## Customizing config.yaml
Echidna uses config files formatted in the [yaml syntax][yaml_syntax]. You can find a [sample config.yaml file here][sample_yaml].


[yaml_syntax]:https://en.wikipedia.org/wiki/YAML#Syntax
[sample_yaml]:https://github.com/trailofbits/echidna/blob/master/solidity/config.yaml

### Default configuration values



| Parameter | Default Value | Description |
|-----------|---------------|-------------|
| `solcArgs`      |  Empty string | Additional arguments to pass to solc. Internally echidna passes `--combined-json=bin-runtime,bin,srcmap,srcmap-runtime,abi,ast`. Avoid this unless you know what you're doing.
| `epochs`        |  2 | Number of iterations the genetic algorithm for coverage guided fuzzing executes. Only used if `printCoverage` is true or `--coverage` is passed on the command line.
| `range`         |  10 | Max number of function calls in a single test
| `contractAddr`  |  0x00a329c0648769a73afac7f9381e08fb43dbea72 | The address of the contract under test
| `sender`        |  0x00a329c0648769a73afac7f9381e08fb43dbea70 | The address from which echidna sends transaction
| `addrList`      |  Empty List | A predefined list of addresses Echidna will use for functions accepting parameters of the address type
| `gasLimit`      |  0xffffffffffffffff | The maximum amount of gas to expend before erroring.
| `testLimit`     | 10000 | The maximum number of tests to execute
| `shrinkLimit`   | 1000 | The maximum number of shrinks to attempt
| `return`        |  Success | Specifies the behavior of Echidna test functions. For a description of values see the "return" Parameters section.
| `prefix`        |  "echidna_" | The function name prefix that identifies echidna tests.
| `printCoverage` |  False | Setting this to true has the same effect as the `--coverage` command line switch
| `outputJson`    |  False | Setting this to true formats the output of echidna as JSON.



## `return` Parameters
|Value | Meaning|
|-|-|
| `Success`        | Tests return true to indicate test success
| `Fail`       | Tests return false to indicate test success
| `Throw`            | Tests revert to indicate test success
| `Fail or Throw` | Tests should revert or return false to indicate test success