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# modbus_new_tcp
## Name
modbus_new_tcp - create a libmodbus context for TCP/IPv4
## Synopsis
```c
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
```c
modbus_t *ctx;
ctx = modbus_new_tcp("127.0.0.1", 502);
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](modbus_tcp_listen.md)
- [modbus_free](modbus_free.md)
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