Geth comes bundled with a choice of tracers that can be invoked via the [tracing API](/docs/interacting-with-geth/rpc/ns-debug). Some of these built-in tracers are implemented natively in Go, and others in Javascript. The default tracer is the opcode logger (otherwise known as struct logger) which is the default tracer for all the methods. Other tracers have to be specified by passing their name to the `tracer` parameter in the API call.
The struct logger (aka opcode logger) is a native Go tracer which executes a transaction and emits the opcode and execution context at every step. This is the tracer that will be used when no name is passed to the API, e.g. `debug.traceTransaction(<txhash>)`. The following information is emitted at each step:
| returnData | []byte | Last call's return data. Enabled via `enableReturnData` |
| storage | map[hash]hash | Storage slots of current contract read from and written to. Only emitted for `SLOAD` and `SSTORE`. Disabled via `disableStorage` |
Note that the fields `memory`, `stack`, `returnData`, and `storage` have dynamic size and depending on the exact transaction they could grow large in size. This is specially true for `memory` which could blow up the trace size. It is recommended to keep them disabled unless they are explicitly required for a given use case.
It is also possible to configure the trace by passing Boolean (true/false) values for four parameters that tweak the verbosity of the trace. By default, the _EVM memory_ and _Return data_ are not reported but the _EVM stack_ and _EVM storage_ are. To report the maximum amount of data:
The following tracers are implement in Go. This means they are much more performant than other tracers that are written in Javascript. The tracers are selected by passing their name to the `tracer` parameter when invoking a tracing API method, e.g. `debug.traceTransaction(<txhash>, { tracer: 'callTracer' })`.
[addressed](https://docs.soliditylang.org/en/develop/abi-spec.html#function-selector) using the first four byte of the Keccak-256 hash of their signature. Therefore when calling the function of a contract, the caller must send this function selector as well as the ABI-encoded arguments as call data.
The `4byteTracer` collects the function selectors of every function executed in the lifetime of a transaction, along with the size of the supplied call data. The result is a `map[string]int` where the keys are `SELECTOR-CALLDATASIZE` and the values are number of occurrences of this key. For example:
The `callTracer` tracks all the call frames executed during a transaction, including depth 0. The result will be a nested list of call frames, resembling how EVM works. They form a tree with the top-level call at root and sub-calls as children of the higher levels. Each call frame has the following fields:
-`onlyTopCall: true` instructs the tracer to only process the main (top-level) call and none of the sub-calls. This avoids extra processing for each call frame if only the top-level call info are required.
-`withLog: true` instructs the tracer to also collect the logs emitted during each call.
The prestate tracer has two modes: `prestate` and `diff`. The `prestate` mode returns the accounts necessary to execute a given transaction. `diff` mode returns the differences between the transaction's pre and post-state (i.e. what changed because the transaction happened). The `prestateTracer` defaults to `prestate` mode. It reexecutes the given transaction and tracks every part of state that is touched. This is similar to the concept of a [stateless witness](https://ethresear.ch/t/the-stateless-client-concept/172), the difference being this tracer doesn't return any cryptographic proof, rather only the trie leaves. The result is an object. The keys are addresses of accounts. The value is an object with the following fields:
There are also a set of tracers written in Javascript. These are less performant than the Go native tracers because of overheads associated with interpreting the Javascript in Geth's Go environment.
`trigramTracer` counts the opcode trigrams. Trigrams are the possible combinations of three opcodes this tracer reports how many times each combination is seen during execution.
It is possible to give temporary state modifications to Geth in order to simulate the effects of `eth_call`. For example, some new bytecode could be deployed to some address _temporarily just for the duration of the execution_ and then a transaction interacting with that address can be traced. This can be used for scenario testing or determining the outcome of some hypothetical transaction before executing for real.
This page showed how to use the tracers that come bundled with Geth. There are a set written in Go and a set written in Javascript. They are invoked by passing their names when calling an API method. State overrides can be used in combination with tracers to examine precisely what the EVM will do in some hypothetical scenarios.
