---
name: "modbus_mapping_new_start_address"
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_mapping_new_start_address"
---

# modbus\_mapping\_new\_start\_address(3)

allocate four arrays of bits and registers accessible from their starting addresses

## NAME

modbus\_mapping\_new\_start\_address - allocate four arrays of bits and registers accessible from their starting addresses

## SYNOPSIS

**modbus\_mapping\_t** modbus\_mapping\_new\_start\_address(int *start\_bits*, int *nb\_bits*, int *start\_input\_bits*, int *nb\_input\_bits*, int *start\_registers*, int *nb\_registers*, int *start\_input\_registers*, int *nb\_input\_registers*);\*

## DESCRIPTION

The *modbus\_mapping\_new\_start\_address()* function shall allocate four arrays to store bits, input bits, registers and inputs registers. The pointers are stored in modbus\_mapping\_t structure. All values of the arrays are initialized to zero.

The different starting addresses make it possible to place the mapping at any address in each address space. This way, you can give access to values stored at high addresses without allocating memory from the address zero, for eg. to make available registers from 10000 to 10009, you can use:

```
mb_mapping = modbus_mapping_new_start_address(0, 0, 0, 0, 10000, 10, 0, 0);
```

With this code, only 10 registers (uint16\_t) are allocated.

If it isn’t necessary to allocate an array for a specific type of data, you can pass the zero value in argument, the associated pointer will be NULL.

This function is convenient to handle requests in a Modbus server/slave.

## RETURN VALUE

The *modbus\_mapping\_new\_start\_address()* function shall return the new allocated structure if successful. Otherwise it shall return NULL and set errno.

## ERRORS

ENOMEM

Not enough memory

## EXAMPLE

```
/* The first value of each array is accessible at the defined address.
   The end address is ADDRESS + NB - 1. */
mb_mapping = modbus_mapping_new_start_address(BITS_ADDRESS, BITS_NB,
                                INPUT_BITS_ADDRESS, INPUT_BITS_NB,
                                REGISTERS_ADDRESS, REGISTERS_NB,
                                INPUT_REGISTERS_ADDRESS, INPUT_REGISTERS_NB);
if (mb_mapping == NULL) {
    fprintf(stderr, "Failed to allocate the mapping: %s\n",
            modbus_strerror(errno));
    modbus_free(ctx);
    return -1;
}
```

## SEE ALSO

[**modbus\_mapping\_new**(3)](/modbus_mapping_new/3.md) [**modbus\_mapping\_free**(3)](/modbus_mapping_free/3.md)

## AUTHORS

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