Confusing definition

[michurin]

C:\Nordic Semiconductor\nrf51_sdk_v4_4_1_31827\nrf51822\Include\ble\softdevice\ble_gatts.h

typedef struct {
  uint16_t     handle;    /**< Attribute Handle. */
  uint8_t       op;          /**< Type of write operation, see @ref BLE_GATTS_OPS. */
  ble_gatts_attr_context_t    context;     /**< Attribute Context. */
  uint16_t      offset;      /**< Offset for the write operation. */
  uint16_t      len;          /**< Length of the incoming data. */
  uint8_t       data[1];    /**< Incoming data, variable length. */
} ble_gatts_evt_write_t;

My question is how to interpret the element data[1]? Is it just uint8_t *data(C-wise it is)? If so why is it defined in such a way? Especially with the size of the data array as one with the following comment about its variable length which all together makes this definition very confusing.

Thanks.

[michurin]

One more thing: if array has predefined length then it needs to be less than uint16_t len; /* Length of the incoming data. */.

The way I see the intention of the developer is:

typedef union {
unit8_t event_x[1];  /* event x message */
uint16_t event_y[2];/* event y message */
void *event_z; /* event z message */
/* more event messages if necessary */
} event_message_t; /* union will have the size of known largest message */
struct
{
...
uint16_t len; /* Length of the incoming data; len <= sizeof(event_message_t); */
event_message_t data;
}ble_gatts_evt_write_t;

[michurin]

Sorry again...How can one implement variable length? Byte by byte with the offset 1? Why then have member len? I have serious trouble understanding your point. Yes one can create ble_gatts_evt_X_write_t, ble_gatts_evt_Y_write_t, ble_gatts_evt_Z_write_t and so on and in this case each event will have its own constant not variable length. Or use ble_gatts_evt_write_t with the length 1 and use offset if more than one byte is required. Or I'm totally misunderstanding what those types are trying to achieve...

Lets say I have LED which is using event write to turn it on and off(bool) and I have a DAC and need to set it up(uint16_t). How to manage this combination?

Thanks!