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]

I'm trying to response as soon as I can to beat the time difference, so excuse me it is little bit aster 6 in the morning here... Your said:"Beware that you are never expected to initialize a struct of this type, as it's only an event structure". There is option static winch forces a compiler to initialize memory with 0 and yes this is static and not dynamic memory. That is exactly what SD is doing. BTW please mention the exact type: we're talking about quite few in this discussion.

http://c-faq.com/struct/structhack.html char data[] is defined in C99 and not in C89 as you said before. char data[1] is defined in both standards. Yet it is very tastefulness to use [1] as commenting it as variable length. In this case adding extra pointer and all hassles which come with it is much better for readability. It worth it from my stand point especially if this code is published for other people reading/using. Don't see any issues with static allocation by SD array of MAX_MTU size and application casting it to the any type it knows and needs.

Thank you for coming back to this discussion.