modbus_mapping_new_start_address

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(
    unsigned int start_bits, unsigned int nb_bits,
    unsigned int start_input_bits, unsigned int nb_input_bits,
    unsigned int start_registers, unsigned int nb_registers,
    unsigned int start_input_registers, unsigned 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 clients to values stored at high addresses without allocating memory from the address zero, for example to make available registers from 340 to 349, you can use:

mb_mapping = modbus_mapping_new_start_address(0, 0, 0, 0, 340, 10, 0, 0);

The newly created mb_mapping will have the following arrays:

tab_bits set to NULL
tab_input_bits set to NULL
tab_registers allocated to store 10 registers (uint16_t)
tab_input_registers set to NULL

The clients can read the first register by using the address 340 in its request. On the server side, you should use the first index of the array to set the value at this client address:

mb_mapping->tab_registers[0] = 42;

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.

Because the Modbus address space is 16-bit, each of the nb_bits, nb_input_bits, nb_registers and nb_input_registers arguments must not exceed 65536 entries. Larger values are rejected to avoid excessively large allocations driven by untrusted configuration.

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

EINVAL, one of the requested array dimensions exceeds 65536 entries.
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;
}