--- layout: docu title: Replacement Scans --- The replacement scan API can be used to register a callback that is called when a table is read that does not exist in the catalog. For example, when a query such as `SELECT * FROM my_table` is executed and `my_table` does not exist, the replacement scan callback will be called with `my_table` as parameter. The replacement scan can then insert a table function with a specific parameter to replace the read of the table. ## API Reference Overview

void duckdb_add_replacement_scan(duckdb_database db, duckdb_replacement_callback_t replacement, void *extra_data, duckdb_delete_callback_t delete_callback);
void duckdb_replacement_scan_set_function_name(duckdb_replacement_scan_info info, const char *function_name);
void duckdb_replacement_scan_add_parameter(duckdb_replacement_scan_info info, duckdb_value parameter);
void duckdb_replacement_scan_set_error(duckdb_replacement_scan_info info, const char *error);

#### `duckdb_add_replacement_scan` Add a replacement scan definition to the specified database. ##### Syntax

void duckdb_add_replacement_scan(
  duckdb_database db,
  duckdb_replacement_callback_t replacement,
  void *extra_data,
  duckdb_delete_callback_t delete_callback
);

##### Parameters * `db`: The database object to add the replacement scan to * `replacement`: The replacement scan callback * `extra_data`: Extra data that is passed back into the specified callback * `delete_callback`: The delete callback to call on the extra data, if any
#### `duckdb_replacement_scan_set_function_name` Sets the replacement function name. If this function is called in the replacement callback, the replacement scan is performed. If it is not called, the replacement callback is not performed. ##### Syntax

void duckdb_replacement_scan_set_function_name(
  duckdb_replacement_scan_info info,
  const char *function_name
);

##### Parameters * `info`: The info object * `function_name`: The function name to substitute.
#### `duckdb_replacement_scan_add_parameter` Adds a parameter to the replacement scan function. ##### Syntax

void duckdb_replacement_scan_add_parameter(
  duckdb_replacement_scan_info info,
  duckdb_value parameter
);

##### Parameters * `info`: The info object * `parameter`: The parameter to add.
#### `duckdb_replacement_scan_set_error` Report that an error has occurred while executing the replacement scan. ##### Syntax

void duckdb_replacement_scan_set_error(
  duckdb_replacement_scan_info info,
  const char *error
);

##### Parameters * `info`: The info object * `error`: The error message