---
name: "modbus_new_tcp"
section: "3"
platform: "linux"
package: "libmodbus-dev"
version: "3.1.6-2.1"
release: "Debian 12 (Bookworm)"
platforms: ["linux"]
url: "https://new.carta.tech/modbus_new_tcp"
---

# modbus\_new\_tcp(3)

create a libmodbus context for TCP/IPv4

## NAME

modbus\_new\_tcp - create a libmodbus context for TCP/IPv4

## SYNOPSIS

**modbus\_t \*modbus\_new\_tcp(const char \****ip***, int** *port***);**

## DESCRIPTION

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.

## ERRORS

**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.

## EXAMPLE

```
modbus_t *ctx;
ctx = modbus_new_tcp("127.0.0.1", 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

[**modbus\_tcp\_listen**(3)](/modbus_tcp_listen/3.md) [**modbus\_free**(3)](/modbus_free/3.md)

## AUTHORS

The libmodbus documentation was written by Stéphane Raimbault \<stephane.raimbault@gmail.com\>