How to get the names of all power schemes in windows 7 in C++?

Question

I have to get the names of all available power schemes in windows 7. I try to enumerate them with the power management functions and I do get the right amount but when I call "PowerReadFriendlyName" (http://msdn.microsoft.com/en-us/library/windows/desktop/aa372740%28v=vs.85%29.aspx) it works sometimes and fails sometimes:

UCHAR   displayBuffer[256];
DWORD   displayBufferSize = sizeof(displayBuffer);
GUID    buffer;
DWORD   bufferSize = sizeof(buffer);
int     index;
int     fail=0,ok=0;
//
for(index = 0 ; ; index++)
{   ZeroMemory(&buffer, sizeof(buffer));
    ZeroMemory(&displayBuffer, sizeof(displayBuffer));
    if (PowerEnumerate(NULL,NULL,NULL, ACCESS_SCHEME,index,(UCHAR*)&buffer,&bufferSize) == ERROR_SUCCESS)
    {   if (PowerReadFriendlyName(NULL, &buffer,&NO_SUBGROUP_GUID,NULL,displayBuffer,&displayBufferSize) == ERROR_SUCCESS)
        {   ok++;
            // stuff to todo
        }
        else
        {   fail++;
            // why?
        }
    }
    else
    {   break;
    }
}

At first I had 2 custom power schemes and the retrieval of their name always failed. The standard 3 power schemes (high performance, balanced, power saver) always worked.

So I thought it had to do with the custom schemes and I manually added 2 more of them. But as it turns out now one of them actually works and I can get its name (both were derived from balanced).

I then manually added another 2 custom schemes (this time derived from power saver) and this time both seemed to work. I now have 9 in total and I can get the names of 6 of them. I cannot get the name of the 2 original custom power schemes (both derived from balanced) as well as the 2nd of the ones I added the first time.

When I type "powercfg -list" in a console I can get the list of all power schemes, but how can I get the names of all power schemes reliably in c++ without redirecting/parsing the console but using the windows power management functions?

Answer 1

The documentation of the PowerReadFriendlyName() function does not mention that the variable holding the length of the buffer gets overwritten in a successful call with a non-NULL buffer. It has therefore be set before each call of PowerReadFriendlyName() or it can fail:

UCHAR   displayBuffer[2048];
DWORD   displayBufferSize;
GUID    buffer;
DWORD   bufferSize = sizeof(buffer);
int     index;
int     fail=0,ok=0;
//
for(index = 0 ; ; index++)
{   ZeroMemory(&buffer, sizeof(buffer));
    ZeroMemory(&displayBuffer, sizeof(displayBuffer));
    if (PowerEnumerate(NULL,NULL,NULL, ACCESS_SCHEME,index,(UCHAR*)&buffer,&bufferSize) == ERROR_SUCCESS)
    {   displayBufferSize = sizeof(displayBuffer);
        if (PowerReadFriendlyName(NULL, &buffer,&NO_SUBGROUP_GUID,NULL,displayBuffer,&displayBufferSize) == ERROR_SUCCESS)
        {   ok++;
            // stuff to todo
        }
        else
        {   fail++;
        }
    }
    else
    {   break;
    }
}