ZeroMemory on PROCESSENTRY32 local variable? - visual-c++

Question

I needed to enumerate running processes and wondered for a while why my code wasn't working:
PROCESSENTRY32 ProcEntry;
ZeroMemory (&ProcEntry, sizeof (PROCESSENTRY32)); //problem
ProcEntry.dwFlags = sizeof(PROCESSENTRY32);
HANDLE Snapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, NULL);
if (Snapshot == INVALID_HANDLE_VALUE)
return false;
if (Process32First(Snapshot, &ProcEntry))
....
Problem was that Process32First always returned FALSE because of ERROR_BAD_LENGTH error.
Once I removed ZeroMemory line, everything started working fine. So the question is, why ZeroMemory caused it? It should just fill memory at the address of X for Z bytes. I use it a lot for winapi pointer-like structures, this time I didnt realise its a local variable but that doesn't explain the problem or does it?
Thanks,
Kra
EDIT: plus I found out code works fine only in Debug version, once I compile it as Release version, its bugged again :/

Answer 1

You should set dwSize, not dwFlags.
ProcEntry.dwFlags = sizeof(PROCESSENTRY32);
should be
ProcEntry.dwSize = sizeof(PROCESSENTRY32);

Answer 2

You cannot zero out the entire PROCESSENTRY32 structure as it is self-describing - you have to set dwSize. From the sample here:
HANDLE hProcessSnap;
HANDLE hProcess;
PROCESSENTRY32 pe32;
DWORD dwPriorityClass;
// Take a snapshot of all processes in the system.
hProcessSnap = CreateToolhelp32Snapshot( TH32CS_SNAPPROCESS, 0 );
if( hProcessSnap == INVALID_HANDLE_VALUE )
{
printError( TEXT("CreateToolhelp32Snapshot (of processes)") );
return( FALSE );
}
// Set the size of the structure before using it.
pe32.dwSize = sizeof( PROCESSENTRY32 );
// Retrieve information about the first process,
// and exit if unsuccessful
if( !Process32First( hProcessSnap, &pe32 ) )
{
printError( TEXT("Process32First") ); // show cause of failure
CloseHandle( hProcessSnap ); // clean the snapshot object
return( FALSE );
}