Using OpenProcess to obatin a valid process handle

Posted on 2004-10-06
Last Modified: 2008-01-09
I am writing an application that is tracking (cpu usage and memory usage) of other applications I wrote.  To do this I need to obatin a valid handle to the process I want to track. I am trying to do this by using some code I got from msdn that takes in the exe name.  The code will loop through the processes and I can obtain the process name to see if it the process I am looking for, but it always returns a process handle of 0x000007cc for all processes.  GetLastError returns 5 or Invalid handel.  Why is it not returning the actual processes handle?

void ProcessTracker::setName(string name)
    _name = name;  //used for cpu needs to be intretated with handle

    DWORD aProcesses[1024], cbNeeded, cProcesses;
    unsigned int i;

    if ( !EnumProcesses( aProcesses, sizeof(aProcesses), &cbNeeded ) )

      cProcesses = cbNeeded / sizeof(DWORD);

    for ( i = 0; i < cProcesses; i++ )
          TCHAR szProcessName[MAX_PATH] = TEXT("<unknown>");

            HANDLE hProcess = OpenProcess( PROCESS_QUERY_INFORMATION |
                                   FALSE, aProcesses[i] );

            DWORD err = GetLastError();

            if (NULL != hProcess )
                  HMODULE hMod;
                  DWORD cbNeeded;

                  if ( EnumProcessModules( hProcess, &hMod, sizeof(hMod),
                         &cbNeeded) )
                        GetModuleBaseName( hProcess, hMod, szProcessName,
                                                   sizeof(szProcessName)/sizeof(TCHAR) );
                              _handle = hProcess;

            CloseHandle( hProcess );
Question by:nicoleheritage
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Author Comment

ID: 12240740
By the way.  I am using Microsoft VC++ 6.0 on Win XP
LVL 56

Accepted Solution

Julian Hansen earned 200 total points
ID: 12241188
Are you sure OpenProcess is failing? OpenProcess should return NULL if it failes - if it doesn't the return value of GetLastError() is meaningless as it will hold the value of the last error encountered.

The fact that OpenProcess returns the same value each time is not an error. OpenProcess returns a handle which is an internal reference to an OS object. Because you are closing the handle after each iteration it is highly likely the next time you open a process you will get the same handle value back.

To illustrate - right after you do an OpenProcess on aProcesses[i] do

                                   FALSE, aProcesses[i+1] );

and add the corresponding close at the end for hP2. If you step through this you will see the second open for the next process in the list gets a different value from the first but always the same different value as you iterate.



Author Comment

ID: 12242942
You are right.  I moved the closehandle from the following into the class's destructor and everything worked.  

                         _handle = hProcess;


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