sd_ble_gatts_value_set with const data

[Adrian Eggenberger]

Hi

When implementing some characteristic handling i noticed, that in the newer softdevices the function "sd_ble_gatts_value_set" has changed to the following declaration: uint32_t sd_ble_gatts_value_set(uint16_t conn_handle, uint16_t handle, ble_gatts_value_t *p_value);

In the structure ble_gatts_value_t the member p_value points to the data to be written. As the type of p_value is "uint8_t *" i got a warning when using the function with a pointer to a constnant value. I ignored that by casting the pointer to (uint8_t *) because i assuemd, that when using the sturcture in sd_ble_gatts_value_set the data space won't be written. This actually work on target without a problem.

Yesterday i found out, that when this is used with serialization it is a problem. The serialization implementation of sd_ble_gatts_value_set somehow tries to write to the area the the p_value pointer points at. I did a quick fix for the probelm by copy the constant data into RAM anf us this area on the call of sd_ble_gatts_value_set.

I'm now not sure what is the right way to use sd_ble_gatts_value_set. If not necessairy i would prefere not to copy a value everytime it needs to be set. From my opinion the serialization should behave as much as possible like the softdevice itself.

How do you use the "sd_ble_gatts_value_set" function with constant data? Can the behavior of the serialisation be fixed to be equal to the softdevice?

Regards Adrian

[Susheel Nuguru]

Hi Adrian,

typedef struct
{
  uint16_t  len;        /**< Length in bytes to be written or read. Length in bytes written or read after successful return.*/
  uint16_t  offset;     /**< Attribute value offset. */
  uint8_t   *p_value;   /**< Pointer to where value is stored or will be stored. 
                             If value is stored in user memory, only the attribute length is updated when p_value == NULL.
                             Set to NULL when reading to obtain the complete length of the attribute value */
} ble_gatts_value_t;

As you can see that the len field will be updated by SD. That is the reason for the member being [in,out] So the behavior or newer softdevice and serialization code are compatible,

[Adrian Eggenberger]

I'm not sure if the softdevice really writes to this structure on sd_ble_gatts_value_set. The type ble_gatts_value_t is used for writing values and reading values. But it makes only sense that the region is written when reading values.

I personaly prefered the old API without this structure ble_gatts_value_t where it was possible to write constant values. As a consequence of the new API I'm forced to copy the value to RAM before calling sd_ble_gatts_value_set.

[Susheel Nuguru]

you are right, it will be used only for reads, i am not sure for the motivation of merging this. But the serialization team just writes codecs on what the API says and they do not know what actually softdevice is doing. Unfortunately your workaround even though looks unnecessary overhead but at this time is unavoidable.