Question

Why does the following code always print out 127 (“The specified procedure could not be found.”) even though it finds “firefox.exe” and terminates it successfully??

#include<Windows.h>
#include <TlHelp32.h>
#include<iostream>
using namespace std;

int main( int, char *[] )
{
    PROCESSENTRY32 entry;
    entry.dwSize = sizeof(PROCESSENTRY32);

    HANDLE snapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, NULL);

    if (Process32First(snapshot, &entry) == TRUE)
    {
        while (Process32Next(snapshot, &entry) == TRUE)
        {
            if (_tcscmp(entry.szExeFile, TEXT("firefox.exe")) == 0)
            {  
                HANDLE hProcess = OpenProcess(PROCESS_ALL_ACCESS, FALSE, entry.th32ProcessID);
                DWORD d = GetLastError();
                cout<<d<<'\n';
                TerminateProcess(hProcess,0);

                CloseHandle(hProcess);
            }
        }
    }

    CloseHandle(snapshot);

    return 0;
}

Answer 1

You need to check return values for specific functions before checking GetLastError.

GetLastError will always return the last error code that was set by by an API function. However, not all functions will set the last-error code when they succeed. So the result from GetLastError may be an error code that was set at some other part of your program.

So in your case, according to the MSDN docs for OpenProcess, the function will return NULL if it fails. Hence there’s no need to look at GetLastError if OpenProcess returns other than NULL.

Note that the last-error code is unique to the current thread.