BluetoothGATTGetDescriptors function (bluetoothleapis.h)

Article
10/13/2021

The BluetoothGATTGetDescriptors function gets all the descriptors available for the specified characteristic.

Syntax

HRESULT BluetoothGATTGetDescriptors(
  [in]            HANDLE                      hDevice,
  [in]            PBTH_LE_GATT_CHARACTERISTIC Characteristic,
  [in]            USHORT                      DescriptorsBufferCount,
  [out, optional] PBTH_LE_GATT_DESCRIPTOR     DescriptorsBuffer,
  [out]           USHORT                      *DescriptorsBufferActual,
  [in]            ULONG                       Flags
);

Parameters

[in] hDevice

Handle to the Bluetooth device or service. If a service handle is passed, then the service must be the grandparent of the descriptor.

[in] Characteristic

Pointer to BTH_LE_GATT_CHARACTERISTIC structure containing the parent characteristic of the descriptors to be retrieved.

[in] DescriptorsBufferCount

The number of elements allocated for the DescriptorsBuffer parameter.

[out, optional] DescriptorsBuffer

Pointer to buffer containing a BTH_LE_GATT_DESCRIPTOR structure into which to return descriptors.

[out] DescriptorsBufferActual

Pointer to buffer into which the actual number of descriptors were returned in the DescriptorsBuffer parameter.

[in] Flags

Flags to modify the behavior of BluetoothGATTGetDescriptors:

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: DescriptorsBuffer is NULL, and DescriptorsBufferCount is 0. DescriptorsBuffer is non-NULL, but DescriptorsBufferCount is NULL. DescriptorsBuffer is non-NULL, and DescriptorsBufferCount 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 DescriptorsBufferActual and NULL in DescriptorsBuffer.

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 characteristic 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 Descriptor Buffer Size
////////////////////////////////////////////////////////////////////////////
GetDescriptors:
            hr = BluetoothGATTGetDescriptors(
                    hCurrService,
                    currGattChar,
                    0,
                    NULL,
                    &descriptorBufferSize,
                    BLUETOOTH_GATT_FLAG_NONE);
            
            if (HRESULT_FROM_WIN32(ERROR_MORE_DATA) != hr) {
                PrintHr("BluetoothGATTGetDescriptors - Buffer Size", hr);
                goto Done; // Allow continuation
            }
            
            if (descriptorBufferSize > 0) {
                pDescriptorBuffer = (PBTH_LE_GATT_DESCRIPTOR)
                        malloc(descriptorBufferSize 
                            * sizeof(BTH_LE_GATT_DESCRIPTOR));
            
                if (NULL == pDescriptorBuffer) {
                    printf("pDescriptorBuffer out of memory\r\n");
                    goto Done;
                } else {
                    RtlZeroMemory(pDescriptorBuffer, descriptorBufferSize);
                }
            
////////////////////////////////////////////////////////////////////////////
// Retrieve Descriptors
////////////////////////////////////////////////////////////////////////////
    
                hr = BluetoothGATTGetDescriptors(
                        hCurrService,
                        currGattChar,
                        descriptorBufferSize,
                        pDescriptorBuffer,
                        &numDescriptors,
                        BLUETOOTH_GATT_FLAG_NONE);
            
                if (S_OK != hr) {
                    PrintHr("BluetoothGATTGetDescriptors - Actual Data", hr);
                    goto Done;
                }

                if (numDescriptors != descriptorBufferSize) {
                    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