S130 Attribute Table Size anomaly

[Chris]

I was looking into changing the S130 attribute table size, which is by default set to BLE_GATTS_ATTR_TAB_SIZE_DEFAULT, and noticed that this default seems to be set to zero (in ble_gatts.h):

#define BLE_GATTS_ATTR_TAB_SIZE_DEFAULT 0x0000 /**< Default Attribute Table size (0x600 bytes for this version of the SoftDevice). */

The define is clearly set to zero but the comment suggests it ought to be 0x600. Could you clarify this? Does a zero value mean that the S130 automatically uses the 0x600 default?

[tesc]

Hi,

Yes, using an attr_tab_size of BLE_GATTS_ATTR_TAB_SIZE_DEFAULT will set the default attribute table size on SoftDevice initialisation. The default value depends on SoftDevice and SoftDevice version, and it is 0x600 for the particular SoftDevice that you are using. You can use another value if you want to, as long as it is not less than BLE_GATTS_ATTR_TAB_SIZE_MIN.

Examples for how the attribute table size is set can be found in the ble_central examples. The attr_tab_size value is part of the ble_enable_params_t structure passed to sd_ble_enable() for initialising the SoftDevice (in main.c).

Regards, Terje

[fluent]

I just ran into this issue and I have to say the implementation is inconsistent and really confusing. If you name both variables BLE_GATTS_ATTR_TAB_SIZE_xxx, the implication is that both variables are defined in the same units. I would suggest renaming BLE_GATTS_ATTR_TAB_SIZE_DEFAULT to USE_DEFAULT_BLE_GATTS_ATTR_TAB_SIZE or similar, and #defining it to 1. Or just #define BLE_GATTS_ATTR_TAB_SIZE_DEFAULT to the default size, 0x600.

[pabigot]

I second the confusion comment, but propose that a simpler solution is to document in ble_gatts_enable_params_t::attr_tab_size that a value of zero uses the SD-specific default. This is implicit in the current software, but you have to dig for it and there's no promise that it will remain that way in the future.

[tesc]

Thank you to both of you for the input on this one. This was changed for version 4 of our SoftDevice API, so that BLE_GATTS_ATTR_TAB_SIZE_DEFAULT is set to the actual size value. This change is not applicable for s130 (still on API version 2), but it is present in s132 versions 4.0.2 and 5.0.0.

[pabigot]

Is it still the case that using zero is short-hand for asking for the default?

[tesc]

No, it is not. Do note that if you use BLE_GATTS_ATTR_TAB_SIZE_DEFAULT there is no change required for this when porting to a newer SoftDevice, as it is defined in the SoftDevice header files.

Edit: Not for SD versions >= 4, that is. The old versions still use the old API where 0 is default.