How to effectively kill a process in C++ (Win32)?

The PID you need for OpenProcess() is not normally easy to get a hold of. If all you got is a process name then you need to iterate the running processes on the machine. Do so with CreateToolhelp32Snapshot, followed by Process32First and loop with Process32Next. The PROCESSENTRY32.szExeFile gives you the process name (not path!), th32ProcessID gives you the PID.

The next consideration is that the process may appear more than once. And there's a chance that the same process name is used for very different programs. Like "Setup". If you don't just want to kill them all, you'll need to try to obtain some runtime info from them. Window caption bar text, perhaps. GetProcessImageFileName() can give you the path to the .exe. It uses the native kernel format, you'd need QueryDosDevice to map a disk drive device name to a drive letter.

The next consideration is the rights you ask for in OpenProcess(). You are unlikely to get PROCESS_ALL_ACCESS, all you need is PROCESS_TERMINATE. Although that's privileged as well. Ensure the account you use to run your program can obtain that right.

Rather than going through all that pain to kill a process with a known name, why not simply call out to "system" and ask the command-line to kill it?

For example,

int retval = ::_tsystem( _T("taskkill /F /T /IM MyProcess.exe") );

The following code works:

const auto explorer = OpenProcess(PROCESS_TERMINATE, false, process_id);
TerminateProcess(explorer, 1);
CloseHandle(explorer);