1 # Civetweb API Reference
3 ### `mg_websocket_write( conn, opcode, data, data_len );`
7 | Parameter | Type | Description |
9 |**`conn`**|`struct mg_connection *`|Connection on which the data must be written|
10 |**`opcode`**|`int`|Opcode|
11 |**`data`**|`const char *`|Data to be written to the client|
12 |**`data_len`**|`size_t`|Length of the data|
16 | Type | Description |
18 |`int`|Number of bytes written or an error code|
22 The function `mg_websocket_write()` sends data to a websocket client wrapped in a websocket frame. The function issues calls to [`mg_lock_connection()`](mg_lock_connection.md) and [`mg_unlock_connection()`](mg_unlock_connection.md) to ensure that the transmission is not interrupted. Data corruption can otherwise happen if the application is proactively communicating and responding to a request simultaneously.
24 The function is available only when Civetweb is compiled with the `-DUSE_WEBSOCKET` option.
26 The function returns the number of bytes written, **0** when the connection has been closed and **-1** if an error occurred.
30 * [`mg_lock_connection();`](mg_lock_connection.md)
31 * [`mg_printf();`](mg_printf.md)
32 * [`mg_unlock_connection();`](mg_unlock_connection.md)
33 * [`mg_websocket_client_write();`](mg_websocket_client_write.md)
34 * [`mg_write();`](mg_write.md)