mongoose/docs/overview/mbufs.md

Memory buffers

Each connection has a send and receive buffer, struct mg_connection::send_mbuf and struct mg_connection::recv_mbuf respectively. When data arrives, Mongoose appends received data to the recv_mbuf and triggers an MG_EV_RECV event. The user may send data back by calling one of the output functions, like mg_send() or mg_printf(). Output functions append data to the send_mbuf. When Mongoose successfully writes data to the socket, it discards data from struct mg_connection::send_mbuf and sends an MG_EV_SEND event. When the connection is closed, an MG_EV_CLOSE event is sent.