使用 unicode 时使用 WideCharToMultiByte 的正确方法

Question

I asked a question recently about using unicode and the problems that arose here:

argument of type "WCHAR *" is incompatible with parameter of type "LPCSTR" in c++

In solving one problem, I encountered another one that has taken me now a literal rabbit hole of differences between ansi and unicode. I have learnt a lot but I still cannot seem to solve this problem after almost a week of trying.

I looked at How do you properly use WideCharToMultiByte as I think this is what I need to convert the find_pid from unicode wchar back to char otherwise I get erros but to no avail.

All help little or small is appreciated as this is driving me mad.

Here is what I am trying to solve:

 DWORD find_pid(const wchar_t* procname) {
// Dynamically resolve some functions
HMODULE kernel32 = GetModuleHandleA("Kernel32.dll");

using CreateToolhelp32SnapshotPrototype = HANDLE(WINAPI *)(DWORD, DWORD);
CreateToolhelp32SnapshotPrototype CreateToolhelp32Snapshot = (CreateToolhelp32SnapshotPrototype)GetProcAddress(kernel32, "CreateToolhelp32Snapshot");

using Process32FirstPrototype = BOOL(WINAPI *)(HANDLE, LPPROCESSENTRY32);
Process32FirstPrototype Process32First = (Process32FirstPrototype)GetProcAddress(kernel32, "Process32First");

using Process32NextPrototype = BOOL(WINAPI *)(HANDLE, LPPROCESSENTRY32);
Process32NextPrototype Process32Next = (Process32NextPrototype)GetProcAddress(kernel32, "Process32Next");

// Init some important local variables
HANDLE hProcSnap;
PROCESSENTRY32 pe32;
DWORD pid = 0;
pe32.dwSize = sizeof(PROCESSENTRY32);

// Find the PID now by enumerating a snapshot of all the running processes
hProcSnap = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
if (INVALID_HANDLE_VALUE == hProcSnap)
    return 0;

if (!Process32First(hProcSnap, &pe32)) {
    CloseHandle(hProcSnap);
    return 0;
}

while (Process32Next(hProcSnap, &pe32)) {
    if (lstrcmp(procname, pe32.szExeFile) == 0) {
        pid = pe32.th32ProcessID;
        break;
    }
}

// Cleanup
CloseHandle(hProcSnap);

return pid;
  }

//changing to std::wstring does not work, already tried
std::string parentProcess = "C:\\hello.exe"
DWORD pid = find_pid(parentProcess.c_str());

Answer 1

Most likely your problem is that you're compiling with UNICODE defined. In this case PROCESSENTRY32 will actually be PROCESSENTRY32W .

But you're calling the ASCII-Version of Process32First instead of the unicode-version Process32FirstW .

Most of the winapi functions that accept both ascii & unicode arguments have 2 separate versions:

• the ascii one, those usually end with A (or nothing)
• the unicode on, those usually end with W
• A macro that switches between the ascii and unicode versions depending on wether UNICODE is defined or not.

In your case that would be:

#ifdef UNICODE
#define Process32First Process32FirstW
#define Process32Next Process32NextW
#define PROCESSENTRY32 PROCESSENTRY32W
#define PPROCESSENTRY32 PPROCESSENTRY32W
#define LPPROCESSENTRY32 LPPROCESSENTRY32W
#endif  // !UNICODE

Also keep in mind that Process32First will also populate your PROCESSENTRY32 (with the first found entry). So with your current implementation you'll always skip over the first process.

---

If you're building a windows app it's best to decide from the start if you want to use ascii or unicode.
(there's also the option to make both compile with TCHAR & friends)

Mixing them within a single app will lead to a lot of conversion problems (since not every unicode character can be represented in your ascii code page)

Also it'll make your life a lot easier if you just rely on the linker to import the functions instead of using GetProcAddress() .

If you want to stick with unicode (the default for new projects), you could write your function like this:

#include <windows.h>
#include <tlhelp32.h>

DWORD find_pid(const wchar_t* procname) {
    // Init some important local variables
    HANDLE hProcSnap = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
    PROCESSENTRY32 pe32;
    pe32.dwSize = sizeof(PROCESSENTRY32);

    // Find the PID now by enumerating a snapshot of all the running processes
    if (hProcSnap == INVALID_HANDLE_VALUE)
        return 0;

    if (!Process32First(hProcSnap, &pe32)) {
        CloseHandle(hProcSnap);
        return 0;
    }

    do {
        if (lstrcmp(procname, pe32.szExeFile) == 0) {
            CloseHandle(hProcSnap);
            return pe32.th32ProcessID;
        }
    } while (Process32Next(hProcSnap, &pe32));

    // not found
    CloseHandle(hProcSnap);
    return 0;
}

and call it like this:

std::wstring parentProcess = L"C:\\hello.exe";
DWORD pid = find_pid(parentProcess.c_str());

// or just:
DWORD pid = find_pid(L"C:\\hello.exe");

---

If you want your application to be able to compile for both unicode & ascii, you'll have to use TCHAR :

#include <windows.h>
#include <tlhelp32.h>
#include <tchar.h>

DWORD find_pid(const TCHAR* procname) {
    // Init some important local variables
    HANDLE hProcSnap = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
    PROCESSENTRY32 pe32;
    pe32.dwSize = sizeof(PROCESSENTRY32);

    // Find the PID now by enumerating a snapshot of all the running processes
    if (hProcSnap == INVALID_HANDLE_VALUE)
        return 0;

    if (!Process32First(hProcSnap, &pe32)) {
        CloseHandle(hProcSnap);
        return 0;
    }

    do {
        if (lstrcmp(procname, pe32.szExeFile) == 0) {
            CloseHandle(hProcSnap);
            return pe32.th32ProcessID;
        }
    } while (Process32Next(hProcSnap, &pe32));

    // not found
    CloseHandle(hProcSnap);
    return 0;
}

and call it like this:

DWORD pid = find_pid(_T("C:\\hello.exe"));