modbus_tcp_listen - Man Page

create and listen a TCP Modbus socket (IPv4)


int modbus_tcp_listen(modbus_t *ctx, int nb_connection);


The modbus_tcp_listen() function shall create a socket and listen to maximum nb_connection incoming connections on the specified IP address. The context _ctx _must be allocated and initialized with modbus_new_tcp(3) before to set the IP address to listen, if IP address is set to NULL or, any addresses will be listen.

Return Value

The function shall return a new socket if successful. Otherwise it shall return -1 and set errno.


For detailed examples, see source files in tests directory:


/* To listen any addresses on port 502 */
ctx = modbus_new_tcp(NULL, 502);

/* Handle until 10 established connections */
server_socket = modbus_tcp_listen(ctx, 10);

/* Clear the reference set of socket */

/* Add the server socket */
FD_SET(server_socket, &refset);

if (select(server_socket + 1, &refset, NULL, NULL, NULL) == -1) {



See Also

modbus_new_tcp(3) modbus_tcp_accept(3) modbus_tcp_pi_listen(3)


The libmodbus documentation was written by Stéphane Raimbault <>

Referenced By

modbus_new_tcp(3), modbus_tcp_accept(3), modbus_tcp_pi_accept(3), modbus_tcp_pi_listen(3).

07/20/2023 libmodbus v3.1.7 libmodbus Manual