Tizen 3.0.0.2下Bluetooth LE GATT Server特征值始终为NULL求助

Hey there, let's dig into why your BlueScanner isn't showing the values for your BLE characteristic and descriptor. I spotted a few key issues in your code that are likely causing this:

1. Incorrect Descriptor UUID Format #

Your descriptor UUID is set as "0x1002" — BLE UUIDs don't use the 0x prefix. For custom 16-bit UUIDs, you should use just the string "1002", but keep in mind non-SIG 16-bit UUIDs can have compatibility issues. For better reliability, use a full 128-bit UUID that matches your service's base pattern (e.g., tweak the last few characters of your service UUID).

2. Missing Read Callback Implementation #

You registered a read callback (__bt_gatt_server_read_value_requested_cb) but didn't provide its code. BLE clients like BlueScanner rely on this callback to receive the characteristic value when they send a read request. Without it, the client won't get any data to display.

3. Initial Value Handling for Characteristic/Descriptor #

• For the characteristic, you passed a string but didn't explicitly cast it to a byte array or specify its length correctly.
• For the descriptor, "50" is a string, but if you intend to send a numeric value of 50, you should pass a byte representation instead of a string literal.

4. Double-Check Advertising Setup (Bonus) #

Make sure your Gear S3 is advertising the service UUID you created. If the client can't discover the service properly, it won't be able to access its characteristics/descriptors at all.

Here's the revised code with fixes applied:

#include <string.h>
#include <dlog.h>
#include <bluetooth.h>

#define LOG_TAG "BLE_SERVER"
#define LE_INITIAL_BUF_SIZE 64

const char *service_uuid = "ADE3D529-C784-4F63-A987-EB69F70EE816";
bt_gatt_service_type_e type = BT_GATT_SERVICE_TYPE_PRIMARY;

// Server handles
static bt_gatt_server_h gattServer = NULL;
static bt_gatt_h gattSvc = NULL;
static bt_gatt_h gattChara = NULL;
static bt_gatt_h gattDescriptor = NULL;

const char *charaUuid = "AD7B334F-4637-4B86-90B6-9D787F03D218";
const char *charaValue = "myCharacteristic";

// Fixed descriptor UUID (matching service base pattern)
const char *DescUuid = "ADE3D529-C784-4F63-A987-EB69F70EE817";
uint8_t descValue = 50; // Numeric value as byte

// Implement the read callback to return characteristic value
static void __bt_gatt_server_read_value_requested_cb(bt_gatt_h characteristic, unsigned int offset, void *user_data) {
    int ret = bt_gatt_server_send_read_value_response(
        characteristic, 
        BT_ERROR_NONE, 
        offset, 
        (unsigned char*)charaValue, 
        strlen(charaValue)
    );
    if (ret != BT_ERROR_NONE) {
        char* err = get_error_message(ret);
        dlog_print(DLOG_ERROR, LOG_TAG, "Failed to send read response: %s", err);
    }
}

void createService() {
    dlog_print(DLOG_INFO, LOG_TAG, "Create Service");
    int ret = bt_gatt_server_initialize();
    if (ret != BT_ERROR_NONE) {
        char* err = get_error_message(ret);
        dlog_print(DLOG_ERROR, LOG_TAG, "Init GATT server failed. err = %s", err);
        return;
    }

    if (!gattServer) {
        ret = bt_gatt_server_create(&gattServer);
        if (ret != BT_ERROR_NONE) {
            char* err = get_error_message(ret);
            dlog_print(DLOG_ERROR, LOG_TAG, "Create GATT server failed. err = %s", err);
            return;
        } else {
            dlog_print(DLOG_INFO, LOG_TAG, "Create GATT server Succeed");
        }
    }

    ret = bt_gatt_service_create(service_uuid, type, &gattSvc);
    if (ret != BT_ERROR_NONE) {
        char* err = get_error_message(ret);
        dlog_print(DLOG_ERROR, LOG_TAG, "Create GATT service failed. err = %s", err);
        return;
    } else {
        dlog_print(DLOG_INFO, LOG_TAG, "Create GATT service Succeed");
    }

    ret = bt_gatt_characteristic_create(
        charaUuid, 
        BT_GATT_PERMISSION_READ | BT_GATT_PERMISSION_WRITE, 
        BT_GATT_PROPERTY_INDICATE | BT_GATT_PROPERTY_READ, 
        (unsigned char*)charaValue, 
        strlen(charaValue), 
        &gattChara
    );
    if (ret != BT_ERROR_NONE) {
        char* err = get_error_message(ret);
        dlog_print(DLOG_ERROR, LOG_TAG, "create characteristic failed. err = %s", err);
        return;
    } else {
        dlog_print(DLOG_INFO, LOG_TAG, "create characteristic Succeed");
    }

    ret = bt_gatt_server_set_read_value_requested_cb(gattChara, __bt_gatt_server_read_value_requested_cb, NULL);
    if (ret != BT_ERROR_NONE) {
        char* err = get_error_message(ret);
        dlog_print(DLOG_ERROR, LOG_TAG, "register read request callback failed. err = %s", err);
        return;
    } else {
        dlog_print(DLOG_INFO, LOG_TAG, "register read request callback Succeed");
    }

    ret = bt_gatt_descriptor_create(
        DescUuid, 
        BT_GATT_PERMISSION_READ | BT_GATT_PERMISSION_WRITE, 
        &descValue, 
        sizeof(descValue), 
        &gattDescriptor
    );
    if (ret != BT_ERROR_NONE) {
        char* err = get_error_message(ret);
        dlog_print(DLOG_ERROR, LOG_TAG, "create descriptor failed. err = %s", err);
        return;
    } else {
        dlog_print(DLOG_INFO, LOG_TAG, "create descriptor server Succeed");
    }

    ret = bt_gatt_characteristic_add_descriptor(gattChara, gattDescriptor);
    if (ret != BT_ERROR_NONE) {
        char* err = get_error_message(ret);
        dlog_print(DLOG_ERROR, LOG_TAG, "add descriptor to characteristic failed. err = %s", err);
        return;
    } else {
        dlog_print(DLOG_INFO, LOG_TAG, "add descriptor to characteristic Succeed");
    }

    ret = bt_gatt_service_add_characteristic(gattSvc, gattChara);
    if (ret != BT_ERROR_NONE) {
        char* err = get_error_message(ret);
        dlog_print(DLOG_ERROR, LOG_TAG, "add characteristic to service failed. err = %s", err);
        return;
    } else {
        dlog_print(DLOG_INFO, LOG_TAG, "add characteristic to service Succeed");
    }

    ret = bt_gatt_server_register_service(gattServer, gattSvc);
    if (ret != BT_ERROR_NONE) {
        char* err = get_error_message(ret);
        dlog_print(DLOG_ERROR, LOG_TAG, "register GATT service failed. err = %s", err);
        return;
    } else {
        dlog_print(DLOG_INFO, LOG_TAG, "register GATT service Succeed");
    }

    ret = bt_gatt_server_start();
    if (ret != BT_ERROR_NONE) {
        char* err = get_error_message(ret);
        dlog_print(DLOG_ERROR, LOG_TAG, "start GATT server failed. err = %s", err);
        return;
    } else {
        dlog_print(DLOG_INFO, LOG_TAG, "start GATT server Succeed");
    }
}

Final Checks #

1. After applying these fixes, restart your Gear S3 app and re-scan with BlueScanner.
2. Tap directly on the characteristic/descriptor in BlueScanner to trigger a read request — some scanners don't auto-fetch values.
3. Verify all your log messages show "Succeed" for each step to ensure no hidden errors.

内容的提问来源于stack exchange，提问作者Mick