BluetoothGATTGetCharacteristics function (bluetoothleapis.h)

Article
03/06/2023

The BluetoothGATTGetCharacteristics function gets all the characteristics available for the specified service.

Syntax

HRESULT BluetoothGATTGetCharacteristics(
  [in]            HANDLE                      hDevice,
  [in, optional]  PBTH_LE_GATT_SERVICE        Service,
  [in]            USHORT                      CharacteristicsBufferCount,
  [out, optional] PBTH_LE_GATT_CHARACTERISTIC CharacteristicsBuffer,
  [out]           USHORT                      *CharacteristicsBufferActual,
  [in]            ULONG                       Flags
);

Parameters

[in] hDevice

Handle to the Bluetooth device or service.

[in, optional] Service

Address of a BTH_LE_GATT_SERVICE structure containing the parent service of the included services to be retrieved. This parameter is required if a device handle is passed to hDevice. This parameter is optional if a service handle was passed to hDevice, in which case the service specified by the service handle will be treated as the parent.

[in] CharacteristicsBufferCount

The number of elements allocated for the CharacteristicsBuffer parameter.

[out, optional] CharacteristicsBuffer

Pointer to a buffer into which to return characteristics as an array of BTH_LE_GATT_CHARACTERISTIC structures.

[out] CharacteristicsBufferActual

Pointer to buffer into which is returned the actual number of characteristics that were returned in the CharacteristicsBuffer parameter.

[in] Flags

Flags to modify the behavior of BluetoothGATTGetCharacteristics:

Flag	Description
BLUETOOTH_GATT_FLAG_NONE	The client does not have specific GATT requirements (default).

Return value

This function returns the following values:

Return code	Description
S_OK	The operation completed successfully.
ERROR_MORE_DATA	The buffer parameter is NULL and the number of items available is being returned instead.
ERROR_ACCESS_DENIED	Returned if both a parent service and a service handle are provided and the service hierarchy does not roll up to the provided parent service handle.
ERROR_INVALID_PARAMETER	One of the following conditions occurred: CharacteristicsBuffer is NULL, and CharacteristicsBufferCount is 0 CharacteristicsBuffer is non-NULL, but CharacteristicsBufferCount is NULL CharacteristicsBuffer is non-NULL, and CharacteristicsBufferCount is 0
ERROR_INVALID_USER_BUFFER	A buffer is specified, but the buffer count size is smaller than what is required, in bytes.
ERROR_BAD_COMMAND	The current data in the cache appears to be inconsistent, and is leading to internal errors.
ERROR_NO_SYSTEM_RESOURCES	The operation ran out of memory.

Remarks

Returned characteristics are cached upon successful retrieval of characteristics from the device directly. Unless a service-change event is received, the list of returned characteristics is not expected to change.

Profile drivers should pre-allocate a sufficiently large buffer for the array of characteristics to be returned in. Callers can determine the necessary buffer size by passing a non-NULL value in CharacteristicsBufferActual and NULL in CharacteristicsBuffer.

Do not modify the returned characteristic structure, and then use the modified structure in subsequent function calls. Behavior is undefined if the caller does this.

The parent service must be present in the cache, otherwise the function will fail. The parent service must be a service returned by either BluetoothGATTGetServices or BluetoothGATTGetIncludedServices.

Example

////////////////////////////////////////////////////////////////////////////
// Determine Characteristic Buffer Size
////////////////////////////////////////////////////////////////////////////

        hr = BluetoothGATTGetCharacteristics(
                hCurrService,
                currGattService,
                0,
                NULL,
                &charBufferSize,
                BLUETOOTH_GATT_FLAG_NONE);
        
        if (HRESULT_FROM_WIN32(ERROR_MORE_DATA) != hr) {
            PrintHr("BluetoothGATTGetCharacteristics - Buffer Size", hr);
            goto Done;
        }
        
        if (charBufferSize > 0) {
            pCharBuffer = (PBTH_LE_GATT_CHARACTERISTIC)
                    malloc(charBufferSize * sizeof(BTH_LE_GATT_CHARACTERISTIC));
        
            if (NULL == pCharBuffer) {
                printf("pCharBuffer out of memory\r\n");
                goto Done;
            } else {
                RtlZeroMemory(pCharBuffer, 
                        charBufferSize * sizeof(BTH_LE_GATT_CHARACTERISTIC));
            }

////////////////////////////////////////////////////////////////////////////
// Retrieve Characteristics
////////////////////////////////////////////////////////////////////////////
    
            hr = BluetoothGATTGetCharacteristics(
                    hCurrService,
                    currGattService,
                    charBufferSize,
                    pCharBuffer,
                    &numChars,
                    BLUETOOTH_GATT_FLAG_NONE);

            if (S_OK != hr) {
                PrintHr("BluetoothGATTGetCharacteristics - Actual Data", hr);
                goto Done; // Allow continuation
            }

            if (numChars != charBufferSize) {
                printf("buffer size and buffer size actual size mismatch\r\n");
                goto Done;
            }
        }

Requirements

Requirement	Value
Minimum supported client	Supported in Windows 8 and later versions of Windows.
Target Platform	Universal
Header	bluetoothleapis.h
Library	BluetoothApis.lib
DLL	BluetoothApis.dll