Using the Device Power API
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Using the Device Power API

Use the Device Power programming elements to manage the way devices perform while the system is in a sleep state.

Opening and closing the device list

Opening and closing the device list is a costly process in terms of CPU time. The DevicePowerOpen and DevicePowerClose functions that do this are only necessary if the application needs to query the device list multiple times.

If DevicePowerOpen is called, DevicePowerClose must be called when device-list queries are complete. DevicePowerEnumDevices and DevicePowerSetDeviceState will not close the device list if it has been explicitly opened by DevicePowerOpen.

Enumerating devices

The DevicePowerEnumDevices function detects whether the device list is open, and if not, opens it. DevicePowerEnumDevices enumerates the devices based on the specified search criteria. If the device list was opened by the function, it is closed before the function returns.

Enabling and disabling a device from waking the system

The DevicePowerSetDeviceState function, similar to DevicePowerEnumDevices, detects whether the device list is open, and if not, opens it. DevicePowerSetDeviceState then sets the specified criteria for the specified device. If the device list was opened by the function, it is closed before the function returns.

Example Code

The following example demonstrates the use of the Device Power API.

#define _WIN32_WINNT 0x0600
#include <Windows.h>
#include <PowrProf.h>
#include <stdio.h>
#include <tchar.h>

#pragma comment(lib, "PowrProf.lib")

int __cdecl main()
 // Define and initialize our return variables.
 LPWSTR  pRetBuf = NULL;
 ULONG bufSize = MAX_PATH * sizeof(WCHAR);
 ULONG uIndex = 0;

 // Open the device list, querying all devices
 if (!DevicePowerOpen(0)) 
   printf("ERROR: The device database failed to initialize.\n");
   return FALSE;

 // Enumerate the device list, searching for devices that support 
 // waking from either the S1 or S2 sleep state and are currently 
 // present in the system, and not devices that have drivers 
 // installed but are not currently attached to the system, such as 
 // in a laptop docking station.

 pRetBuf = (LPWSTR)LocalAlloc(LPTR, bufSize);

 while (NULL != pRetBuf && 
        0 != (bRet = DevicePowerEnumDevices(uIndex,
   printf("Device name: %S\n", pRetBuf);

   // For the devices we found that have support for waking from 
   // S1 and S2 sleep states, disable them from waking the system.
   bRet = (0 != DevicePowerSetDeviceState((LPCWSTR)pRetBuf, 

   if (0 != bRet) 
     printf("Warning: Failed to set device state.\n");

 // Close the device list.
 if (pRetBuf!= NULL) LocalFree(pRetBuf);
 pRetBuf = NULL;

 return TRUE;

In this example the device list is opened once, and closed once. If the DevicePowerOpen and DevicePowerClose functions were not called, the device list would have been opened and closed by each call to DevicePowerEnumDevices and DevicePowerSetDeviceState. By using DevicePowerOpen and DevicePowerClose we saved opening the device list two unnecessary times.



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