modbus_new_tcp - Man Page

create a libmodbus context for TCP/IPv4


modbus_t *modbus_new_tcp(const char *ip, int port);


The modbus_new_tcp() function shall allocate and initialize a modbus_t structure to communicate with a Modbus TCP IPv4 server.

The ip argument specifies the IP address of the server to which the client wants to establish a connection. A NULL value can be used to listen any addresses in server mode.

The port argument is the TCP port to use. Set the port to MODBUS_TCP_DEFAULT_PORT to use the default one (502). It’s convenient to use a port number greater than or equal to 1024 because it’s not necessary to have administrator privileges.

Return Value

The function shall return a pointer to a modbus_t structure if successful. Otherwise it shall return NULL and set errno to one of the values defined below.



An invalid IP address was given.


Out of memory. Possibly, the application hits its memory limit and/or whole system is running out of memory.


modbus_t *ctx;

ctx = modbus_new_tcp("", 1502);
if (ctx == NULL) {
    fprintf(stderr, "Unable to allocate libmodbus context\n");
    return -1;

if (modbus_connect(ctx) == -1) {
    fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
    return -1;

See Also

modbus_tcp_listen(3) modbus_free(3)


The libmodbus documentation was written by Stéphane Raimbault <>

Referenced By

libmodbus(7), modbus_new_rtu(3), modbus_new_tcp_pi(3), modbus_tcp_listen(3).

01/25/2024 libmodbus v3.1.7 libmodbus Manual