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;

[Ole Morten]

This isn't really feasible, as the value within this array can be any data, of any format. If you look into the on-air operations, there is no requirement that this data is structured in any way. Also, even if the characteristic value have a certain structure, the data used here can be offset, and hence have no structure after all. A variable sized array is therefore the only reasonable option as far as I can see.