Why don't I get a BLE_GATTS_EVT_SYS_ATTR_MISSING event?

[brianreinhold]

This discussion got mixed up with a discussion about what Nordic means by a 'system attribute' (which is still not clear). In any case, in that discussion I was trying to save pairing/bonding information so I could restore it on a reconnect. The pairing keys worked. I grabbed the keys on the pairing complete event, saved them to a file, read them from that file, and gave them to the peer when asked on a reconnect in the BLE_GAP_EVT_SEC_INFO_REQUEST event.

But the CCCD information doesn't work that way. I can't restore the CCCD state using the same approach, because I cannot initialize the CCCD myself from the application (even prior to a connection). Instead I was told I need to get the 'system attributes' by calling this method sd_ble_gatts_sys_attr_get(), save them to a file, and restore them by calling this method sd_ble_gatts_sys_attr_set() when I get a BLE_GATTS_EVT_SYS_ATTR_MISSING event.

Fantastic! Sounds easy enough. However, I never get a BLE_GATTS_EVT_SYS_ATTR_MISSING event. I get this event BLE_GAP_EVT_SEC_INFO_REQUEST to restore the keys, so that works. What is missing?

[tesc]

Hi,

You will get the BLE_GATTS_EVT_SYS_ATTR_MISSING event if the peer requests a read on any of the system attributes.

In the case where you send indications or notifications before the peer requests such a read, for instance you want to perform a sd_ble_gatts_service_changed(), you need to do the sd_ble_gatts_sys_attr_set() first.

In our SDK, this is done for instance inside service_changed_send_in_evt() in gatt_cache_manager.c (part of Peer Manager), where the function local_db_apply_in_evt() calls gscm_local_db_cache_apply() in gatts_cache_manager.c, which eventually calls sd_ble_gatts_sys_attr_set().

Regards,
Terje

[brianreinhold]

I do make the sd_ble_gatts_sys_attr_set() first and then attempt the service changed indication.

I get invalid handle for values of the handle from 0 to 13 and when I hit 14 I get the BLE _ERROR missing system attributes.