Skip to content



modbus_new_tcp - 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.


  • EINVAL, an invalid IP address was given.
  • ENOMEM, 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