How to call WMI function correctly - c++

Question

The following code fails when ExecMethod is called. Can anyone pinpoint what I am doing wrong?
#define _WIN32_DCOM
#include <iostream>
using namespace std;
#include <comdef.h>
#include <Wbemidl.h>
# pragma comment(lib, "wbemuuid.lib")
int main(int iArgCnt, char ** argv)
{
HRESULT hres;
// Step 1: --------------------------------------------------
// Initialize COM. ------------------------------------------
hres = CoInitializeEx(0, COINIT_MULTITHREADED);
if (FAILED(hres))
{
cout << "Failed to initialize COM library. Error code = 0x"
<< hex << hres << endl;
return 1; // Program has failed.
}
// Step 2: --------------------------------------------------
// Set general COM security levels --------------------------
// Note: If you are using Windows 2000, you must specify -
// the default authentication credentials for a user by using
// a SOLE_AUTHENTICATION_LIST structure in the pAuthList ----
// parameter of CoInitializeSecurity ------------------------
hres = CoInitializeSecurity(
NULL,
-1, // COM negotiates service
NULL, // Authentication services
NULL, // Reserved
RPC_C_AUTHN_LEVEL_DEFAULT, // Default authentication
RPC_C_IMP_LEVEL_IMPERSONATE, // Default Impersonation
NULL, // Authentication info
EOAC_NONE, // Additional capabilities
NULL // Reserved
);
if (FAILED(hres))
{
cout << "Failed to initialize security. Error code = 0x"
<< hex << hres << endl;
CoUninitialize();
return 1; // Program has failed.
}
// Step 3: ---------------------------------------------------
// Obtain the initial locator to WMI -------------------------
IWbemLocator *pLoc = NULL;
hres = CoCreateInstance(
CLSID_WbemLocator,
0,
CLSCTX_INPROC_SERVER,
IID_IWbemLocator, (LPVOID *) &pLoc);
if (FAILED(hres))
{
cout << "Failed to create IWbemLocator object. "
<< "Err code = 0x"
<< hex << hres << endl;
CoUninitialize();
return 1; // Program has failed.
}
// Step 4: ---------------------------------------------------
// Connect to WMI through the IWbemLocator::ConnectServer method
IWbemServices *pSvc = NULL;
// Connect to the local root\cimv2 namespace
// and obtain pointer pSvc to make IWbemServices calls.
hres = pLoc->ConnectServer(
_bstr_t(L"ROOT\\wmi"),
NULL,
NULL,
0,
NULL,
0,
0,
&pSvc
);
if (FAILED(hres))
{
cout << "Could not connect. Error code = 0x"
<< hex << hres << endl;
pLoc->Release();
CoUninitialize();
return 1; // Program has failed.
}
cout << "Connected to ROOT\\CIMV2 WMI namespace" << endl;
// Step 5: --------------------------------------------------
// Set security levels for the proxy ------------------------
hres = CoSetProxyBlanket(
pSvc, // Indicates the proxy to set
RPC_C_AUTHN_WINNT, // RPC_C_AUTHN_xxx
RPC_C_AUTHZ_NONE, // RPC_C_AUTHZ_xxx
NULL, // Server principal name
RPC_C_AUTHN_LEVEL_CALL, // RPC_C_AUTHN_LEVEL_xxx
RPC_C_IMP_LEVEL_IMPERSONATE, // RPC_C_IMP_LEVEL_xxx
NULL, // client identity
EOAC_NONE // proxy capabilities
);
if (FAILED(hres))
{
cout << "Could not set proxy blanket. Error code = 0x"
<< hex << hres << endl;
pSvc->Release();
pLoc->Release();
CoUninitialize();
return 1; // Program has failed.
}
///////////////////////////////////////////////////////////////////////////////////////////////////////////////
// set up to call the WmiSetBrightness Method
BSTR MethodName = SysAllocString(L"WmiSetBrightness");
BSTR ClassName = SysAllocString(L"WmiMonitorBrightnessMethods");
IWbemClassObject* pClass = NULL;
hres = pSvc->GetObject(ClassName, 0, NULL, &pClass, NULL);
IWbemClassObject* pInParamsDefinition = NULL;
hres = pClass->GetMethod(MethodName, 0,
&pInParamsDefinition, NULL);
IWbemClassObject* pClassInstance = NULL;
hres = pInParamsDefinition->SpawnInstance(0, &pClassInstance);
VARIANT var1;
VariantInit(&var1);
V_VT(&var1) = VT_BSTR;
V_BSTR(&var1) = SysAllocString(L"1000");
hres = pClassInstance->Put(L"Timeout", 0, &var1, CIM_UINT32); //CIM_UINT64
VARIANT var2;
VariantInit(&var2);
V_VT(&var2) = VT_BSTR;
V_BSTR(&var2) = SysAllocString(L"30");
hres = pClassInstance->Put(L"Brightness", 0, &var2, CIM_UINT8);
// Execute Method
IWbemClassObject* pOutParams = NULL;
hres = pSvc->ExecMethod(ClassName, MethodName, 0,
NULL, pClassInstance, &pOutParams, NULL);
if (FAILED(hres))
{
cout << "Could not execute method. Error code = 0x"
<< hex << hres << endl;
//VariantClear(&varCommand);
SysFreeString(ClassName);
SysFreeString(MethodName);
pClass->Release();
pInParamsDefinition->Release();
pOutParams->Release();
pSvc->Release();
pLoc->Release();
CoUninitialize();
return 1; // Program has failed.
}
// To see what the method returned,
// use the following code. The return value will
// be in &varReturnValue
VARIANT varReturnValue;
hres = pOutParams->Get(_bstr_t(L"ReturnValue"), 0,
&varReturnValue, NULL, 0);
// Clean up
//--------------------------
// VariantClear(&varCommand);
VariantClear(&varReturnValue);
SysFreeString(ClassName);
SysFreeString(MethodName);
pClass->Release();
pInParamsDefinition->Release();
pOutParams->Release();
pLoc->Release();
pSvc->Release();
CoUninitialize();
return 0;
}

Answer 1

Since the question was tagged "C++" I tidied up your code by using C++ techniques for failure checking and cleanup. It turned out that you've forgotten to check for failure in several places. The code below reports first failure at GetObject(className, ...).
#define _WIN32_DCOM
#pragma comment(lib, "wbemuuid.lib")
#include <iostream>
#include <string>
#include <stdexcept>
#include <stdlib.h> // EXIT_FAILURE, EXIT_SUCCESS
using namespace std;
#include <comdef.h>
#include <Wbemidl.h>
#define DEF_SMARTPTR_TYPE( Interface ) \
_COM_SMARTPTR_TYPEDEF( Interface, __uuidof( Interface ) )
DEF_SMARTPTR_TYPE( IWbemLocator );
DEF_SMARTPTR_TYPE( IWbemServices );
DEF_SMARTPTR_TYPE( IWbemClassObject );
bool throwX( string const& s ) { throw std::runtime_error( s ); }
string hexFrom( unsigned long v )
{
char buf[40];
sprintf( buf, "%08lx", v );
return buf;
}
struct Fail
{
string message;
explicit Fail( string const& aMessage ): message( aMessage ) {}
};
void operator ||( HRESULT hr, Fail const& failure )
{
SUCCEEDED( hr )
|| throwX( failure.message + " (Error code 0x" + hexFrom( hr ) + ")" );
}
struct ComLibUsage
{
struct Threading
{
enum Enum {
singleThreaded = COINIT_APARTMENTTHREADED,
multiThreaded = COINIT_MULTITHREADED
};
};
ComLibUsage( Threading::Enum threading = Threading::multiThreaded )
{
::CoInitializeEx( 0, threading )
|| Fail( "Failed to initialize COM library." );
}
~ComLibUsage() { ::CoUninitialize(); }
};
void cppMain()
{
// Step 1: --------------------------------------------------
// Initialize COM. ------------------------------------------
ComLibUsage comLibUsage( ComLibUsage::Threading::multiThreaded );
// Step 2: --------------------------------------------------
// Set general COM security levels --------------------------
// Note: If you are using Windows 2000, you must specify -
// the default authentication credentials for a user by using
// a SOLE_AUTHENTICATION_LIST structure in the pAuthList ----
// parameter of CoInitializeSecurity ------------------------
CoInitializeSecurity(
NULL,
-1, // COM negotiates service
NULL, // Authentication services
NULL, // Reserved
RPC_C_AUTHN_LEVEL_DEFAULT, // Default authentication
RPC_C_IMP_LEVEL_IMPERSONATE, // Default Impersonation
NULL, // Authentication info
EOAC_NONE, // Additional capabilities
NULL // Reserved
) || Fail( "Failed to initialize security" );
// Step 3: ---------------------------------------------------
// Obtain the initial locator to WMI -------------------------
IWbemLocatorPtr pLoc;
CoCreateInstance(
CLSID_WbemLocator,
0,
CLSCTX_INPROC_SERVER,
IID_IWbemLocator, (LPVOID *) &pLoc
) || Fail( "Failed to create IWbemLocator object." );
// Step 4: ---------------------------------------------------
// Connect to WMI through the IWbemLocator::ConnectServer method
// Connect to the local root\cimv2 namespace
// and obtain pointer pSvc to make IWbemServices calls.
IWbemServicesPtr pSvc;
pLoc->ConnectServer(
_bstr_t(L"ROOT\\wmi"),
NULL,
NULL,
0,
NULL,
0,
0,
&pSvc
) || Fail( "Could not connect." );
cout << "Connected to ROOT\\CIMV2 WMI namespace" << endl;
// Step 5: --------------------------------------------------
// Set security levels for the proxy ------------------------
CoSetProxyBlanket(
pSvc, // Indicates the proxy to set
RPC_C_AUTHN_WINNT, // RPC_C_AUTHN_xxx
RPC_C_AUTHZ_NONE, // RPC_C_AUTHZ_xxx
NULL, // Server principal name
RPC_C_AUTHN_LEVEL_CALL, // RPC_C_AUTHN_LEVEL_xxx
RPC_C_IMP_LEVEL_IMPERSONATE, // RPC_C_IMP_LEVEL_xxx
NULL, // client identity
EOAC_NONE // proxy capabilities
) || Fail( "Could not set proxy blanket" );
////////////////////////////////////////////////////////////////
// set up to call the WmiSetBrightness Method
_bstr_t methodName = L"WmiSetBrightness";
_bstr_t className = L"WmiMonitorBrightnessMethods";
IWbemClassObjectPtr pClass;
pSvc->GetObject(className, 0, NULL, &pClass, NULL)
|| Fail( "GetObject(className, ...) failed" );
IWbemClassObjectPtr pInParamsDefinition;
pClass->GetMethod(methodName, 0, &pInParamsDefinition, NULL)
|| Fail( "GetMethod(methodName, ...) failed" );
IWbemClassObjectPtr pClassInstance;
pInParamsDefinition->SpawnInstance(0, &pClassInstance)
|| Fail( "SpawnInstance failed" );
_variant_t var1( L"1000" );
pClassInstance->Put(L"Timeout", 0, &var1, CIM_UINT32) //CIM_UINT64
|| Fail( "Put failed for 'Timeout'" );
_variant_t var2( L"30" );
pClassInstance->Put(L"Brightness", 0, &var2, CIM_UINT8)
|| Fail( "Put failed for 'Brightness'" );
// Execute Method
IWbemClassObject* pOutParams = NULL;
//hres = pSvc->ExecMethod(className, methodName, 0,
//NULL, pClassInstance, &pOutParams, NULL)
//|| Fail( "Could not execute method" );
// To see what the method returned,
// use the following code. The return value will
// be in &varReturnValue
_variant_t varReturnValue;
pOutParams->Get(_bstr_t(L"ReturnValue"), 0, &varReturnValue, NULL, 0)
|| Fail( "Get failed" );
}
int main()
{
try
{
cppMain();
return EXIT_SUCCESS;
}
catch( exception const& x )
{
cerr << "!" << x.what() << endl;
}
return EXIT_FAILURE;
}
Cheers & hth. (even if it's a bit on the side of your question!),

Answer 2

Instead of the Step 4 & 6 from Example: Calling a Provider Method, it can set the brightness ABSOLUTELY.
Step 4:
IWbemServices *pSvc = NULL;
// Connect to the local root\cimv2 namespace
// and obtain pointer pSvc to make IWbemServices calls.
hres = pLoc->ConnectServer(
_bstr_t(L"ROOT\\WMI"),
NULL,
NULL,
0,
NULL,
0,
0,
&pSvc
);
if (FAILED(hres))
{
cout << "Could not connect. Error code = 0x"
<< hex << hres << endl;
pLoc->Release();
CoUninitialize();
return 1; // Program has failed.
}
cout << "Connected to ROOT\\WMI WMI namespace" << endl;
Step 6:
BSTR ClassName = SysAllocString(L"WmiMonitorBrightnessMethods");
BSTR MethodName = SysAllocString(L"WmiSetBrightness");
BSTR bstrQuery = SysAllocString(L"Select * from WmiMonitorBrightnessMethods");
IEnumWbemClassObject *pEnum = NULL;
hres = pSvc->ExecQuery(_bstr_t(L"WQL"), //Query Language
bstrQuery, //Query to Execute
WBEM_FLAG_FORWARD_ONLY | WBEM_FLAG_RETURN_IMMEDIATELY, //Make a semi-synchronous call
NULL, //Context
&pEnum /*Enumeration Interface*/);
hres = WBEM_S_NO_ERROR;
ULONG ulReturned;
IWbemClassObject *pObj;
DWORD retVal = 0;
//Get the Next Object from the collection
hres = pEnum->Next(WBEM_INFINITE, //Timeout
1, //No of objects requested
&pObj, //Returned Object
&ulReturned /*No of object returned*/);
IWbemClassObject* pClass = NULL;
hres = pSvc->GetObject(ClassName, 0, NULL, &pClass, NULL);
IWbemClassObject* pInParamsDefinition = NULL;
hres = pClass->GetMethod(MethodName, 0, &pInParamsDefinition, NULL);
IWbemClassObject* pClassInstance = NULL;
hres = pInParamsDefinition->SpawnInstance(0, &pClassInstance);
VARIANT var1;
VariantInit(&var1);
BSTR ArgName0 = SysAllocString(L"Timeout");
V_VT(&var1) = VT_BSTR;
V_BSTR(&var1) = SysAllocString(L"0");
hres = pClassInstance->Put(ArgName0,
0,
&var1,
CIM_UINT32); //CIM_UINT64
printf("\nPut ArgName0 returned 0x%x:", hres);
VariantClear(&var1);
VARIANT var;
VariantInit(&var);
BSTR ArgName1 = SysAllocString(L"Brightness");
V_VT(&var2) = VT_BSTR;
V_BSTR(&var2) = SysAllocString(L"80"); //Brightness value
hres = pClassInstance->Put(ArgName1,
0,
&var2,
CIM_UINT8);
VariantClear(&var2);
printf("\nPut ArgName1 returned 0x%x:", hres);
// Call the method
VARIANT pathVariable;
VariantInit(&pathVariable);
hres = pObj->Get(_bstr_t(L"__PATH"),
0,
&pathVariable,
NULL,
NULL);
printf("\npObj Get returned 0x%x:", hres);
hres = pSvc->ExecMethod(pathVariable.bstrVal,
MethodName,
0,
NULL,
pClassInstance,
NULL,
NULL);
VariantClear(&pathVariable);
printf("\nExecMethod returned 0x%x:", hres);
printf("Terminating normally\n");
// Clean up
//--------------------------
//VariantClear(&varCommand);
//VariantClear(&varReturnValue);
SysFreeString(ClassName);
SysFreeString(MethodName);
pClass->Release();
pClassInstance->Release();
pInParamsDefinition->Release();
//pOutParams->Release();
pLoc->Release();
pSvc->Release();
CoUninitialize();
system("pause");
return 0;
ref: 通过WMI的方式去设置LCD背光亮度 Author: Tody