mongoose/docs/c-api/net.h/mg_broadcast.md

title	decl_name	symbol_kind	signature
mg_broadcast()	mg_broadcast	func	void mg_broadcast(struct mg_mgr *mgr, mg_event_handler_t cb, void *data, size_t len);

Passes a message of a given length to all connections.

Must be called from a thread that does NOT call mg_mgr_poll(). Note that mg_broadcast() is the only function that can be, and must be, called from a different (non-IO) thread.

func callback function will be called by the IO thread for each connection. When called, the event will be MG_EV_POLL, and a message will be passed as the ev_data pointer. Maximum message size is capped by MG_CTL_MSG_MESSAGE_SIZE which is set to 8192 bytes.