C++ 指针，无法读取内存

Question

为什么，当我启动我的指针时，我可以读取分配的值：

DWORD *pBytesReturned = new DWORD[0];
_result =  EnumProcesses(pProcessIds, 1000, pBytesReturned);

但是当我像这样初始化它时，我无法读取分配的值:

DWORD *pBytesReturned = 0;
_result =  EnumProcesses(pProcessIds, 1000, pBytesReturned);

如果需要，这里是完整的代码：

#include "stdafx.h"
#include <Psapi.h>

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR pCmdLine, int iCmdShow)
{
    bool _result;
    DWORD *pProcessIds = new DWORD[1000];
    DWORD cb;
    DWORD *pBytesReturned =0;
    _result =  EnumProcesses(pProcessIds, 1000, pBytesReturned);
}

Answer 1

函数的意图

BOOL
WINAPI
EnumProcesses (
    _Out_writes_bytes_(cb) DWORD * lpidProcess,
    _In_ DWORD cb,
    _Out_ LPDWORD lpcbNeeded
    );

是要有一个指针作为第三个参数，这个指针必须指向一个有效的内存位置，函数将在该位置存储一个双字。预期的函数调用应如下所示

DWORD *pProcessIds = new DWORD[1000];
DWORD bytesReturned = 0;
bool _result = EnumProcesses(pProcessIds, 1000, &bytesReturned);

或者像这样

DWORD *pProcessIds = new DWORD[1000];
DWORD *pBytesReturned = new DWORD[1];
bool _result = EnumProcesses(pProcessIds, 1000, pBytesReturned);

但您不应该使用 NULL 指针并期望调试器取消引用它。

C++ 指针，无法读取内存

C++ Pointers, Unable To Read Memory

c++

pointers

initialization