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modbus_new_tcp_pi.md

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modbus_new_tcp_pi

Name

modbus_new_tcp_pi - create a libmodbus context for TCP Protocol Independent

Synopsis

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

Description

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. "192.168.0.5" , "::1" or "server.com". 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.

:octicons-tag-24: 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.

Errors

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

Example

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));
    modbus_free(ctx);
    return -1;
}

See also