Dynamically Allocated GATT Service Not Visible in nRF Connect for mobile

[Mahmoud Fadel]

When dynamically allocating BLE GATT services and characteristics using custom structures and Zephyr’s dynamic memory functions (e.g., k_malloc, k_calloc), the service registration using bt_gatt_service_register function reports success and the internal GATT database enumeration (via bt_gatt_foreach_attr) shows all attributes with correct UUIDs and permissions. However, the dynamically allocated service does not appear in external BLE scanning tools (e.g., nRF Connect for mobile).
Additional Notes:
The issue does not occur with statically defined services (using BT_GATT_SERVICE_DEFINE), but I'm asking about dynamic allocation and if there any example show steps of dynamic service allocation

[Hung Bui]

Hi Mahmoud, 
Have you made sure you included: 

CONFIG_BT_GATT_SERVICE_CHANGED=y
Without service changed characteristic, the phone won't update the attribute database and may show the cached one. You can try to turn off and on Bluetooth on the phone to check . 

CONFIG_BT_GATT_DYNAMIC_DB=y

Dynamic DB is needed to change the att database. 

You can take a look at this sample: https://github.com/zephyrproject-rtos/zephyr/pull/84401

[Mahmoud Fadel]

Thank you for your response!
I have included the flag CONFIG_BT_GATT_SERVICE_CHANGED=y, but the issue still persists. Below is a snippet of the code I used to dynamically add services and characteristics, which I hope will be helpful.

static uint32_t ble_add_chars(tstr_service str_svc)
{
	uint32_t return_value = _ERROR;
	uint8_t total_attrs = 1;  // for the service declaration attribute
    for (uint8_t counter = 0; counter < str_svc.u8_no_of_char; counter++) {
        // Declarations + Value => 2
        // If notify or indicate => +1 for CCCD
        uint8_t needed = 2;
        if (str_svc.pstr_chars[counter].str_char_perm.u8_notify ||
            str_svc.pstr_chars[counter].str_char_perm.u8_indicate) {
            needed++;
        }
        total_attrs += needed;
    }

	struct bt_gatt_attr *attrs = k_calloc(total_attrs, sizeof(struct bt_gatt_attr));
    if (!attrs) {
        printk("No memory for allocate attrs\n");
        return_value = _ERROR;
    }

	struct bt_gatt_service *svc = k_malloc(sizeof(struct bt_gatt_service));
    if (!svc) {
        printk("No memory for allocate service\n");
        k_free(attrs);
        return_value = _ERROR;
    }

	struct bt_uuid *primary_svc_uuid = alloc_uuid16(str_svc.u16_uuid);
	service_val.uuid = primary_svc_uuid;
    uint8_t idx = 0;
    attrs[idx].uuid = BT_UUID_GATT_PRIMARY;       // "Primary Service"  
    attrs[idx].perm = BT_GATT_PERM_READ;         // read only service
    attrs[idx].read = bt_gatt_attr_read_service;  // Standard read func
	attrs[idx].user_data = &service_val;
    idx++;

	for (uint8_t counter = 0; counter < str_svc.u8_no_of_char; counter++)
    {
        tstr_char *p_char = &str_svc.pstr_chars[counter];
		struct bt_uuid *dyn_char_uuid = alloc_uuid16(p_char->u16_uuid);

        // 1) Characteristic Declaration
        attrs[idx].uuid = BT_UUID_GATT_CHRC;       // "Characteristic Declaration"
        attrs[idx].perm = BT_GATT_PERM_READ;       // Usually read only
        attrs[idx].read = bt_gatt_attr_read_chrc; //bt_gatt_attr_read

        struct bt_gatt_chrc *chrc_decl = k_malloc(sizeof(struct bt_gatt_chrc));
        if (!chrc_decl) {
            printk("No memory for chrc_decl\n");
            break;
        }

        // Map permission bits to Zephyr's properties
        uint8_t properties = 0;
        if (p_char->str_char_perm.u8_read) {
            properties |= BT_GATT_CHRC_READ;
        }
        if (p_char->str_char_perm.u8_write) {
            properties |= BT_GATT_CHRC_WRITE;
        }
        if (p_char->str_char_perm.u8_write_wo_resp) {
            properties |= BT_GATT_CHRC_WRITE_WITHOUT_RESP;
        }
        if (p_char->str_char_perm.u8_notify) {
            properties |= BT_GATT_CHRC_NOTIFY;
        }
        if (p_char->str_char_perm.u8_indicate) {
            properties |= BT_GATT_CHRC_INDICATE;
        }
		attrs[idx].user_data = (void *)(uintptr_t)properties;
		idx++;

		struct bt_uuid *dyn_char_uuid = alloc_uuid16(p_char->u16_uuid);
        if (!dyn_char_uuid) {
            return_value= -ENOMEM;
        }
        // 2) Characteristic Value
        attrs[idx].uuid  = dyn_char_uuid;
        attrs[idx].perm  = 0;
        attrs[idx].perm |= BT_GATT_PERM_READ;
        attrs[idx].perm |= BT_GATT_PERM_WRITE;
        
		
		attrs[idx].read  = read_cb;		
        attrs[idx].write = write_cb;

        // store initial data or a pointer in user_data
        attrs[idx].user_data = "InitialZephyrValue"; // for test
        idx++;

        // 3) Optional CCCD if notify/indicate is set
        if (p_char->str_char_perm.u8_notify || p_char->str_char_perm.u8_indicate) {
            attrs[idx].uuid  = BT_UUID_GATT_CCC;
            attrs[idx].perm  = BT_GATT_PERM_READ | BT_GATT_PERM_WRITE;
            attrs[idx].read  = bt_gatt_attr_read_ccc;
            attrs[idx].write = bt_gatt_attr_write_ccc;
            // Typically store a ccc_cfg array + callback
			struct bt_gatt_ccc_cfg *ccc_cfg = k_malloc(sizeof(struct bt_gatt_ccc_cfg) * BT_GATT_CCC_MAX);
            struct _cccfg_with_cb {
                struct bt_gatt_ccc_cfg *cfg;
                bt_gatt_ccc_cfg_changed_cb cb;
            }; 
			struct _cccfg_with_cb *ccc_data = k_malloc(sizeof(struct _cccfg_with_cb));
            if (!ccc_data) {
                printk("Out of memory for ccc_data\n");
                break;
            }
            ccc_data->cfg = ccc_cfg;
            ccc_data->cb  = cccd_cfg_changed;

            attrs[idx].user_data = ccc_data;
            idx++;
        }

    }

	svc->attrs         = attrs;
    svc->attr_count    = idx;
	return_value = bt_gatt_service_register(svc);
    if (_SUCCESS != return_value) {
        printk("Failed to register service with error : %d\n", return_value);
        k_free(attrs);
        k_free(svc);
        return_value = _ERROR;
    }
	else{
		printk("Service with %d attributes registered successfully.\n", idx);
    	return_value =  _SUCCESS;
	}

	return (return_value);
}

[Hung Bui]

Hi Mahmoud, 

After you add the service if you do a disconnect and connect again do you see the service ? 

Could you take a screenshot of the attribute table on the phone ? 
Please also try using  nRF Connect for Desktop to check the attribute table as it doesn't cache the service. 

Could you explain what you are planning to do when adding dynamic service ? It's not very common to have a service being added dynamically. 

[Mahmoud Fadel]

Thank you for your response.

The attached screenshot from nRF Connect for Mobile shows only the default services, with no additional ones added. Despite multiple attempts to disconnect and reconnect, the issue remains unchanged. I have also tried using nRF Connect for Desktop but observed the same behavior.

Regarding your question about why I am trying to add dynamic service like this way, I want to add services dynamically based on specific conditions in my app

Have you noticed anything in the code I provided that might be incorrect and potentially causing the issue?

[Hung Bui]

Hi, 
Please click on Generic Attribute. 

Have you added CONFIG_BT_GATT_DYNAMIC_DB=y ? 
I don't see a problem with your code. But could you take a look at this test sample : 
\zephyr\tests\bsim\bluetooth\host\gatt\sc_indicate

It does exactly what you want to do (see peripheral.c). 

[MarkLaloo]

Hi,

I've run into the same issue, NCS v2.9.0, nRF54L15. 

Initially, I defined my attribute structs statically, which works as expected.

I'm now trying to do something similar to Mahmoud; dynamic allocation. bt_gatt_service_register shows no error (err = 0), though I am unable to see the services in nRF Connect.

To sanity check, I passed the static defined attrs variable into my service and this works; so there is definitely something missing in how the bt_gatt_attr structs are being defined or configured.

Is there any sample that displays this approach? Thus far, I've only come across samples with statically defined Attributes and Services.

* Generic Attribute and Generic Access contents show no change regardless of bt_gatt_service_register being called with dynamic attributes, CONFIG_BT_GATT_DYNAMIC_DB is enabled, as well as CONFIG_BT_GATT_SERVICE_CHANGED. I've also referenced this Ticket, CONFIG_SETTINGS=y made no difference. I've also confirmed toggling BT on the phone changes nothing, force refresh of services also does nothing, and no Bond information to delete.

Thanks in advance!

Kind regards,
Mark Laloo

[MarkLaloo]

Hi,

Stumbled across this ticket, which helped solve the issue (see sample in the Answer response)

Kind regards,
Mark Laloo