# Logging

The driver’s logging system uses `stderr` by default and the log level `CASS_LOG_WARN`. Both of these settings can be changed using the driver’s `cass_log_*()` configuration functions.

**Important**: Only setting the log level before any other interaction with the driver’s API is fully supported. Support for altering the log level during the driver’s operation is experimental, might misbehave (by displaying more or fewer logs than expected), and may be removed in the future.

## Log Level

To update the log level use `cass_log_set_level()`. Pass `CASS_LOG_DISABLED` to silence the driver completely.

Set the level once, before calling any other driver function. Setting it later, or more than once, is experimental - see the notice above. Note also that events emitted before a custom logging callback is installed are handled by the default one, i.e. printed to `stderr`.

```c
cass_log_set_level(CASS_LOG_INFO);

/* Create cluster and connect session */
```

## Custom Logging Callback

The use of a logging callback allows an application to log messages to a file, syslog, or any other logging mechanism. This callback must be thread-safe because it is possible for it to be called from multiple threads concurrently. The `data` parameter allows custom resources to be passed to the logging callback.

```c
void on_log(const CassLogMessage* message, void* data) {
  /* Handle logging */
}

int main() {
  void* log_data = NULL /* Custom log resource */;
  cass_log_set_callback(on_log, log_data);
  cass_log_set_level(CASS_LOG_INFO);

  /* Create cluster and connect session */

}
```