == * 5) Run Script section * 6) Derive new state root
func NewStateTransition ¶
func NewStateTransition(env vm.Environment, msg Message, coinbase *state.StateObject) *StateTransition
func (*StateTransition) AddGas ¶
func (self *StateTransition) AddGas(amount *big.Int)
func (*StateTransition) BuyGas ¶
func (self *StateTransition) BuyGas() error
func (*StateTransition) Coinbase ¶
func (self *StateTransition) Coinbase() *state.StateObject
func (*StateTransition) From ¶
func (self *StateTransition) From() (*state.StateObject, error)
func (*StateTransition) To ¶
func (self *StateTransition) To() *state.StateObject
type TxPool ¶
type TxPool struct {
// contains filtered or unexported fields
}
TxPool contains all currently known transactions. Transactions enter the pool when they are received from the network or submitted locally. They exit the pool when they are included in the blockchain.
The pool separates processable transactions (which can be applied to the current state) and future transactions. Transactions move between those two states over time as they are received and processed.
func (*TxPool) Add ¶
func (self *TxPool) Add(tx *types.Transaction) error
Add queues a single transaction in the pool if it is valid.
func (*TxPool) AddTransactions ¶ added in v0.8.4
func (self *TxPool) AddTransactions(txs []*types.Transaction)
AddTransactions attempts to queue all valid transactions in txs.
func (*TxPool) GetQueuedTransactions ¶ added in v0.9.20
func (self *TxPool) GetQueuedTransactions() types.Transactions
GetQueuedTransactions returns all non-processable transactions.
func (*TxPool) GetTransaction ¶ added in v0.9.20
func (tp *TxPool) GetTransaction(hash common.Hash) *types.Transaction
GetTransaction returns a transaction if it is contained in the pool and nil otherwise.
func (*TxPool) GetTransactions ¶ added in v0.8.4
func (self *TxPool) GetTransactions() (txs types.Transactions)
GetTransactions returns all currently processable transactions. The returned slice may be modified by the caller.
func (*TxPool) RemoveTransactions ¶ added in v0.9.20
func (self *TxPool) RemoveTransactions(txs types.Transactions)
RemoveTransactions removes all given transactions from the pool.
func (*TxPool) State ¶ added in v0.9.32
func (pool *TxPool) State() *state.ManagedState
type TxPostEvent ¶
type TxPostEvent struct{ Tx *types.Transaction }
TxPostEvent is posted when a transaction has been processed.
type TxPreEvent ¶
type TxPreEvent struct{ Tx *types.Transaction }
TxPreEvent is posted when a transaction enters the transaction pool.
type VMEnv ¶
type VMEnv struct {
// contains filtered or unexported fields
}
func (*VMEnv) AddStructLog ¶ added in v0.9.32
func (*VMEnv) BlockNumber ¶
func (*VMEnv) Create ¶
func (self *VMEnv) Create(me vm.ContextRef, data []byte, gas, price, value *big.Int) ([]byte, error, vm.ContextRef)
func (*VMEnv) Difficulty ¶
func (*VMEnv) StructLogs ¶ added in v0.9.32
type ValidationErr ¶
type ValidationErr struct {
Message string
}
Block validation error. If any validation fails, this error will be thrown
func ValidationError ¶
func ValidationError(format string, v ...interface{}) *ValidationErr
func (*ValidationErr) Error ¶
func (err *ValidationErr) Error() string
type ValueTransferError ¶ added in v0.9.20
type ValueTransferError struct {
// contains filtered or unexported fields
}
func ValueTransferErr ¶ added in v0.9.20
func ValueTransferErr(str string, v ...interface{}) *ValueTransferError
func (*ValueTransferError) Error ¶ added in v0.9.20
func (self *ValueTransferError) Error() string