Allow read/write callbacks to respond with modbus exception codes

[grantramsay]

Hi, I'm using a protocol that runs on top of modbus.
It has requirements like: "If attempting to write a register to an invalid value, modbus Illegal-Data-Value exception must be returned".
This was not possible to do from the read/write callbacks, so I've made a minor change to allow it.

This is a great libmodbus fork btw

[grantramsay]

Alternatively could get the user to set errno in the callback, instead of returning the negated error number if you think that's better

[grantramsay]

I found a workaround to this.
For anyone interested, you can pass the request data via callback context (or static buffer if not multi-instance) and use modbus_reply_exception:

struct callback_context {
    modbus_t *modbus;
    const uint8_t *req;
};

static int write_callback(void *user_ctx, int slave, int function, uint16_t address, int nb, const uint8_t *req)
{
    struct callback_context *cb_ctx = (struct callback_context *)user_ctx;
    modbus_reply_exception(cb_ctx->modbus, cb_ctx->req, MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE);
    return -1; // No additional reply will be sent if negative
}

int main() {
    modbus_t *modbus = modbus_new_...
    struct callback_context cb_ctx = {
        .modbus = modbus
    };
    int rc = modbus_set_reply_callbacks(modbus, &callbacks, &cb_ctx);
    uint8_t req[260];
    int rx_length = modbus_receive(modbus, req);
    cb_ctx.req = req; // Pass the request data to callbacks
    rc = modbus_reply_callback(modbus, query, rx_length);
    ...