compute_response_size() no entry for read_input_status()

[Chris Hellyar]

libmodbus1.2.2 :

When using modbus function 0x02 - Read Discrete Inputs the reply frame is the same length as for command 0x01.

compute_response_size() had no entry for function 0x02.

I added it into modbus.c as per:

< --------------- snip ---------------->
static unsigned int compute_response_size(modbus_param_t *mb_param,
                                          unsigned char *query)
{
        int response_size_computed;
        int offset;

        offset = mb_param->header_length;

        switch (query[offset + 1]) {
        case 0x01: /* function 0x01 and 0x02 have same response size. */
        case 0x02: {
                /* Header + nb values (code from force_multiple_coils) */
                int coil_count = (query[offset + 4] << 8) | query[offset + 5];
                response_size_computed = 3 +
                        (coil_count / 8) + ((coil_count % 8) ? 1 : 0);
        } break;
< --------------- snip --------------->

Thanks for a really well put together library, I had started writing my own after looking at the 'other' libmodbus, then found this.

[Chris Hellyar (chris-trash)]

Got a bit further through my RTU stack coding, and found this also effects function 04, so I've added a similar fix for that, as per:

< --------------- snip --------------->
                        (coil_count / 8) + ((coil_count % 8) ? 1 : 0);
        } break;

        case 0x03: /* function 0x03 and 0x04 have same response size. */
        case 0x04:
                /* Header + 2 * nb values */
                response_size_computed = 3 +
< --------------- snip --------------->

Cheers, Chris H.

[Stéphane Raimbault (sra)]

You're right and this change is already on trunk.
I applied your patch on stable branch to do a new stable release.

Thank you for your bug report.

Snip from trunk branch:

/* Computes the size of the expected response */
static unsigned int compute_response_size(modbus_param_t *mb_param,
                                          unsigned char *query)
{
        int response_size_computed;
        int offset;

        offset = mb_param->header_length;

        switch (query[offset + 1]) {
        case FC_READ_COIL_STATUS:
        case FC_READ_INPUT_STATUS: {
                /* Header + nb values (code from force_multiple_coils) */
                int coil_count = (query[offset + 4] << 8) | query[offset + 5];
                response_size_computed = 3 +
                        (coil_count / 8) + ((coil_count % 8) ? 1 : 0);
                }
                break;
        case FC_READ_HOLDING_REGISTERS:
        case FC_READ_INPUT_REGISTERS:
                /* Header + 2 * nb values */
                response_size_computed = 3 +
                        2 * (query[offset + 4] << 8 | query[offset + 5]);
                break;
        case FC_READ_EXCEPTION_STATUS:
                response_size_computed = 4;
                break;
        default:
                response_size_computed = 6;
        }

        response_size_computed += offset + mb_param->checksum_size;

        return response_size_computed;
}