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modbus_new_tcp_pi - create a libmodbus context for TCP Protocol Independent


*modbus_t *modbus_new_tcp_pi(const char *node, const char *service);


The modbus_new_tcp_pi() function shall allocate and initialize a modbus_t structure to communicate with a Modbus TCP IPv4 or IPv6 server.

The node argument specifies the host name or IP address of the host to connect to, eg. "" , "::1" or "". A NULL value can be used to listen any addresses in server mode.

The service argument is the service name/port number to connect to. To use the default Modbus port, you can provide an NULL value or the string "502". On many Unix systems, it's convenient to use a port number greater than or equal to 1024 because it's not necessary to have administrator privileges.

v3.1.8 handles NULL value for service (no EINVAL error).

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.


  • 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_pi("::1", "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