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Man Pages
MODBUS_MAPPING_NEW_S(3) libmodbus Manual MODBUS_MAPPING_NEW_S(3)

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

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);*

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 adresses 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 adresses without allocating memory from the address zero, for eg. to make available registers from 10000 to 10009, you can use:
mb_mapping = modbus_mapping_offset_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.

The modbus_mapping_offset_new() function shall return the new allocated structure if successful. Otherwise it shall return NULL and set errno.

ENOMEM
Not enough memory

/* The first value of each array is accessible at the defined address.
   The end address is ADDRESS + NB - 1. */
mb_mapping = modbus_mapping_offset_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;
}

modbus_mapping_new(3) modbus_mapping_free(3)

The libmodbus documentation was written by Stéphane Raimbault <stephane.raimbault@gmail.com>
12/20/2018 libmodbus v3.1.4

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